Arc welder triac controler

[rodrigopires]

Hi, i have made this triac power controller for my homemade MOT arc welder, but the first time i plugged it in on mains, 230VAC, with no load, one of the 27K resistors (R6 / R7) blow up. What could be the problem?

Thanks!

[IconicPCB]

what is the purpose of the two resistors in the circuit?

[rodrigopires]

what is the purpose of the two resistors in the circuit?

DB1 diode bridge and R6-R7 resistors are used to discharge C1 at each line zero voltage. This resets the voltage charge at each new half line cycle, and keeps the same turn-on delay.

[rodrigopires]

Help? I really want to finish this project and i don't want to blow more parts, so if someone could help, i would appreciate it. Thanks!

[TheElectricChicken]

are photos of the board possible, to check for mistakes, just in case the circuit is not consistent from paper to real world.

[TheElectricChicken]

it is difficult to understand the function of the first rectifier, it is a strange configuration for a rectifier. It doesn't turn AC into DC as most rectifiers do.

I think this one is beyond me.

[TheElectricChicken]

this one has a more conventional easy to understand cct, might be a backup as you'd have the parts already.



https://sites.google.com/site/jfpayeur/welder

[eneuro]

What could be the problem?

I do not like this design at all, while you have potentiometers P1, P2 (I guess) NOT galvanic insulated from AC mains 

Anyway, In spot welder I've used two thyristors in anti-parallel configuration and they can be easy trigered using optotriac from small MPU powered from 12V battery or low power small trafo <$2 
AC mains zero crossing can be detected using two PC817 in anti-parallel.
I see no reason to use oldschool analog dimmers those days, since small 8 pin MPU <$1 can be easy programmed to do this job in more sophisticated way.

I think, galvanic insualtion from AC mains of anything used to controll arc welder is a must, especially in DIY home made projects....while so many things can go wrong and it is better have a few kV insulation, not only for human safety, but also to limit possible circuit failure to power triacs/thyristors and make it easier to debug and test, while we have optoisolated subcircuits and we can test them alone to ensure they works as expected 

[TheElectricChicken]

I expect it is possible however, to bury the potentiometer deep inside the case and glue an insulator to the shaft. The insulator can run to the outside as the knob.... ?

[IconicPCB]

1W rating of the resistors is on the low side.

Resistors are not the most expensive components. Buildup a 27K resistor from four 27K 1w resistors by runing them in aeries parallel combination ( two series groups of two parallel resistors).

Presently the resistor are operating close to their upper dissipation rating.

[rodrigopires]

Thanks for the help, here are some photos of the circuit.

this one has a more conventional easy to understand cct, might be a backup as you'd have the parts already.



https://sites.google.com/site/jfpayeur/welder

I have seen this circuit but Q1 is too expensive for me, so i tried this one instead.

1W rating of the resistors is on the low side.

I was just testing for the first time, with no load, is t normal to pop 1W resistor with no load, and also my 16A circuit breaker shuts off.

[TheElectricChicken]

when you say no load, do you mean no transformer connected ?

is it possible to get a closer-up view from direct above the components and a view of the underside. People could use the two to check the wiring I think..

Strong light may help make the picture clear too...

[rodrigopires]

yes, with nothing on the output wires.

[Andy Watson]

I was just testing for the first time, with no load, is t normal to pop 1W resistor with no load, and also my 16A circuit breaker shuts off.

I would guess that either you've not wired the circuit as per your diagram and/or there is a short on the board.

Either way: 27k at 1W rating is pushing it. Are the resistors rated for 325V operation? Are you sure it was a 27k - looks like it popped with some violence.

In the last picture you posted they look like 27 Ohm resistors.

[TheElectricChicken]

In the last picture you posted they look like 27 Ohm resistors.

I agree. 27k off the top of my head has a orange stripe rather than black. Those resistors are not 27k they are 27 Ohms

0 = black
3 = orange

27 x zero tens = 27 ohms

[rodrigopires]

I was just testing for the first time, with no load, is t normal to pop 1W resistor with no load, and also my 16A circuit breaker shuts off.

I would guess that either you've not wired the circuit as per your diagram and/or there is a short on the board.

Either way: 27k at 1W rating is pushing it. Are the resistors rated for 325V operation? Are you sure it was a 27k - looks like it popped with some violence.

In the last picture you posted they look like 27 Ohm resistors.

OMG  , they are actually 27 Ohms, not 27K, the must have been a mistake in the shipping when i bought them, because i ordered 27K. Thank you so much! i will replace the resistors, and next time check every component!

Thank you all for the help, i really appreciate it! 

[TheElectricChicken]

[oldway]

I know well this circuit because I use it in a battery charger project.
The circuit is very simple and works well.

It must however comply with certain precautions.
1)For safety,  the potentiometer must have a plastic shaft and be mounted on an epoxy insulating plate.
2) the circuit generates a pulse train, which is correct for an inductive load.
But pulses are very short (2 to 3 microseconds) and are insufficient to ensure that the current reaches the holding/latching current with highly inductive load.
For proper and safe operation, add an incandescent lamp 100W 220V in parallel with the transformer.
3) I replaced the MOV by a bridge rectifier and 4 zener diodes in serie of 15V 1,3W.

Choose 3 quadrants snuberless triac (with snubber added for more reliability).

To test the circuit, use only the 100W lamp as load, without the transformer.

Nb: for auxiliary triac, be sure to use the Z0103, not the Z0107, nor Z0109, nor Z0110 because it will not generate pulse train with lower sensitivity auxiliary triacs. (higher gate current)
With transformer, you must have a symetrical triggering of the triac to avoid create a DC voltage on the primary of the transformer.
If this occur, transformer will saturate and a very high current will blow your mains fuse and perhaps also your triac.
With a 100W  incandescent lamp in parallel with the primary of the transformer (resistive load low enough to reach the latching current), you are sure that no false triggering can occur and it will works with reliability.

[rodrigopires]

I know well this circuit because I use it in a battery charger project.
The circuit is very simple and works well.

It must however comply with certain precautions.
1)For safety,  the potentiometer must have a plastic shaft and be mounted on an epoxy insulating plate.
2) the circuit generates a pulse train, which is correct for an inductive load.
But pulses are very short (2 to 3 microseconds) and are insufficient to ensure that the current reaches the holding/latching current with highly inductive load.
For proper and safe operation, add an incandescent lamp 100W 220V in parallel with the transformer.
3) I replaced the MOV by a bridge rectifier and 4 zener diodes in serie of 15V 1,3W.

Choose 3 quadrants snuberless triac (with snubber added for more reliability).

To test the circuit, use only the 100W lamp as load, without the transformer.

Nb: for auxiliary triac, be sure to use the Z0103, not the Z0107, nor Z0109, nor Z0110 because it will not generate pulse train with lower sensitivity auxiliary triacs. (higher gate current)
With transformer, you must have a symetrical triggering of the triac to avoid create a DC voltage on the primary of the transformer.
If this occur, transformer will saturate and a very high current will blow your mains fuse and perhaps also your triac.
With a 100W  incandescent lamp in parallel with the primary of the transformer (resistive load low enough to reach the latching current), you are sure that no false triggering can occur and it will works with reliability.

Hi, thank you for the information, i will use the 100W bulb in parallel with the transformers, my pots have long plastic knobs, and i used the Z0103 that is on the circuit.

Once again, thanks for all the help 

[TheElectricChicken]

Please do let us know if it works !!!!!

all the best.

