Author Topic: Stahl Soldering Station (Read 21881 times)

jobog · « **on:** February 04, 2015, 10:06:15 pm »

I have a Stahl TCSS soldering station and the .33ohm, 3W resistor that goes from the gate of a BT136 triac to ground has burnt up. The station uses a ZD 919-2 board. I have checked everything on the board except the two ICs, which I have no way of checking, and everything checks OK. How do these stations detect the temperature of the iron tip and know when to shut the heater off? When I replace the .33 resistor the iron will heat up but never shuts off and in a short time the .33 resistor burns out. The ICs are a 555 and an HA17358. Could one of these ICs be causing the problem or could it be in the iron itself? The iron measures about 5ohms across the heater.

Paul Price · « **Reply #1 on:** February 04, 2015, 11:57:16 pm »

If the .33 resistor burns out, my best guess without an exact schematic is that you have a shorted Triac and/or the Triac triggering circuit is itself shorted. If you can somehow remove the gate connection of the Triac to the circuit board, and the iron still gets hot, then you will know that the Triac is shorted.

picandmix · « **Reply #2 on:** February 05, 2015, 09:52:50 am »

Your station looks to be a 936 clone so if you search on '936 schematic' you will find a number of circuits, one may match yours though suspect yours will be a slightly different and simpler version of the original hakko 936 circuit.

The iron contains both the heater and a sensor, if you look at the instruction manual shown on this ebay page it shows how to check the sensor with a meter.
http://www.ebay.co.uk/itm/312087-936-Digital-Electronic-Soldering-Rework-Station-60W-Temperature-Control-/291045808706?pt=UK_Home_Garden_PowerTools_SM&hash=item43c3ae9a42

jobog · « **Reply #3 on:** February 05, 2015, 07:01:30 pm »

Thanks for the link and the info.

Joe

jobog · « **Reply #4 on:** February 09, 2015, 04:21:37 am »

Picandmix:

Finally got around to checking out the info you sent and I still have a question as to how this Stahl station detects the temperature. Unlike the Hakko 936 the Stahl iron only has 3 leads going to it. One is the earth ground that is connected to the iron tip for the ESD protection and the other 2 are connected to the heater. The heater measures just short of 4 ohms so I'm assuming the heater is okay. There is no sensor in the tip.

I was wrong about this .33 resistor that is burning out. It is not connected to the gate but is connected to pin 1 of the triac. The triac tests okay.

Can anyone tell me how this thing is sensing the temperature and what might possibly cause it not to shut off?

picandmix · « **Reply #5 on:** February 09, 2015, 10:07:27 am »

Assume from the lack of a sensor in the iron then you have no true temperature control.

The lower form of 'controlled' stations just use what is basically a pot controlling the flow though the triac, like a light dimmer, which does produce a basic temperature control.

Have used one like that, but recently bought a 936 clone like the one shown eariler; think you will find they are a good step up.

Think you will find its a better move than messing around repairing your old one.

Just ensure you get a true 936 type which does have the two extra wires for the temp sensor in the iron.

jobog · « **Reply #6 on:** February 10, 2015, 08:40:04 pm »

Again, thanks for the reply.

I have purchased a new station. I got the Weller WES51. Now wish I had bought the Hakko for the same money.

Repairing the old station has now become a project without a good reason. Just want to be able to say I fixed it and understand how it works. The Stahl did hold a set temperature pretty well when it was working and the LED would cycle on and off with the heater. Still don't know how it sensed a temperature to turn the heater off. I'm assuming that in stations such as my new Weller, with a sensor, that the sensor is some type of varistor in series with the pot and when the resistance reaches a certain value the triac kicks the heater off. Then, when the varistor cools down the total resistance changes and the heater comes back on.

Like I said, the Stahl station worked well until the resistor on the board burnt out. Could there be a sensor in the same line as the heater buried somewhere in the ceramic heater? Since the unit heats up but doesn't shut off any sensor in that line would have to be shorted so I don't know how I would be able to measure that.

I think I will keep plugging away off and on but it would be much simpler if I could find out if there is indeed a sensor and if not how it knows to turn the heater off and on.

