Author Topic: kWeld - "Next level" DIY battery spot welder  (Read 93431 times)

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Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #475 on: December 06, 2017, 09:17:06 pm »
Well if you want accurate current measurement, one way would be to measure the resistance of the cable via 4 wire method and measure voltage on portion of the cable like 10cm apart then use ohms law to calculate current.

You could add these to the code also.
Hope this helps.


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The tempco of copper is a in the same order as that of a MOSFET (4m / K), but the wiring also heats up much more. So there is no advantage with that respect. The absolute accuracy could be better, but at additional cost and mechanical complexity. This is why i decided against that in the beginning. I also thought of adding a dedicated shunt resistor, but didn't like the additional cost and complexity either.
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Online anishkgt

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #476 on: December 06, 2017, 09:18:26 pm »
I wouldn’t argue about franks design but just sharing idea for current measurement.


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Online anishkgt

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #477 on: December 06, 2017, 09:23:34 pm »
Quote
The tempco of copper is a in the same order as that of a MOSFET (4m / K), but the wiring also heats up much more. So there is no advantage with that respect. The absolute accuracy could be better, but at additional cost and mechanical complexity. This is why i decided against that in the beginning. I also thought of adding a dedicated shunt resistor, but didn't like the additional cost and complexity either.

Yes a bit of work too but reading voltage “ 10cm apart” is like using a shunt. Since you also provide the cable too the resistance would be a constant but as mentioned by others “how important is current accuracy?”


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Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #478 on: December 06, 2017, 09:36:00 pm »
the resistance would be a constant
No it's not. It is copper, which has a huge temperature coefficient  ;)
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Online anishkgt

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #479 on: December 06, 2017, 09:38:19 pm »
Hmmm yea. Oooopsy again that wouldn’t be accurate but would give an idea of it.


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Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #480 on: December 07, 2017, 02:26:09 am »
Hmmm yea. Oooopsy again that wouldn’t be accurate but would give an idea of it.
Nothing that the on-state resistance of the MOSFET's wouldn't give - except better absolute accuracy.
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Offline TopQuark

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #481 on: December 07, 2017, 06:36:41 pm »
Frank, really appreciate your answer :D using the mosfets as a shunt is quite a clever idea. How about using the fuse as a shunt? Does the resistance of the fuse change much from one to another, or does the temperature rise of the fuse contribute too much to resistance instability?

If I end up building one for myself, my OCD will probably force me to place a 'real' shunt resistor to get a slightly better current reading. Maybe a 100 microOhm resistor with an amplifier circuit similar to what Dave used for the uCurrent project. But now I am quite occupied with another project I have on hand, so maybe I'll just place Frank's welder on the top of my Christmas wishlist... :P
 

Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #482 on: December 08, 2017, 04:28:05 am »
How about using the fuse as a shunt? Does the resistance of the fuse change much from one to another, or does the temperature rise of the fuse contribute too much to resistance instability?
Both, as I don't always buy the fuses from the same place. And they heat up significantly, way more than the MOSFETs.
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Offline JohnG

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #483 on: December 09, 2017, 02:14:48 am »
For what it's worth, I have found that Infineon MOSFETs have pretty accurate resistance values, maybe within 10% or better, and the tempco is well specified. My experience has mostly been 100V FETs and above, so it may not be directly applicable here.

I would guess that if you know the FET temp, you can do a pretty good job. For those who have to know, there could be an option to calibrate the current using an external shunt. Not sure if it would be worthwhile, because I suspect the current varies a lot during the weld process.

John

 

Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #484 on: December 09, 2017, 10:21:37 am »
For what it's worth, I have found that Infineon MOSFETs have pretty accurate resistance values, maybe within 10% or better, and the tempco is well specified. My experience has mostly been 100V FETs and above, so it may not be directly applicable here.
Did you find this by measuring, or were these datasheet figures?

I would guess that if you know the FET temp, you can do a pretty good job. For those who have to know, there could be an option to calibrate the current using an external shunt. Not sure if it would be worthwhile, because I suspect the current varies a lot during the weld process.
I was considering calibrating Rdson during production testing, but as mentioned before the absolute accuracy is not that important for this application. And as the MOSFETs only heat up by 25K (accounting for ~10% of error), I didn't go for measuring their temperature either. If I would add temperature monitoring, then to protect the power stage in first place.

In the end, kWeld has proven to make very repeatable welding results, and that's all it has to do  ;)
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Offline ovnr

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #485 on: December 11, 2017, 12:34:08 am »
In the end, kWeld has proven to make very repeatable welding results, and that's all it has to do  ;)

Let's have a massive round of applause for tatus1969. Congratulations, you've avoided the feature creep hole I invariably spend my days digging even deeper...
 
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Offline Fraser

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #486 on: December 11, 2017, 01:06:46 am »
Yes, the scourge of many projects. Frank has released a perfectly functional design that he is now enhancing in a controlled manner to provide sensible improvement. The wish list for many projects can become a nightmare as the BoM and design man hours can spiral out of control.