[rodrigopires]

yah, just have to wait for my new resistors and probably will post a video on yt to show the project, and also leave a link in this post.

[TheElectricChicken]

SWEET! Thanks, can't wait 

[eneuro]

Please do let us know if it works !!!!!

It will be interesting to see, but it lacks a lot of things like overcurrent detection and protection to do not triger home mains fuse when it wil draw too high current for longer period of time, so that is why I've also Hall current sensor and MPU which averages input current (10000 samples per second) and when it is above preset limit than adjusts phase to be able lower it down to safe levels 



Anyway, I've also reinforced 5mm mains PCB SCRs tracks with two copper wires 1.5mm2 each in parallel, so it is 3mm2 + solder per PCB power track, so now I think that in the case of disaster high current those SCRs will explode first and this PCB should survive-just replace them with brand new SCRs and weld again



Working now on synchronous rectification on secondary side, to be able get high DC current with low power losses on transformer secondary (classic bulky 230VAC -> 24VAC ).

Another story is HV arc ignition pulses and this is in my interest too and looking for good ways to do it. I consider also preheating welded parts with spot welder high current, while higher metal temperature shoould also help get stable arc easy, while air will break at lower voltage when it is hot 

[oldway]

Hi, i have made this triac power controller for my homemade MOT arc welder, ...

Rodrigopires did not explain what transformer he is using.
I hope he knows that the welding transformer must have a high short circuit voltage (20%) to limit short circuit current and an open voltage above 40V to work correctly.

[eneuro]

I hope he knows that the welding transformer must have a high short circuit voltage (20%) to limit short circuit current and an open voltage above 40V to work correctly.

What do you mean work correctly?
For me this means for example limit input power to be below 5kVA.

This bulky 10kVA 200AC/DC Inverter Operating Manual by WELDSKILL has 76V open circuit voltage.

I can build up such voltage even from 24VAC but what short circuit current is required at this voltage above 40V-to ignite arc I guess, while later welding  closed circuit voltages are <=~24V in the case of this welder shown above 

Update, As always patents can explain many things-sounds good 
Patent US 6156999 A: Method and device for welding arc ignition for arc welding apparatus

Anyway, from this linked manual:

Suppose a Welding Power Source is designed to operate at a 20% duty cycle, 200 amperes at 18.0 volts.

they says about 18V*200A=3.6kVA , so when we assume 70% efficiency we get ~5kVA input power, which means 5kVA/230V~22A (for 240VAC it is ~20A), but they says about 15A effective input current which means 3.6kVA @ 240VAC they specified in their datasheet, but when we take 170A*25.8V~4.4kVA and it is higher than their effective current..   
100A*24V~2.4kVA, so it could be ~67% efficiency when we take 3.6kVA input power...



The first thing when looking for welder probably should be home grid power capabilities in the case of single phase, so sensing input power (current) was a must in my design to leave some power for other home equipment and avoid blackout while welding  :-D

[oldway]

40V is the minimum open circuit voltage of the welding transformer to ignite the arc.
Commercial low cost units have an open circuit voltage of 45 or 50V.
It would be better to have an open circuit voltage between 60 to 70V.

[tautech]

40V is the minimum open circuit voltage of the welding transformer to ignite the arc.
Commercial low cost units have an open circuit voltage of 45 or 50V.
It would be better to have an open circuit voltage between 60 to 70V.

+1
But at the risk of minor shocks for the user. (wet work area, use outdoors, etc)

Been here myself.

[oldway]

I would recommand to rodrigopires to not waste his time with this kind of project.

I have some experience with MMA welding, both with welding transformer and DC welding inverter...there is simply no match at all....DC welding inverter is 1000% better than welding transformer.

Lets try welding with a welding inverter and you will understand what I mean.

There are now cheap welding inverters available, I recommand to buy one.
I have a MOS100 DECA inverter and I am very satisfied with it for hobby welding.

[eneuro]

It would be better to have an open circuit voltage between 60 to 70V.

I think, that I will try add in series with my welder transformer a few turns on this MOT core, which has broken crappy aluminium primary coil:



Than I will make the same amount of turns with provided HV low current added MOT core primary at much higher frequency than 50Hz using one of those step-up DC-DC converter with high voltage gain:


In the case of AC welding, since I know current direction from my Hall efect sensor, I can step-up voltage in series with welder transformer synchronized in polarity in a way that this HV low current MOT voltage will create sparks between welding elctrode and base metal regardless of transformer low voltage 

What do you think, If I made usch plasma speaker talking during welding to take care and do not touch welded metals and electrode it has enougth ignited arc to start welding easy, providing current will be vey low and kept on safe levels even if human touched this think-only small shock to remeber to do not touch this in this welder? 
Plasma Speaker - Singing Arc: HV ZVS Flyback driver project


So, question is how much ignition arc energy (we can break air at given distance with different current) is needed to start welding easy, because probably it matters, while HV in series with welding electrone and metal can be made in many ways.

I like this idea with additional MOT core and I'd like to test this while I've low voltage bulky transformer (its secondary is now used with a few thick turns of wire as spot welder) , but Thinking if it could be used as classic welder too, since its oryginal secondary has quite thick 2 wires in parallel of copper...

Those copper pipes used to help dispate heat on spot welder secondary are 15mm in diameter on this photo, to get some idea of trafo dimensions 

Idea is add afew lones of code to support not only spot welder, but add classic welder mode too with not removed trafo oryginal secondary 

[rodrigopires]

Rodrigopires did not explain what transformer he is using.
I hope he knows that the welding transformer must have a high short circuit voltage (20%) to limit short circuit current and an open voltage above 40V to work correctly.

I'm using MOT's, i used two, but now with the power controler (not yet finished) i'm going to ad a third one. With two MOT's i had about 30V and it welded about anything, the only limitation was my home circuit breakers .

I would recommand to rodrigopires to not waste his time with this kind of project.

I have some experience with MMA welding, both with welding transformer and DC welding inverter...there is simply no match at all....DC welding inverter is 1000% better than welding transformer.

Lets try welding with a welding inverter and you will understand what I mean.

There are now cheap welding inverters available, I recommand to buy one.
I have a MOS100 DECA inverter and I am very satisfied with it for hobby welding.

You are right, there are some cheap and good welders out there, and with the money i have already spent on this project i would be able to by one, but whats the fun? Why buy one when i can make one, yes it not as good, but its homemade.

[eneuro]

With two MOT's i had about 30V and it welded about anything, the only limitation was my home circuit breakers .

That is why I prefere active monitoring of welder transformer primary AC current, so using Hall effect current sensor we can calculate average current drawn in lets say 100ms periods, so when it is higher than given percentage of home circuit breaker triac/thyristors fire angle can be adjusted in both half AC periods to avoid overloading home grid 

Simple circuits as yours might work, but blackout at home while welding for fun this is not what circuit designer wants to happen, even in hobby project....

[oldway]

MOT transformers are ok because the windings are wound side by side, which give a high short circuit voltage.
15V by secondary seems reasonable given the power rating of the MOT's.

So yes, it takes 3 MOT's with the secondaries in series and primaries in parallel.

The short circuit current is limited naturally by the short circuit voltage: 20%, this means that the short circuit current is equal to 5x the rated current.

You should know what open your home Circuit breaker: inrush current or short circuit current ?

Regarding to the primary phase control, it can not be fast because a rapid change in phase angle produces a DC component that saturates the transformer and produces a huge current in the primary.
The cure can be worse than the problem it is trying to eliminate.