Thanks again for the reply.

Paul Price · « **Reply #7 on:** February 10, 2015, 10:51:08 pm »

Could be that the heating element has a temperature coefficient and the circuit switches from powering the heater to measuring the resistance. Could be that the ground wire and one of the heater leads are the two leads of a thermistor.

If the power to the heating element is unisolated 120 or 240VAC then the ground lead is only a ground.

You can heat the old handle up with your new iron or even give it a treatment on the kitchen stove to find out the truth..let us all know.

It is also possible to use only two wires and a reversing voltage, so by using steering diodes it is possible to switch between the thermistor temperature sensor(low current negative voltage) and powering the heating element(rectified full-wave positive voltage controlled by PWM or a phase triggered SCR/TRIAC).

picandmix · « **Reply #8 on:** February 11, 2015, 09:48:44 am »

The best thing to do, is draw out the full circuit of that board, then you can see exactly how its intended to work, either as a basic dimmer or some form of feedback control.

Upload your diagram so we can have a look.

The WES51, whats wrong with it ?

Would have liked to buy one of those myself at $100 but seems its a US only model; over here in the UK /EU the cheapest Weller Station is about $300+

jobog · « **Reply #9 on:** February 13, 2015, 02:42:24 am »

It will take me awhile but I will draw out the circuit and post it here.

The Weller station works just fine but, as it turns out, I could have bought the Hakko with the digital readout for the same money. I think I paid $119.00 for the Weller with 5 additional tips. I'm finding the digital Hakko for around $90.00 with one tip. The Hakko also has a ceramic heating element while the Weller is Nichrome wire. All the reviews I'm reading now, after it's too late, prefer the Hakko.

picandmix · « **Reply #10 on:** February 13, 2015, 10:03:05 am »

Weller has been the world #1 for so many years, don't think it will disappoint.

Digital displays are not going to make it perform any better, the more parts, the more to go wrong.

Reviews, are only as good as the reviewer and their opinion.
If you saw Davys review of the cheapo $16 solder station ( aka 936) you would never touch one , yet I recently got one and find it more than good enough, ok it may not last a lifetime but at the price ... and yes, I have used Wellers at work.

If you are having trouble with your circuit board layout, can you take a clear pic of both sides of the board and post them up ?

jobog · « **Reply #11 on:** February 14, 2015, 04:02:36 am »

Here are pictures of the board. I will keep plugging away on the schematic.

I know Weller was good and that's why I bought the WES51 but now I'm reading that since the assembly has moved out of the US the quality has really suffered.

picandmix · « **Reply #12 on:** February 14, 2015, 11:27:14 am »

Should not spend your time worrying about the Weller, just use it, if it ever fails then your supplier or Weller direct should sort it out.

Nice clear photos, just a couple of points, is ic2 the 555 ?
The resistor below R4 , is that partially burnt as well ?
Does the station have a mains transformer, typically 24vac output , or its the board just fed straight from the mains voltage ?

You say the heating element shows 4 ohms, but with the iron disconnect from the station have you measured across the heating element and the irons metal body in case it is shorting to ground ? You should not get a reading of less than 2-3 meg ohms if ok.

picandmix · « **Reply #13 on:** February 14, 2015, 04:52:36 pm »

Been able to do a very rough diagram , see attachment.

Need you to identify a few things to complete it and lay it out more logically.

First, details of how and what power wires are connected to the two sockets.

The positive /negative orientation of the Electrolytic caps.

The component number of some items and some values, like the Zener Voltage and transistor code where I have left a ? mark

A bit difficult to see all the tracks around the pot, but think I have worked out most of them apart from where R10 and C3 go diagonally down behind the pot ?

Find when working on transformerless designs like that you often get a chain reaction where one part blows the other etc etc or it damages another part only for the next item to fail when you power up.
Generally find its better and quicker to simply replace all the active parts, partic the Zener,.
So a list of codes for the Triac, transistor, diodes , 2 ics (lm358 should do) and the electrolytic caps would all be on my shopping list.
( on some very old boards I will literally strip it and replace all parts to ensure its reliable long term, (commercial use) )

However whats the cost of those parts v a replacement ?