Great work Frank  :-+

Fraser
 
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Offline rhb

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #487 on: December 11, 2017, 05:12:55 am »
I went back through the thread quite a bit, but it's a long thread, so please forgive me if this was already addressed.

As I understand it, you're discharging a capacitor bank and you're measuring the energy of the pulse by measuring the current.  The accuracy of the  measurement is limited by the difficulty of measuring a large current under dynamic conditions.

If the charging current is disconnected prior to the pulse and the capacitor bank voltage measured before and after the pulse that should give an accurate measure of the energy from first principles.  There is likely some voltage variation after the pulse is cutoff related to the degree to which the capacitors do not discharge uniformly.  However, this might provide a means for assessing the condition of the capacitor bank.

Is there a problem with using the relationship among capacitance, charge and voltage to determine the energy of a pulse that I have overlooked?  This seems to me a good bit easier and more accurate.  One still has heating losses, but those should not be too hard to measure or estimate using suitably placed thermistors.
 

Offline JohnG

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #488 on: December 11, 2017, 09:17:54 am »
For what it's worth, I have found that Infineon MOSFETs have pretty accurate resistance values, maybe within 10% or better, and the tempco is well specified. My experience has mostly been 100V FETs and above, so it may not be directly applicable here.
Did you find this by measuring, or were these datasheet figures?



At one point I did (i.e. my tech did) a LOT of voltage drop measurements on Si and SiC MOSFETs and IGBTs at 25, 75, and 125C. The Infineon ones all measured within 10% of that specified in the data sheet. Note that they were easier to measure than high current, low voltage FETs since the Ron was 10s to hundreds of milliohm, but I was surprised at how close they were to the data sheet. Interestingly, so were the measured CV curves.

Not all the FETs were like this... 

John
 
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Offline ovnr

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #489 on: December 11, 2017, 10:02:51 am »
Is there a problem with using the relationship among capacitance, charge and voltage to determine the energy of a pulse that I have overlooked?  This seems to me a good bit easier and more accurate.  One still has heating losses, but those should not be too hard to measure or estimate using suitably placed thermistors.

That only estimates how much energy you drain from the capacitors - not how much gets delivered to the weld site. A majority of the energy consumption is in the capacitors themselves (ESR).

Also, kWeld supports both supercaps and high-energy LiPo cells; the latter are more cost-effective, IMO, and would not work with your proposed measurement method.

Using Rds(on) to estimate current is actually a really common method. In fact, if you're reading this on a computer you're already using a device that does it, namely the CPU core voltage regulator on your motherboard.

 

Offline rhb

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #490 on: December 11, 2017, 12:18:05 pm »
ESR is measurable as is the cable resistance.  The relationship between charge and voltage for a LiPo is more complex than the linear relationship of a capacitor,  but I don't see why it is harder than the time variant  Rds(on) of a MOSFET passing a large current for a short period of time. 

A comparison to the measurement of CPU drains is fatuous at best.  That does not involve going from 0 to 1000A and back in a fraction of a heartbeat.
 

Offline ovnr

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #491 on: December 11, 2017, 12:58:43 pm »
ESR is measurable as is the cable resistance.  The relationship between charge and voltage for a LiPo is more complex than the linear relationship of a capacitor,  but I don't see why it is harder than the time variant  Rds(on) of a MOSFET passing a large current for a short period of time. 
Wait. You think that estimating how many joules - with a precision of 0.1J or better - remain in a LiPo based solely on the pack voltage is easier than Rds(on) sensing? Really? Besides, as tatus1969 has said: The weld energy is not calibrated in any way. You're supposed to find the correct energy level by trial and error, at which point the welder will produce consistent results.

Also, the welder does the joule counting during the welding process, cutting off the current the instant the correct energy level has been reached. That means you can't very well measure the voltage before and after.

A comparison to the measurement of CPU drains is fatuous at best.  That does not involve going from 0 to 1000A and back in a fraction of a heartbeat.
True; 1 kA CPUs are still a bit out. Still, we do have 200+ amp CPUs like AMDs Threadripper (with a healthy overclock). And the VRM switches at 500+ kHz.
 

Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #492 on: December 11, 2017, 05:54:30 pm »
As I understand it, you're discharging a capacitor bank and you're measuring the energy of the pulse by measuring the current.  The accuracy of the  measurement is limited by the difficulty of measuring a large current under dynamic conditions.
The kWeld firmware uses a measurement cycle of 10 microseconds, that is enough bandwidth to sample the current waveform.

If the charging current is disconnected prior to the pulse and the capacitor bank voltage measured before and after the pulse that should give an accurate measure of the energy from first principles.  There is likely some voltage variation after the pulse is cutoff related to the degree to which the capacitors do not discharge uniformly.  However, this might provide a means for assessing the condition of the capacitor bank.
There are quite a few CD welders that use a similar mechanism. But instead of measuring voltage before and after, they let the user dial in the desired stored energy, calculate the corresponding charging voltage for them, and firing all the energy at once until the capacitors are drained completely. This has the advantage that you can use an SCR as the switch, as it does not have to have the capability to interrupt the flowing current again.