This is one of the major limitations of the phase control by the primary voltage of the transformers.
Whenever possible, I recommend to always do the control in the secondary than in the primary.

As I said, this circuit is very simple but also very imperfect.
For small transformers, it is acceptable.
I have used it for the project of a small battery charger with 12V 8A ajustable output current and charging voltage limited to 14.4V. (see pictures)
But for higher power, you should use a more sophisticated circuit.

[Circlotron]

As I said, this circuit is very simple but also very imperfect.
For small transformers, it is acceptable.
<snip>
But for higher power, you should use a more sophisticated circuit.

You'll love this then. In the late 80's I worked for a company that made SCR based UPSs up to 20KVA, and over the road was the electric furnace division. The used to make their own transformers from scratch too. Anyway, there was this furnace that was IIRC about 20KW, maybe even a bit more, and there was a 1:1 isolating transformer between the incoming 3x240VAC and the heating elements. The tranny had a star primary with the star point unconnected, and a delta secondary. The phase control part was 3 x SCRs and 3 x anti-parallel diodes. Most definitely the two-stroke topology of the phase control world. At some settings that transformer made this dirty, sickening growl. My protests went unheeded.

[oldway]

This is a modification I made in 1993 of a 6000A 12V water cooled electrolysis rectifier which was split in two 3000A 12V rectifiers each with his own control.
I made the new 3 phase control circuits and on site modifications.
Phase control was made in the primary of the transformer by a 6 SCR's.

[eneuro]

Regarding to the primary phase control, it can not be fast because a rapid change in phase angle produces a DC component that saturates the transformer and produces a huge current in the primary.

Thanks for this hint.
I'm talking about 1ms adjustements, so while @ 50Hz we have 10ms half AC mains period in 10 steps can be reduced to small currents 

Anyway, while I've AC mains voltage zero crossing based phase shift timing possible, but I've also Hall current sensor like this below and MPU makes 10000 current senses per second, so I know not only when voltage zero crossing happends, but also when current goes to zero



Trying to optimize my phase shift controll and I've used so far voltage zero crossing to detect positive AC mains pulses, take measurements within 2 seconds to find exact period time and calculate needed phase shifts to fire two SCRs in anti-paralle shown earlier.

But, what about using current sensor? I've 60W light bulb on SCRs switch and the same 60W in parallel with transformer primary, so even when SCRs switch is OFF this 60W light bullb in series with welder/spotwelder transformer, limits circuit current, but I can detect current maximum, so period, than expected zero crossing, etc...

Maybe I could  take advantage somehow of fusion data from voltage zero crossing and current sensing (crossing) when SCRs switch is off but, small Ac current present in circuit, while 60W light bulb bypasses SCRs switch? 
I've upgraded my Hall effect sensor for higher current sensitivity and in th ecase of spot welder I'd like to try detect for example rise in current on traf primary, when spot welder electrodes are closed on welded steel, so within 1s-2s activate choosen spot welding program without need to press any spot start buttons etc.

Maybe I will try the same with classic welding-touching electrode will close welder transformer secondary )increase primary current and heat up welding start point) and withiin 1s-2s welder transformer will start increase its power and HV circuit activated to help ignite arc....   

I need experiment more with this enhanced Hall current sensor, if I will be able detect closed welder transformer secondary using DSP processing in MPU with 10kHz current sampling rate 

[rodrigopires]

With two MOT's i had about 30V and it welded about anything, the only limitation was my home circuit breakers .

That is why I prefere active monitoring of welder transformer primary AC current, so using Hall effect current sensor we can calculate average current drawn in lets say 100ms periods, so when it is higher than given percentage of home circuit breaker triac/thyristors fire angle can be adjusted in both half AC periods to avoid overloading home grid 

Simple circuits as yours might work, but blackout at home while welding for fun this is not what circuit designer wants to happen, even in hobby project....

that happened with my old setup, just two MOT's with secondaries in series and primaries in parallel straight to the wall socket with no protection or regulation,thats why the only limitation was my home circuit breaker. Thats why im building this regulator, to eliminate that problem.

[eneuro]

Thats why im building this regulator, to eliminate that problem.

But this simple regulator circuit has no current sensor, so still you have no idea what average and maximum current is, so even if welder transformers will not triger circuit breaker, someone will turn on microwave oven during your welding and.... blackout again 

But ok, for serious welding no way three phase additional grid needed, to protect single phase 230VAC home grid from overcurrent blackouts when playing with high power tools 

[oldway]

that happened with my old setup, just two MOT's with secondaries in series and primaries in parallel straight to the wall socket with no protection or regulation,thats why the only limitation was my home circuit breaker. Thats why im building this regulator, to eliminate that problem.

You still did not answered my question: when does the circuit breaker trip ?
At power on ? (this means inrush current)
Or when you short circuits output of the 2 MOT's ?
With 3 MOT's, things will be worse.
Perhaps are your home circuit breakers too low intensity for this purpose.
With commercial 150A welding transformers and 230V mains voltage, you need at least 25A circuit breakers.
These commercial transformers have an optimum transformation ratio, that's not the case of yours MOT's.

This phase angle control was intended for adjusting welding current (for exemple, from 50 to 150A), not for eliminate circuit breaker trip.

[oldway]

Thats why im building this regulator, to eliminate that problem.

But this simple regulator circuit has no current sensor, so still you have no idea what average and maximum current is, so even if welder transformers will not triger circuit breaker, someone will turn on microwave oven during your welding and.... blackout again 

But ok, for serious welding no way three phase additional grid needed, to protect single phase 230VAC home grid from overcurrent blackouts when playing with high power tools 

As I already explained, current regulation can only be slow with phase control in the primary.
It is not able to limit instant peak current. Circuit breaker is faster than phase control current limitation and it is not possible to avoid circuit breaker tripping by this way.
No need of a current sense, it is useless.

NB: The conventional solution of magnetic shunt welding current setting is far better than the phase control by triac.

[eneuro]

It is not able to limit instant peak current.
...
No need of a current sense, it is useless.

Nope, current sense is needed, and no need to limit transient current, but I've in this calibrated custom Hall sensor current limit set to 16A (avaregae of 10ms 10kHz sampling moving average-half period of 50Hz mains) on welder transformer primary which at 230VAc means 3.7kW, so it will not protect against short circuit, but when welding current will be inceased by potentiometer, safe level phase shift can be easy set, while this thing will turn off in current zero crossing for a few seconds when such overpower event happends.
Without current sensor you have no idea what this phase shift limit can be...until you make blackout in house, trying to find this point with potentiometer 



Another story is adaptive phase shift adjustments to rising input power (current). I will test those 1ms time phase shifts adjustments, hoping that below current limit set to 20A per 10ms half 50Hz period it will not create horrible waveforms, due to this phase shift fine tunning 

Update: Tested overcurrent protection software set to 3A with this 21W light bulb powered from DC 12V, so multimeter showd 1.63A current draw.