But don't forget to first check out the iron itself for shorts !

jobog · « **Reply #14 on:** February 15, 2015, 06:25:44 am »

Here are the answers to your questions. If you need anything else just let me know. I really appreciate all the time you are putting into this.

Thanks,
Joe

picandmix · « **Reply #15 on:** February 15, 2015, 10:12:12 am »

Quote from: jobog on February 15, 2015, 06:25:44 am

Here are the answers to your questions. If you need anything else just let me know. I really appreciate all the time you are putting into this.

Thanks,
Joe

No problem, it gave me an excuse to escape the girly chatter for half a hour when friends came around.

Will update the diagram and post it later, but just one more bit of info I need on the tracks surrounding the pot.
Can you check visually or with your meter these highlighted tracks around the pot.
Does the upper red track join the yellow track ?
Does the blue track just go as shown.
Do the lower red and purple track just go as shown.

J1 and J2, they are just jumpers, just pieces of wire with a ceramic body, also called zero ohm links, hence the single black band (0) ,expect its so the auto insertion machines could handle them like resistors.

picandmix · « **Reply #16 on:** February 15, 2015, 09:20:44 pm »

Revised diagram attached, just needs you to confirm those tracks mentioned in the last post.

jobog · « **Reply #17 on:** February 16, 2015, 03:07:16 am »

Here are a couple pics with the pot removed. It looks like they run just as you showed.

picandmix · « **Reply #18 on:** February 16, 2015, 09:41:27 am »

Ok, have updated the diagram, so subject to your final check that its correct and putting in the resistor values for future reference, my job done.

So, what route are you going to use to repair it , just the triac , res, and transistor or all the active parts ?

Also , short of the proper meters, do a visual check on the 4 electrolytic caps, are they secure, any leaking substances and are the tops of the tubes flat ? if they are slightly bowing upwards thats a sign they are failing.

good luck

Paul Price · « **Reply #19 on:** February 16, 2015, 10:58:29 am »

The original enigma was concerning how temperature control seemed to be working, from the schematic I see no feedback, so the temperature control is not temperature regulation an all you end up with is a cheap station with phase control of power using a triac.

I was hoping to see some feedback to create temperature regulation, phase control of a triac guarantees poor performance.

My conclusion then is to well, Weller your way back to better joints, this Stahl iron is always only at the right temperature when not soldering, too cold when heat is needed or too hot if you stop soldering for a few seconds. Its a fiddle box.

jobog · « **Reply #20 on:** February 18, 2015, 03:04:35 am »

I already have a triac and transistor along with the burnt out .33 resistor and the 200 ohm that's showing signs of heat. I will replace these items and see what happens. If it still doesn't shut off I will move on to the electrolytics and the ICs since I can get all of them from Newark for just a couple bucks. I also have a Newark coupon for 20% off that expires the end of the month so an even better deal and I can see what else I need (want really) to make the shipping worthwhile.

Thanks for all the assistance. I still don't understand how it knows when to shut off and will have to do some more studying till I do.

Joe

picandmix · « **Reply #21 on:** February 18, 2015, 09:37:38 am »

You really do want to replace that Zener diode before replacing IC2.
If the Zener blows, the overvolts can damage IC2 then in turn damage IC1 etc and so a chain reaction.

Ic2 A senses either side of the Triac and then passes its output to Ic2 B but notice cap C3, that will create a little time delay .
The other input of Ic2B is the control pot which it compares with the other signal and in turn triggers the 5 second heat pulse from the 555.

Exactly how Ic2 A senses around the triac I'm not sure, but think some other cleverer folk will be able to explain that for you.

jobog · « **Reply #22 on:** February 18, 2015, 10:20:38 pm »

Okay, I will replace the zener as you recommend. Thanks again for all the work you have done on this. I can't edit your schematic or I would add the values for the components myself. If you are up for it here are all the missing values.

The board is a ZD 919-2 which seems to be a pretty common Chinese board for cheap soldering stations.