Is there a problem with using the relationship among capacitance, charge and voltage to determine the energy of a pulse that I have overlooked?  This seems to me a good bit easier and more accurate.  One still has heating losses, but those should not be too hard to measure or estimate using suitably placed thermistors.
The idea behind kWeld is to control the energy that goes into the weld. (To achieve that, measuring the flowing current is just one requisite.) With your idea (or the way standard CD welders work) you can only control the total energy consumption. A large part of that goes into the capacitor (or battery) itself, the connecting leads, and the switch.
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Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #493 on: December 11, 2017, 05:59:08 pm »
ESR is measurable as is the cable resistance.
Yes, but measuring ESR is technically easier, as the MOSFET that you use as the current shunt is typically GND referenced. This way you only need to measure one ground referenced voltage, instead of having to measure a voltage differential. In this way I am following the same rationale that has brought up ESR current sensing in DC/DC converters.
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Offline rhb

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #494 on: December 12, 2017, 11:44:33 am »
Interesting.  Thanks for the explanation.
 

Offline carracing111

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #495 on: January 01, 2018, 04:39:01 am »
Hello everyone.

I am waiting to buy a KWeld spot welder. It has an incredible appearance!

Meanwhile I'm doing some capacitor testing with my Arduino Spot Welder.

I have purchased two Maxwell 58F units to build two different welders and power my Arduino Spot Welder, and I also hope to use them with my future  Kweld !

At the moment I have not obtained any acceptable welding with Maxwell 58F. It seems that the capacitors do not have enough power.
I have charged the capacitors with a small CC CV power supply (12V, 5A).
I have connected the Arduino Spot Welder directly to the capacitors to perform the tests. The set works, but the welds are very loose, the solders hardly leave a mark on the nickel.

The arduino spot welder is working perfectly with a car lead battery.
Of course I have checked that the capacitors have reached their working voltage, 12.15v. I do not understand why even the first weld is not acceptable.

I just made some tests with the same configuration but with 5 car audio capacitors, they are Chinese and of poor quality. In theory it is assumed that 2 parallel capacitors are 1 farad.

With the Chinese capacitors, the Arduino Spot Welder works very well!





I can not understand why the Maxwell capacitors do not work! I bought them on Mouser and they are not cheap! Can the capacitors be defective?

I have tried both units and the result is just as bad.
Do they need more than 12V to work properly? I can not think of anything else.

I would greatly appreciate any ideas or advice.

Happy New Year !
« Last Edit: January 01, 2018, 04:40:40 am by carracing111 »
 

Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #496 on: January 02, 2018, 12:54:28 am »
I have purchased two Maxwell 58F units to build two different welders and power my Arduino Spot Welder, and I also hope to use them with my future  Kweld !
Happy New Year to you too!

I assume that it is this module:  BMOD0058 E016 B02

A quick look in the datasheet tells that they have 22mOhm ESR. This is way too much for welding. The modules seem to be optimized towards energy density, not power density. If you still want to use them, I would suggest ripping both modules apart and reconfigure them in a 3S4P configuration. That way you'd achieve 2.75mOhm ESR at 5.3V, which should result in 870A weld current with kWeld. That should be good enough for 0.1mm nickel, maybe 0.2mm.
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Offline carracing111

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #497 on: January 02, 2018, 04:10:51 am »
Thank you very much for the reply.

That's right, it's the Maxwell BMOD0058 E016 B02.

I chose these modules inspired by this design: https://www.thingiverse.com/thing:2588371

In the article he says that they work well and that's why I chose them.
These modules are not cheap, If I can not get the spot welder to work with just one module, it might be worth it to return them and order different capacitors ...

In the electrical diagram of the article where these capacitors are used, I have seen that there are 2 resistors of 10ohm and 100W in parallel with the capacitors.

I thought that the purpose of the resistors was only to discharge the capacitors after using the soldering iron. Maybe they are necessary for it to work properly?

I'm sorry, but my knowledge of electronics is still very basic.

Regards.
 

Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #498 on: January 02, 2018, 04:26:05 am »
this design: https://www.thingiverse.com/thing:2588371
Maybe you contact the author of that unit, he is talking about 0.22mOhm ESR but the datasheet clearly states 22mR. That is a factor 100 off.

In the electrical diagram of the article where these capacitors are used, I have seen that there are 2 resistors of 10ohm and 100W in parallel with the capacitors.
I thought that the purpose of the resistors was only to discharge the capacitors after using the soldering iron. Maybe they are necessary for it to work properly?
Thats not relevant, they are for discharge. Just make sure to use the thickest wires that you can afford. Use AWG8 or better.

I'm sorry, but my knowledge of electronics is still very basic.
Pay attention to what you do, you are dealing with high energies. You can easily hurt yourself or burn down your house if you do not fully understand what you are doing. Also take great care of handling Lithium batteries, they are potentially dangerous.
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Offline carracing111

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #499 on: January 02, 2018, 05:13:43 am »
Thanks, I have contacted the author of the article.

Although I believe that I will return the Maxwell modules and I will obtain the ones you recommend or if you sell them directly.

Regards.
 


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