However, when 10ms averaging perido was set.... yep inruch current when cold light bulb was turned on trigered this current threshold of couse 
Than changed this time to ~0.1s (1000 averaged current samples) and it was enougth time to warm up bulb filnament to the point, average current sampled at 10kHz within 0.1s from cold start was below 3A overvoltage protection 
later, I've connected 55W car lights bulb and of course overcurrent worked all the time, since multimeter showed ~4.44 A current draw 

This experiment showed, that probably 0.1s might be fine during welding too, since I'll add also soft start (it will be used in spot welder in the same trafo) and maybe demagnetization of trafo core (in spotwelder where we have short circuit on secondary from th ebegining to the end).
Classic welding is slightly different, but I think it will be nice have primary nominal current set potentiometer, and in software automatically overwoltage current will be set based on those settings as lets say 25% higher-it turns off wedling for 10 seconds when something like this happends-enougth time to cut glued weld electrode from steel plate 

Now, calibrated this my new Hall effect sensor and upgrading software to implement this adaptive phase shift controll-overcurrent (overpower)  protection is already done and testing this right now, while preparing to myth buster of crimped soldered connectors vs only crimped   

BTW: Mag amp on this welder transformer primary in on my todo list-I have to upgrade MOT iron core with two windings and add thin contoll coil in the middle-It will be interesting to see if it works   

[Circlotron]

I'll bet that the power factor your transformer presents to the mains is low while actually welding because of harmonics from the triac phase control circuitry and lagging current because of (intentionally) poor coupling between primary and secondary of a MOT. Put a scope on the primary voltage and current and see if you can pull the current into line with a few power factor caps. Get yourself a few extra free amps before you trip the breaker.

[oldway]

so it will not protect against short circuit, but when welding current will be inceased by potentiometer,

It is during the short circuit that the circuit breaker will trip because short circuit current is only limited by the stray inductance of the transformer, that's more or less 5 times the nominal secundary current.
Yourself prove that current limiting is useless because it will not protect against short circuit and short circuit occurs in MMA to start the arc.

Why choose a Hall sensor?
With alternating current, it is better to use a current transformer, it is simpler, more reliable and more stable. (no drift, no need of power supply, ...)

[eneuro]

With alternating current, it is better to use a current transformer, it is simpler, more reliable and more stable.

Could you show us your simply CT circuit schematics connected to MPU pin?

I've shown in photo above: I take +5Vcc and GND from MPU and output from radiometric Hall effect sensor connect directly to MPU ADC pin with a few nF caps if I want additional filtering, but software moveing average at 10kHz sampling rate does the job in this (spot)welder MPu controller 

BTW: Forget about make CT at home, while with manufactured PCB my Hall current sensor PCB need to be only soldered one part: Hall effect sensor and ferrite core to guide mgnetic flux througth sensor....

Now, I'm working on improoved version, where opamps will be added, so with the same footprint shown above 25mm x 25mm this thing will be able drive additionally half bridge mosfets optoisolated gates in high power low voltage (safe 36V) many phase (three at least) synchronous rectifier drivers 

[oldway]

Could you show us your simply CT circuit schematics connected to MPU pin?

Do you really need a circuit schematics of a CT, a bridge rectifier and a load resistance with // filter capacitor ? You are kidding ? 

Forget about make CT at home

Are you kidding again ? 
Every 10 to 25VA transformer, without secondary (only the 230V primary), is a low cost CT.
You pass one turn in the free space and you only have to ajust the load resistance to have the right ratio current/voltage.

[eneuro]

Every 10 to 25VA transformer, without secondary (only the 230V primary), is a low cost CT.

It is useless-there is not too much space eg. to put 6mm in diameter copper wire 
This is quite nice, but costs.. 10x more than my <$3 Hall effect based current sensor with 10mm hole 

http://www.digikey.com/product-detail/en/C-CT-6/563-1390-ND/4428778


Anyway, BE CAREFULL when playing with CTs-DEADLY high voltages can be present on those not shorted CT output wires !!!!

Why the CT secondary should not be kept open?

I've choosen custom made Hall effect based current sensors for my high power EV subsystems and it works...

[oldway]

It is useless-there is not too much space eg. to put 6mm in diameter copper wire 

And what about toroïdal transformers ?
They are cheap, secundary can easily be removed and there is a lot of space.

In power electronics, you must choose the most reliable solution.

Here, we are not speaking about a project to be produced in thousands of units, only about a prototype of a welding transformer made with modified MOT's with phase control by triac.

That a secundary of a CT can't be kept open, that's basic electricity you learn at the very first year of study.
If somebody don't know this, better he does not deal with power electronics.

@rodrigopires: Be aware that the potentiometer is at mains voltage. For security reasons, it must be a plastic potentiometer (not only a plastic shaft) as PIHER type PC16 (http://www.conrad.fr/ce/fr/product/445663/Potentiometre-rotatif-Piher-PC16SH-10IP06474A2020MTA-Mono-02-W-470-k-1-pcs) and, as you can see on the pictures of my battery lader project, it is also mounted on an isolated epoxy board.

[rodrigopires]

You still did not answered my question: when does the circuit breaker trip ?
At power on ? (this means inrush current)
Or when you short circuits output of the 2 MOT's ?
With 3 MOT's, things will be worse.
[/quote]

The circuit breaker tripped when i was welding, after a while when the transformers got a bit warm, but it was very rarely, it did not happen in most of the time, just when i'm welding for a long time without stops, to cool the transformers down. And i hoped that with this regulator i would be able to reduce the heating on the transformers. An i chose to add a third transformer to reduce the load on the other two.

[/quote]
@rodrigopires: Be aware that the potentiometer is at mains voltage. For security reasons, it must be a plastic potentiometer (not only a plastic shaft) as PIHER type PC16 (http://www.conrad.fr/ce/fr/product/445663/Potentiometre-rotatif-Piher-PC16SH-10IP06474A2020MTA-Mono-02-W-470-k-1-pcs) and, as you can see on the pictures of my battery lader project, it is also mounted on an isolated epoxy board.
[/quote]

Yes, actually the pots i got are exactly like those, and i also got them in a separate board.

[oldway]

An i chose to add a third transformer to reduce the load on the other two.

That's not a good option: you will change de turns ratio and increase the mains current for the same welding current.
The circuit breakers will trip more often.
For the highest welding current without tripping the circuit breaker, the best choice is a welding inverter with active PFC (power factor correction).

Try to answer these questions and you will understand that what you try to do is not possible:
1) What's the minimun no load voltage of a welding transformer ? (NB: see output voltage of commercial units)
2) How much max welding current you need ?
3) Calculate the transformer power rating (even with 60% working factor)
4) How much mains current for max welding current ? Compare with your mains circuit breaker.
5) What's the nominal power rating of a MOT ?
6) How much MOT's you need ? (for the same power rating as 3))

[eneuro]

The circuit breaker tripped when i was welding, after a while when the transformers got a bit warm, but it was very rarely, it did not happen in most of the time, just when i'm welding for a long time without stops, to cool the transformers down.

I was thinking also on some tricky, magnetic shunt to add to MOT or even my E core type transformer to... add additional magnetic flux path at the top of MOT/trafo used in welding
I mean, put another mot on top of other to have its iron cores magnetic  contact and create addidional magnetic flux paths, so it should lower down flux in oryginal MOT secondary and current, but didn't tested it yet 
I'm thinking of adding steel sheet on the top, hoping it will decrease magnetic flux in oryginal transformer, so we could have output current regulation without  oryginal MOT/trafo core...
It will be interesting to see if it works, but I noticed, when turned last time my spot welder  that iron screw was heavy attracted to transformer core, so maybe it could work.

BTW: Of course be carefull to do not touch mains part of MOT reversed, when you put it on top of another one, since I expect HV there and maybe dangereus voltages...we'll see if it works in th ecase of my welder trafo, but magnetic circuit theory says, maybe? 

[oldway]

If you want to reduce output current (increasing the short circuit voltage of the transformer), why not use simply the original magnetic shunts of the MOT ?