R1 52ohm, 5%, 3W
R2 300ohm 5%
R3 .33ohm 5% wirewound
R4 15K 1%
R5 13K 5%
R6 200K 1%
R7 3.3K 5%
R8 1.3K 5%
R9 51K 5%
R10 530K 1%
R11 100K 5%
R12 2K 1%
No R13
R14 130K 5%
R15 1.5K 1%
R16 5.1K 5%
No R17
R18 15K 5%
R19 10K 5%
R20 200ohm 5%
W2 75K 1%
Variable resistor 1.2K full range, linear
The 4 diodes are N4007's
The 2 small caps are .1uF
C4 47uF, 25V
C5 330uF, 25V

I think I've included everything here. Thanks again.
Joe

picandmix · « **Reply #23 on:** February 19, 2015, 12:31:16 am »

No problem, will do tomorrow, though will you check the resistance of the Variable, 1.2k is not a common value for pot.

Are you measuring it out of circuit and across the outer connections ?

There is often a stamp or lettering on the body stating its resistance.

The wattage for R3 might be hard to establish but its always better to get the biggest wattage that will fit in the space; plus it dissipate the heat better.

jobog · « **Reply #24 on:** February 19, 2015, 01:52:34 am »

It's a 1K pot, just measures a little larger. I'm replacing the .33 with a 3W as they are about .40c each. Going up to a 4 or 5 watt raises the price to a couple dollars.

picandmix · « **Reply #25 on:** February 19, 2015, 10:21:17 am »

Values added, let me know if anything is wrong.
Did spot an error on R1 and R2 labeling , was R2 and R13.

Have specified all 1% resistors as those types are so cheap its not worth getting both 5% and 1%.

edit
Since posting the above, now just looked at your resistor values and what I can see from your original pictures.
Not easy to be sure so of some of the resistor colour bands eg , red or orange, but I read 3 different from the values you give.
Have you measured in circuit or read off the colour codes or both ?

R5 I read as 1.2k you say 13k
R8 1.2k you say 1.3k
R19 1k you say 10k

jobog · « **Reply #26 on:** February 20, 2015, 03:01:32 am »

Your latest has R1 and R2 reversed. R1 is 52 ohm and is connected from the diode D1 to pin 4 of the 555. R2 is 300 ohm and is connected from pin 2 of the connector to diode D2.

R5 is 1.2K
R8 is 1.2K
R19 is Brown, Black, Orange and measures 10K

picandmix · « **Reply #27 on:** February 20, 2015, 10:06:25 am »

These should be right then...

Did you see your PM inbox ?

jobog · « **Reply #28 on:** February 21, 2015, 12:01:05 am »

Still one minor problem. R1 is the large 3W resistor. Other than that it looks right.

I spent $1.50, replaced the 4 electrolytics and the zener and this thing is now working. Still don't know how it works but, Hurrah!!!

picandmix · « **Reply #29 on:** February 21, 2015, 10:12:11 am »

Should be right

paiusgeorge · « **Reply #30 on:** November 22, 2015, 04:44:34 pm »

Hello guys. Please...can you help me ? Can you write what part did you change exactly ? Because i read 10 times the conversation but i did't understand in the final which part did you change... i changed the zener,the r3,the r20...but r20 keeps burning. Thank you very much.

Deathwish · « **Reply #31 on:** November 22, 2015, 05:13:40 pm »

Quote from: paiusgeorge on November 22, 2015, 04:44:34 pm

Hello guys. Please...can you help me ? Can you write what part did you change exactly ? Because i read 10 times the conversation but i did't understand in the final which part did you change... i changed the zener,the r3,the r20...but r20 keeps burning. Thank you very much.

the 4 electrolytic capacitors

jobog · « **Reply #32 on:** November 23, 2015, 03:09:49 am »

Yes, the 4 electrolytics and the zener. Replaced the 2 burnt resistors.