 @rodrigopires: About overheating of the transformers, use forced cooling.
See how the welding current setting is done in the inexpensive welding transformer by adjusting the position of the magnetic shunt.

[oldway]

Why the current control solution by movable magnetic shunt is better than by a triac controller?

For ease of starting arc, welding transformer must have the highest possible no load voltage.
It takes at least 50V and good welding transformers have a no load voltage of 80V.

The current adjustment by magnetic shunt does not change the open circuit voltage, while the triac controller decreases the primary voltage, and therefore also that of the secondary.

Furthermore, ajustement by magnetic shunt change the output regulation curve of the transformer, making it steeper at low current.
That's a great benefice for stability of the arc at low current settings.

Internet is full of supposedly miraculous solutions: car running with water, devices with 100% or higher eficiency, miraculous electrolysis through the vibration of water molecules, battery regeneration with devices of a few watts, ... and so on .. .

These miraculous "inventions" are contrary to the basic laws of physics and electricity and are coming from two sources:
- either they were created by people who know nothing and who believe, in good faith, to have invented the wheel.
- either they are created by crooks who want to earn a lot of money by exploiting the gullibility of people who are not specialists in the field.

So pay attention to what you find on the Internet, there is good information, but also technical and scientific nonsense.

[eneuro]

The current adjustment by magnetic shunt does not change the open circuit voltage, while the triac controller decreases the primary voltage, and therefore also that of the secondary.

When we replace HV coil with something desired for high current, not too much space left for oryginal MOT magnetic shunts, so probably I'll try to make this tricky additinal magnetic circuit on top of this thing



Anyway, during testing my new upgraded (spot) welder software, I forgot change current limit set for testing in mains Hall current sensor configuration and left it at.. 0.5A RMS  level and I've almost rewritten turn on/off button software averaging and contoll, since... software was good for button event processing and starting transformer SCR's, but of course inrush currents were much bigger than 0.5A rms (100ms), so when I had no idea howto write better MPU pin interface, after a few hours of  and debuging, I've found this bloody definition which puzzled me for so long time   

Code: [Select]

#define FUSE_CURSEN_LIMIT    (0.5)
Beauty of software development - one line of code can ruin your day or let  magic to happen other time 

Yep, using phase controll to limit output current can degrade voltage, too, but in my case It shouldn't be a problem- I'll have HV high frequency pulses in series with welder transformer.

However, phase shift control can be very usefull for demagnetization of transformer core and developing such soft start code for my ATTiny85 MPU right now 

[oldway]

I still not understand what spot welder has to do in a topic about ARC welder !
But anyway,
- it is stupid to use a wire with such an isolation for more than 1000V in a secondary coil of few volts.
It is also very bad for power dissipation.
Use enameled wire and you will have plenty of space to place the original magnetics shunts again.
You may also use several wires in // if necessary.
- So far I remember, spot welding transformers were not fase controlled, but time controlled.
Thyristors (2 in anti-paralel for better dV/dt than triacs) were full conducting during a variable number of complete sine waves, this number depending on the desired current.
- MOT is not enough powerfull to make any serious spot welder. (800 to 1100W, or 1000VA to 1375 VA) (need 10 times more)

[eneuro]

I still not understand what spot welder has to do in a topic about ARC welder !

Both (spot) welder share huge amount of the same software code, use the similar hardware (additional custom windings for spot welder), both use the same Hall effect current sensor, both are controlled using one smart button, which allows multiply wedling time from 20ms up to 10 minutes (connected with remote RF controll in next upgrade, when my home automation project is finished).
Only small diferencies how (spot) welding process is started and finished with optional phase shift control and HV circuit for arc welder 

BTW: We'd like to see some photos of OPs MOTs setup, because a few images are worth thousands of words...and always interesting to see how it was made 

[oldway]

Absolutely nonsense 
A spot welder of such a low power does not even have to be phase controlled nor need a Hall effect current sensor because you do'nt need to know what's the current.  (and furthermore, I already explain to you that using a CT is far more reliable and stable than an Hall effect current sensor).
All what it need is a timer and a solid state relay...

As I said, technologies of MMA welding and of spot welding are fully different...nothing in commun.

[eneuro]

A spot welder of such a low power does not even have to be phase controlled nor need a Hall effect current sensor because you do'nt need to know what's the current.

I didn't said it have to be (many commercial available spot welders have only controll of amount of full wave pulses) , but it is neat feature I wanted to have, since I might want to use its output as high current power source, but with controlable output power, so I've did it, because of ... I can 

I will post photos of my welding machine when upgrade is finished and it looks much better than many "commercial" available and have features I wanted to have at high power (I mean a few kW) low voltage output 

BTW: My (spot) welder is not based on any MOT, but bulky transformer and it has thick copper in his primary/secondary winndings, not China aluminium wires plated with copper, which I've found in MOT I've posted a few posts above 
Yep, I've MOT too, so the same software recompiled for this smaller spot welder/solder iron heater will do other less demanding tasks, but not too much changes in source code needed...

[oldway]

Why do you invade this topic with your spot welder project?
You can not open a new topic?

Once again, I repeat, this has nothing to do with an arc welder and rodrigopires phase controller does not even have any µc ...

No need of "huge amount of software code".

[eneuro]

Once again, I repeat, this has nothing to do with an arc welder and rodrigopires phase controller does not even have any µc ...

He might quicly realize, that this simply phase controller is too simple, and galvanic insulated small DIP 8 pin ATTiny85 MPU can help automate welding and do many other things for him, however it will be as smart as its programmer is 

Try to make such (spot) wedling possible, using ...foot pedal 

[oldway]

He might quicly realize, that this simply phase controller is too simple, and galvanic insulated small DIP 8 pin ATTiny85 MPU can help automate welding and do many other things for him, however it will be as smart as its programmer is 

Then, why don't you share yours schematics and software with rodrigopires ?

Try to make such (spot) wedling possible, using ...foot pedal 

That's not SPOT welding ! This is resistance seam welding.

This is a spot welding machine with foot pedal.
I already used one in the past.

And an another one.

[rodrigopires]

Hi, sorry for the long wait but i have been really busy, anyway, here is my final unit!

[oldway]

With resistive load (incandescent bulb) every schematic of dimmer works well, even far more simple schematic.

But with highly inductive load as a welding transformer, there are a lot of troubles.

If there is any asymetrical triggering of the triac, the transformer will saturate and a very high current will flow throught the primary winding, with magic smoke and / or trip of the circuit breaker.
One of the greatest weakeness of this simple schematic is that the pulses are very short, only 2 or 3µs.
With highly inductive load, this it not enough to ensure that current reach the value of the holding current at the end of the triggering pulse and the triac can loose the conduction.
Even worse, the + and - holding currents are not exactly the same, so triac can trigger during one alternance and not during the other, saturating the transformer.

It would be very important to verify with an oscilloscope if this trigger circuit is really generating a pulse train.
But for doing this, you will need a diferential probe or to feed the dimmer and load with an isolation transformer.

In every case, don't try to dim your welding transformer without a resistive load in parallel with the primary. (your incandescent bulb)

[aburastas]

Hi... New to the thread but since I had followed all the mesaages I think that the part number of the transient suppresor is for the unidirectional part but in the circuit the bidirecciital type is needed. The part number that end in 68CA not 68A.

Anyway very interested if rodrigo managed to use the welding machine with this controller... Waiting for results

[oldway]

I have used a bridge rectifier with 4 zener diodes (15V 1W) in serie instead of this transient suppressor.
So I am sure that it is perfectly symetrical.

[oldway]

Use the two ac terminals of the rectifier bridge in place of the terminals of voltage supressor and the four zener diodes connected between + and - polarities of the bridge...(the 4 zener diodes connected in series, with the correct polarities, of course)

[aburastas]

Yes... I deleted the post when I understand what you want to say.... I was confused with the diode bridge... I belive that was refering to the one in the schematic... Any way the datasheets says that tvs are much better in time response and watts capabilities... The only critical point is to get the bidirectional part. 1.5KE68CA But..   Perhaps it isn't so critical in this schematic..

[oldway]

It is very critical with highly inductive load ...if there is any asymmetry, transformer will saturate and a high current will flow in the primary.
Even diac is a problem as it is not fully symmetrical.
Philips as used diac in only one direction also with bridge rectifier to be sure there is no asymmetry.

About "better in time response", no problem at all, it only works at 50 or 60 Hz.
Power dissipation of the four zener diodes is 4W, it is enough.

[aburastas]

Well... I know that my english can be... "funny?" jajaja but seem that we can understand us.. Following your advices I think in this way of chop the ac signal... In a "old way" without programing with the 555. Trying to first, make the trigger pulse longer with the astable oscilator and second, make that all components run always in the same direction with your brige rectifier technique... Is only a general idea... What do you think about? Can be functional? Since rodrigo seem has abandoned us.... I hope he hasn't electrocuted...

[oldway]

Dirty-cad ?

[aburastas]

Very good the emoticon... It's a freehand analogic sketch jajaja

[oldway]

Very good the emoticon... It's a freehand analogic sketch jajaja

Even so, you could do better...you are only showing us that you don't care !

[rodrigopires]

thanks for all the support on my project, i will make a more accurate video of testing the controller, with the two transformer and the resistive load (light bulb). i should have it done by the end of the week. 

[rodrigopires]

Since rodrigo seem has abandoned us.... I hope he hasn't electrocuted...

sorry but i only found out today that the post has already a fifth page, so i was checking on the fourth and there was nothing there... 
Still alive!

[moya034]

Welding machine built from a car alternator:

https://web.archive.org/web/20130410190142/http://myweb.cableone.net/rschell/TIG.htm

[oldway]

Welding machine built from a car alternator:

https://web.archive.org/web/20130410190142/http://myweb.cableone.net/rschell/TIG.htm

Only for TIG welding, MMA need a higher voltage.
It is non sense, you need expansive alternator and motor: you can buy good MMA inverters for less than 100$...

[moya034]

Welding machine built from a car alternator:

https://web.archive.org/web/20130410190142/http://myweb.cableone.net/rschell/TIG.htm

Only for TIG welding, MMA need a higher voltage.
It is non sense, you need expansive alternator and motor: you can buy good MMA inverters for less than 100$...

TIG and stick welding both use constant current power supplies, it will happily do either. That is a project meant for people that already have the parts around. Great idea if you install it in a vehicle, then you have a mobile welder powered by your truck engine

[oldway]

I do not agree: 14V is too low voltage for MMA / stick welding.
You should use at least a 24V alternator, not a 12V one.
TIG and MMA DC inverters have an open voltage of about 80V to iniciate the arc easily.

MAG need a welding machine with constant voltage of low value and it should work well with this 12V alternator.

[moya034]

I do not agree: 14V is too low voltage for MMA / stick welding.
You should use at least a 24V alternator, not a 12V one.
TIG and MMA DC inverters have an open voltage of about 80V to iniciate the arc easily.

MAG need a welding machine with constant voltage of low value and it should work well with this 12V alternator.

I'm not so sure you actually read the site... he has a circuit which controls the signal going to the primary in the alternator. Being a constant current power supply when no current is flowing open circuit voltage will be higher until you strike the arc. He stated his open circuit voltage is right around 50v.

Commercial versions of these welders based on alterantors are sold and installed in lots of farm and off road equipment.

If you can strike an arc on TIG, you can strike an arc on stick. The PSU's are interchangeable (assuming you are doing scratch start TIG on steel, and no HF start or aluminum work )

[aburastas]

Rodrigopires: good to hear from you! If your circuit is good for control welding machines will be great! Because it's simplicity...

Oldway: uhmmm since you don't like my graphics capabilities... I'll try with my english grammar... Although it isn't much better... My big first question is: it's possible substitute the principal triac with a diode bridge and a scr triggered by an optotriac in order to switch always with the same current in both ways?

[oldway]

It is not a problem of graphics capabilities at all.

I hoped that you would at least have the decency to do new diagrams of a neater way in respect for yourself and for the readers of this forum.
You made a drawing filled with erasures and almost unreadable, you do not give a good impression of yourself.

Answering to your question: Schematic used by rodrigopires is very simple and I don't see how you could replace the triac by a diode bridge and an scr in such a diagram.
For beeing a very simple schematic, it has a lot of shortcomings.
one of them is the problem of safety.
Potentiometers isolation is not intended to withstand full mains voltage.
Use only full plastic potentiometers (axle and case) as PIHER PC16.

http://www.tme.eu/en/details/6pmi-2k2/carbon-single-turn-axial-potentiometers/piher/pc16sh10ipo6-2k2a/

[aburastas]

Well.. I hope I haven't offended nobody... That wasn't my intention. I thought that the previus sketch will suffice for a general idea. I hope the new schematic will be more clear... Without values. Only general idea. And sorry. I haven't near the pc so I haven't any cad program....

[oldway]

Yes, YOU CAN DO IT......      
(I knew it   )

You have only ofended yourself but it is allright now..."errare humanum est, perseverare diabolicum"

About your schematic: The original schematic of Rodrigopires is from an application note of SGS-THOMSON.
http://www.iascaled.com/docs/psu-400w/an308.pdf
It is intended to be a non isolated low cost circuit without transformer and without optocoupler.
If you choose the new 3 quadrants snubberless triacs, this circuit works very well with low power motors and transformers.
You must remember that Rodrigopires is making a welding machine with free MOT transformers and he is looking for the lower cost possible.

If you choose a more expansive solution with isolated control, there are hundreds, perhaps thousands of possible solutions.
To trigger the triac, you can use a triac optocoupler as a MOC3023 or you can use a pulse transformer
In this case, with inductive load, you must have a pulse train to trigger the triac.

In your schematic, you can't use a bridge rectifier with a SCR instead of a triac.
The SCR will probably stay fully conducting with inductive load without any control.
Choose a new 3 quadrants triac, they have a high dV/dt caracteristic adequate for high inductive load.
The photo-triac of the MOC3023 has a lower dV/dt, read the application note to see the right schematic to protect it from high dV/dt.
SCR need also a 470R resistance between gate and cathode.
All the schematics are generally comparing a dc adjustable voltage with a 100 or 120 Hz sawtooth sincronised with mains to generate the triggering pulses.
This may be done with µc, dedicated IC as TCA785 (absolete), OP amp, 555, ....
If you use bipol 555, you can't use 3 output pin to drive directly a transistor as you did, you need a resistance between base and emitter of this transistor.
You should also provide a softstart to prevent saturating the transformer.

[aburastas]

Ok... Once I'm exorcisted from my diabolic possesion we can continue with the discussion...

Really the scr with diodes bridge... Seemed to me a bit strange but... Always is better to know others opinion. Following your advices I have readed the literature you purpose and I've arrived to the following conclusions..