paiusgeorge · « **Reply #33 on:** November 24, 2015, 03:23:19 pm »

hello dear friends. My english is not that good, and i'm a beginner in electronics,and i think that i changed the wrong parts ( r3,r20,zener and bt136),and it still not work...the r20 is burning .
Can you please make me a list with the 4 electrolytics that you changed ( for example R1 , R3, R20 etc ) what zener and what you put in that place for replacement( what value has new parts) , because i think that you put a bigger electrolytics for example on r3 i put 33 ohm with 5 w, instead of 33 ohm and 3w.
thank you so much.

ross57 · « **Reply #34 on:** February 17, 2016, 11:39:08 am »

Hello to the forum mates,
I have found this thread very helpful since I'm fixing my Velleman VTSSC50N (ZD-919-2) soldering station.
Thank you all for the very useful tips and pcb.
As my contribution, here attached is the schematic received directly form the Velleman Customer Support.
My station had the R3 (0.33 Ohm) resistor and the BT 136 triac burned because was powered with the stylus NOT attached.
An unacceptable lack of protection.
It would be interesting if some expert would give directions to avoid this in the future (opto coupler?), as also to add a green led that lights when the set temperature has been reached (even though I do know that my station has NOT a real tip temperature control since the stylus has only the heater leads).

SkyMaster · « **Reply #35 on:** February 18, 2016, 10:29:05 pm »

Quote from: jobog on February 10, 2015, 08:40:04 pm

I have purchased a new station. I got the Weller WES51. Now wish I had bought the Hakko for the same money.

Quote from: jobog on February 13, 2015, 02:42:24 am

The Weller station works just fine but, as it turns out, I could have bought the Hakko with the digital readout for the same money. I think I paid $119.00 for the Weller with 5 additional tips. I'm finding the digital Hakko for around $90.00 with one tip. The Hakko also has a ceramic heating element while the Weller is Nichrome wire. All the reviews I'm reading now, after it's too late, prefer the Hakko.

Jobog,

I have both, so I can compare.

I have a Weller WES50 (made in USA, 2001 vintage) and a Hakko FX-888D (bought recently).

WES50:
The soldering pencil on the WES50 (looks identical to the current one) is bit thinner than the one on the Hakko.
The tips for the Weller sell for about half the price of the Hakko tip.
The analog temperature control of the Weller is nicer to use than the digital control of the Hakko FX-888D.

FX-888D
The user interface of the Hakko FX-888D, with its two buttons, is awful (disgusting would also be an appropriate word).
The power switch on the side is less convenient than the power switch on the front of the Weller.
The brass coil compartment built into the pen holder is a nice feature.

My conclusion is that you have no reason to regret buying the Weller WES51

jobog · « **Reply #36 on:** February 19, 2016, 02:56:12 am »

Quote from: SkyMaster on February 18, 2016, 10:29:05 pm

Quote from: jobog on February 10, 2015, 08:40:04 pm
I have purchased a new station. I got the Weller WES51. Now wish I had bought the Hakko for the same money.

Quote from: jobog on February 13, 2015, 02:42:24 am
The Weller station works just fine but, as it turns out, I could have bought the Hakko with the digital readout for the same money. I think I paid $119.00 for the Weller with 5 additional tips. I'm finding the digital Hakko for around $90.00 with one tip. The Hakko also has a ceramic heating element while the Weller is Nichrome wire. All the reviews I'm reading now, after it's too late, prefer the Hakko.

Jobog,

I have both, so I can compare.

I have a Weller WES50 (made in USA, 2001 vintage) and a Hakko FX-888D (bought recently).

WES50:
The soldering pencil on the WES50 (looks identical to the current one) is bit thinner than the one on the Hakko.
The tips for the Weller sell for about half the price of the Hakko tip.
The analog temperature control of the Weller is nicer to use than the digital control of the Hakko FX-888D.

FX-888D
The user interface of the Hakko FX-888D, with its two buttons, is awful (disgusting would also be an appropriate word).
The power switch on the side is less convenient than the power switch on the front of the Weller.
The brass coil compartment built into the pen holder is a nice feature.