About the second 555 seem to be inefective since choping the triggering signal it isn't neccesary for opto trigger. Perhaps it's better to do a simple trigger signal until final of semiperiod. Don't you think so?

Another question is if you think it's neccesary to put the opto triac inside a diodes-bridge???

In the motorola application note an780 explain how to make the snubber for the optotriac depending of the cosphi of the load. I haven't this number of my welder... It's a cheap ac welder of 160A max so I'll suppose may be 0.5

About the soft start... I think that I have a solution for the monostable to start softly as for change the capacitor rate of charge softly also.. But I'll wait for your opinion before I'll make another "Super Clear" schematic... Jejeje

Really I have seen a lot of such schematics but the most utilize uC and I prefer the analogic solution and the others don't convince me.. So I prefer to do my own... With your help of course and if it isn't funtional i have learned somthing inbthe way.

[oldway]

For controlling AC with inductive load, you can use a triac or 2 SCR's connected in opposite.(anti - parallel)

Phase control can be very simple....you can do this with only 4 transistors.
(NB: in the schematic, OK1 is a triac optocoupler...I had no one in the EAGLE library)

[rodrigopires]

Hi, today i went trying the controller with two MOT's and a 100W LB. The standby amperage was from 0 to 5A. Using a 2.5mm electrode i adjusted the controller to the lowest power possible while still able to weld. But them i found a problem! No matter how low i put it, when the arc started the amperage jumped to 10 A! At 5A, 3A, 1A, it all jumped to 10A  What is happening here? 

[oldway]

You probably loose the control of the triac...it goes to full conduction by too high dV/dt or too high temperature.
What triac are you using ?
What temperature does it reach ?
What are the values of your rc snubber ?

[rodrigopires]

I'm using a BTA40-700B with a 220 Ohm and a 0.1uF cap. The temperature i dont know because i have it inside the box, but with two fans i think it wouldn't get too hot.

[oldway]

BTA40 is a standart triac, I recommand using a snubberless 3 quadrants triac with highly inductive load.
A 25A 3 quadrants triac is enough, you could choose a snubberless BTA25-600W

Your snubber is not enough for such a high current inductive load.
Try 22R 10W(or 15W) in serie with 1.5µF (3 x 0.47µF in //) 275Vac X2.

[aburastas]

Many thanks for your help! Who need uc??? As an old proverb says... "the more you know, the less you need" however studing your schematic... I think that the trigger mechanism (T1-T2) may fail to reset the capacitor C5 when noise is present in the line. The discharge time may not be enough. The 555 solution perhaps is better because it has an flip flop inside... When I Have a litte time I'll post my schematic idea.... I hope you like...

[oldway]

If you worry about the reset time of C5, you can increase it lowering the value of R3 as 1K5 or 1K instead of 2K2.
I used this circuit in low cost battery charger and never had any issue with that.

I made several trigger circuits using LMC555 (Cmos version of LM555), but the simpliest circuit is often the best and the more reliable.

One of the commun error of beginners is that they want to make very complicated design...and there is a good reason for this: it is easier to design an elaborate circuit than a simple one doing the same function because you must use more imagination and trick to succeed high performance with few simple components.

NB: as you can see with latest post of rodrigopires, you can't focus only on phase control circuit....power circuit is also critical....
For instance, photo-triac of the MOC2031 is a "weak" point because max dV/dt of this triac is low.
For controlling high power transformers, I prefer to use a pulse transformer with 2 secundaries and two thyristors head to tail.
Even when it is possible, avoid to make phase control in the primary of the transformer and make it in the secondary instead.

[aburastas]

Hi! I've been searching information about triggering scrs with pulse transformers but.... No luck.. Do you know any application note or similar that teach scr triggering with pulse transformers from begining? That would be geat!

[oldway]

Pulse transformers for triggering SCR's or triac:
http://fusiblesysemiconductores.com.mx/semikron/14access/Accessor.pdf
http://www.vacuumschmelze.com/fileadmin/Medienbiliothek_2010/Produkte/Kerne_und_Bauelemente/Anwendungen/Uebertrager/Ansteueruebertrager/Ansteueruebertrager_en_2012.pdf
You can also make it yourself with 10 turns isolated wire (primary) and 10 turns isolated wire (secondary) wound on a ferrite toroid.

ratio 2/1 with 24Vdc and 1/1 with 12Vdc power supply.
Pulses of 10µs with 20µs delay between pulses.
For high power SCR's, it is recomended to have no load pulses of 10V peak and short circuit pulses of 1A peak.
With low power and sensitive triacs and SCR's, this can be reduced a lot.

[aburastas]

Well... Here we go again! this is the power control unit I'm thinking. But before do the maths do you think that it can be funtional? PErhaps to put a 470 resistor between scr gate and catode has you sugested in an post before? And put a diode in serie with the scr's catode to protect them from reverse voltage conduction?... Or to put an diode in antiparalell to the mosfet?... What's your opinion?

[oldway]

Very good, that's allright.
Why a mosfet ? use a BC337 ! (low power mosfet are very sensitive to ESD, bipolar are more secure)
Zener diode is not necessary, you have 20µs to desmagnetize the transformer...with losses in 0.6V of the forward diode voltage, that's enough.
470R needed between G and K as you said.

R1 = 22R
C1 = 0.1µF
R2 = R3 = 10R
Diodes in the secondaries are not needed. (no high reverse voltage possible because you have a diode in the primary.)

NB: with triacs, use negative pulses for triggering (quadrants II and III) . Triacs don't like to work in quadrant IV...
Snubberless can only work in I, II and III quadrants, never in IV !

[aburastas]

Jeje.. We are close to the final solution but you say r1= 22r and.. It must supply 1A of secondary current plus the magnetizing current at 10us on time that depend of transformer inductance... So.. Maybe 1.5A. 12v/1.5A=8R...

[oldway]

I think you are not trying to trigger 1000A scr's....
This value of 1A peak is for high power scr's....

[aburastas]

Ok. Ok... I'm just begining with scrs... Searching for scrs I have found this reasonably priced "IXYS CS45-16IO1 Tiristor 1600V 100mA 48A 75A 4_32euro" and seem that with 100ma has a 10us delay until 300ma that has a flat line at 2us and Vgt=1.6. So assuming that R2 must be 1.6/0.3= about 5R?

The other thing that worry me is that in the scr search I've readed a semikron document that says that while the scr is blocking voltage you can't put gate voltage in order to avoid "a non permissible increase in off-state power losses and the formation of  hot spots in the thyristor chip"so perhaps the double secondary may be wrong and will be better to do the thing with two transformers and only send gate voltage to one scr in each half cycle.... What do you think about?

[oldway]

For low price scr's, look on ebay.de, there are a lot of them.
I like the "littlefuse", previous Teccor, TO218X isolated package S6065J (600V) or S8065J (800V) scr's but they are expensive.
I think that terminals of TO247 package are a little weak for high current.

For triggering, be aware to not exceeed the max Pgav (average gate power dissipation).
With CS45-16IO1, you need an Ig greater than 0.3Apeak to correctly trigger this scr.

For triggering circuit, better choose R1= 12R
R2 = R3 = 3R3
BC337 is perhaps too weak, use BD139, eventualy in darlington with BC337

you can't put gate voltage in order to avoid "a non permissible increase in off-state power losses

It is not recommended with high reverse voltage, but not forbidden...

Think a little ...
With 2 scr's connected head to tail is it possible that a scr has a gate signal and a high reverse voltage at the same time ?
NO because the other scr is conducting and reverse voltage is only about 2V ....

[aburastas]

Wow... Scrs seem to be more dificult that seemed in the begining... But expensive it is not my favourite word. As your experience and knowledge are priceless. We can do it from begining to avoid missmatchs. My welding machine has has a primary max curent of 26A and an "I1eff=8.4A" that I don't know it's mean and and primary Voltage of 230V. So. In order to search the cheapest correct scr what is the minimum Irms and Vblocking of the scr you recomend? I think that my safety margin is too much...

[oldway]

You have to respect certain principles into a project with scr's.

For example, always trigger the thyristor with sufficient gate current.

I worked in a large industrial electronics company where I developed a line of three-phase industrial battery chargers with full bridge of thyristors.
These chargers were manufactured up to 330V 600A.

My colleague was developing a single phase thyristorised battery charger for telecomunications.
I took a look at his project (he did not like that!) And I got him noticed that the trigger current was far too low.
He became angry, saying that it was working fine like this and I had to take care of it looking at me.
He did not modify his project and the charger was manufactured as is.

Result: The customer complained that the charger was not working in cold weather .... no wonder the scr sensitivity decreases with decreasing temperature ...

As regards the choice of the SCR's, there are many possible solutions between a small thyristor with a large heatsink or a large thyristor with a small heatsink.
This choice is also influenced by the possibility of protection by ultra fast fuse (ferraz example).

The i²dt of the UF fuse must be at least 30% higher than that of the SCR it must protect.

We often first choose the heatsink and the mounting type (1 scr/heatsink or all the scr's on the same heatsink).

Then we select a scr with higher rms current  than nominal rms current.
We calculates the junction temperature in the worst conditions and check whether it is compatible or not with a safe operation, including in case of overload.

Otherwise, we select a scr with a higher rms current or with less thermal resistance case.

For 230V, a 400V scr is enough with resistive load.
But with highly inductive load, a 600V or 800V is better choice.

[aburastas]

Well.... I'll assure you that where I live the scr need a lot of gate current in winter... Following your advices I arrived to the following conclusions.... Realy I hope I'm wrong...

To improve thermal dissipation I'd changed to a to220. LITTELFUSE  SK055R  Tiristor 1 kV 40mA 35A 55A TO-220 3,85euro

It has Vgt=1,5; Igt=40ma; Igm=4A; Tjmax=125C; Pgav=0,8; Rthj-c=0,5; Rthc-h=1,4 (screw, no grease, no mica)

It must pass 26A RMS but only half wave is 18,2A so it's dissipation will be 15W
To give a safe margin... We can put Tj=120C and Ta=35;

All that give us Rthh-a=(120 -35 -0,5*15 -1,4*15)/15= 3,77K/W

I find a 3.3k/w by 2.45euro...plus 3,85 of the scr is a reasonable value for each scr

Another conclusion is... Being D duty cycle. The pulse you purpose is 10us on 20us off so duty cycle is D=1/3. In that short on pulse must be D*Pg<=Pav

So Pg<=Pav/D Pg<=2,4W

Pg=Vgt*Ig; Igmax=2,4/1,5= 1,6A

I'm right for now???

[oldway]

If the max rms current of your welding transformer is 26Arms, The max rms current of each scr is 26/1.41 = 18.44A (conducting only half wave)

TO220 pack is too weak for such a high rms current: terminals are 0.5mm thick and 0.75mm wide, this is only 0.375mm².
This mean a current density of 18.44/0.375 = 49.17A/mm²...This is very high...to compare, we use 4 to 5.5A/mm² in transformers.

Better to choose a TO247 or a TO3P pack.

Ta: must be at least 65° for convection cooling and 45° with force cooling because your heatsinks are connected with mains voltage and MUST be protected in a housing.

Tj = 120°: ok

Pg: ok...NB: with this duty cycle, it hardly happen to exceed the max gate power.

[aburastas]

Well.. After a few tries I return to "IXYS CS45-16IO1 Tiristor 1600V 100mA 48A 75A TO247 4`32euro" that give me the cheapest combo. it has:
Tjmax=125C; Vgt=1.6V; RTHj-s=0.82K/W; Vto=0.85v; Rts=11mR; Pgav=0.5W

The regulator must pass 26Arms so each scr must pass half a cycle each time so this give us 18.2A(rms) and 11.7A(av) so this give us P=Vto*Iav+Rts*Irms^2= 13.6W

With Ta=65C; and Ta=120C result in RTH<=3.22K/W and we can find the RA-T2X-64E with 3.1K/W by 5.37euro... Summing around 10euro each scr set. Not cheap but reasonable. Others scr with more consistents leads may pass more current but I don't usually put my welder to the max power. I think it will suffice.

Following the calculations as before with 1/3 duty cycle give an Igmax=0.9A so now begin the resistors calculations... Depreciating Rgk that only draws about 3ma If we put Ig=0.9A with Vsecondary=around 12V gives R2=R3= around 10R
And R1 must supply each half cycle the 0.9A+0.9A draws by the two scr plus the magnetizing current so perhaps around 2A that gives R1=12/2=6R. I know your values are calculated in a different manner so... What I'm missing??

[rodrigopires]

Hi guys! So since i had to open the controller i figured i would improve my fan power supply (12V) to feed the two small fans in the box and a big 36W fan for the transformers. I build this schematics in a breadboard and it seems to work good, but i want to know your opinion before i start soldering!

C0 and C1 = 1000uf each
C3 = 3.3uf
C4 = 1uf
R1 = 3ohm
Q1 = TIP32C
Regulator = LM7812

[aburastas]

Hi Rodrigopores! Nice to read you again!!! Really I hope you have any progress with your circuit because my brain is drying out with the design I'm planning. Finally ... Have you updated the triac by an 3 cuadrants? And the tvs by an bidirecctional type? And the snubber?

About your 12v regulator ... What I can tell you is that... R1 must be of more than 1w because being Vbe on the transistor about 1.8v... V^2/R1 is just a bit more than 1W so 2 or better 3W will be ok. About the transistor hfe=20; Vcemax=3A and 7812 has a max Iout=1A... You don't say what is your final load but being 36w/12v=3A of the big fan. Lets assume that transistor drive 3A this implies that Ireg=Vbe/R1 + Vcemax/hfe=0.75A. So your small fans can't sink more than 0.75A*12V=8W

To calculate the final thermal tolerances you must know the output voltage of transformer at C0. Anyway if you had made a long time run test with the fans on and T1, 7812 and R1 don't get very hot.... It will be ok.

[aburastas]

Well... Merry christmas for all!!! Although seem that not much people continue reading this. I know that some times my obsessive compulsive disorder can bore anyone so ... A toast for electrons that make life easier! 

[rodrigopires]

Well... Merry christmas for all!!! Although seem that not much people continue reading this. I know that some times my obsessive compulsive disorder can bore anyone so ... A toast for electrons that make life easier! 

Haha Aburatas... i'm still here! Merry christmas to you all! Working on the project and as soon as i have some news i let you guys know!
Sorry for the long wait! Here's an electron trying to reverse biased a diode!

[eneuro]

Welding machine built from a car alternator:

Really?
How is it possible that without synchronized rectifiers in DC stage of alternator one can have decent welding current ?  At  least 50A needed for thin 2mm welding electrodes? 
My MMA has  about 80V open circuit..at lower volatges spotwelder melts welding electrode - no arc no welding with electrodes