My conclusion is that you have no reason to regret buying the Weller WES51

Thanks for the comparison. After using the Weller for awhile now I really like it. At the time I thought the digital readout might be nice but no problems with the dial on the Weller. Thanks again.

romdos · « **Reply #37 on:** March 09, 2016, 05:25:56 am »

Hi,
I know this is a year later, but thankfully I found this forum and thread since I recently had the same problem as the OP, but I noticed mine also burnt up the transistor there. Do you have a value for that? I didn't see that one listed in your posting. All identifying markings were destroyed from the heat, so I can't ID it.

R1 52ohm, 5%, 3W
R2 300ohm 5%
R3 .33ohm 5% wirewound
R4 15K 1%
R5 13K 5%
R6 200K 1%
R7 3.3K 5%
R8 1.3K 5%
R9 51K 5%
R10 530K 1%
R11 100K 5%
R12 2K 1%
No R13
R14 130K 5%
R15 1.5K 1%
R16 5.1K 5%
No R17
R18 15K 5%
R19 10K 5%
R20 200ohm 5%
W2 75K 1%
Variable resistor 1.2K full range, linear
The 4 diodes are N4007's
The 2 small caps are .1uF
C4 47uF, 25V
C5 330uF, 25V

I think I've included everything here. Thanks again.
Joe
[/quote]

romdos · « **Reply #38 on:** March 10, 2016, 02:35:44 am »

Quote from: jobog on February 18, 2015, 10:20:38 pm

Okay, I will replace the zener as you recommend. Thanks again for all the work you have done on this. I can't edit your schematic or I would add the values for the components myself. If you are up for it here are all the missing values.

The board is a ZD 919-2 which seems to be a pretty common Chinese board for cheap soldering stations.

R1 52ohm, 5%, 3W
R2 300ohm 5%
R3 .33ohm 5% wirewound
R4 15K 1%
R5 13K 5%
R6 200K 1%
R7 3.3K 5%
R8 1.3K 5%
R9 51K 5%
R10 530K 1%
R11 100K 5%
R12 2K 1%
No R13
R14 130K 5%
R15 1.5K 1%
R16 5.1K 5%
No R17
R18 15K 5%
R19 10K 5%
R20 200ohm 5%
W2 75K 1%
Variable resistor 1.2K full range, linear
The 4 diodes are N4007's
The 2 small caps are .1uF
C4 47uF, 25V
C5 330uF, 25V

I think I've included everything here. Thanks again.
Joe

jobog · « **Reply #39 on:** March 21, 2016, 12:21:12 am »

Quote from: romdos on March 09, 2016, 05:25:56 am

Hi,
I know this is a year later, but thankfully I found this forum and thread since I recently had the same problem as the OP, but I noticed mine also burnt up the transistor there. Do you have a value for that? I didn't see that one listed in your posting. All identifying markings were destroyed from the heat, so I can't ID it.

R1 52ohm, 5%, 3W
R2 300ohm 5%
R3 .33ohm 5% wirewound
R4 15K 1%
R5 13K 5%
R6 200K 1%
R7 3.3K 5%
R8 1.3K 5%
R9 51K 5%
R10 530K 1%
R11 100K 5%
R12 2K 1%
No R13
R14 130K 5%
R15 1.5K 1%
R16 5.1K 5%
No R17
R18 15K 5%
R19 10K 5%
R20 200ohm 5%
W2 75K 1%
Variable resistor 1.2K full range, linear
The 4 diodes are N4007's
The 2 small caps are .1uF
C4 47uF, 25V
C5 330uF, 25V

I think I've included everything here. Thanks again.
Joe

[/quote]
Sorry this is so late but I just saw your question about the transistor. It was replaced with a C9013.

Shredhead · « **Reply #40 on:** June 08, 2016, 04:31:49 am »

Quote from: picandmix on February 09, 2015, 10:07:27 am

Assume from the lack of a sensor in the iron then you have no true temperature control.

The lower form of 'controlled' stations just use what is basically a pot controlling the flow though the triac, like a light dimmer, which does produce a basic temperature control.

I have this Stahl Station and it must use some kind of feedback to sense the load. My handle's element failed after about 2 years of moderate use so I hooked up different kinds of elements and dummy loads to the station and scoped the output of the station. I found that it reacts much differently with each different load attached.

Stahl is apparently a product of Parts Express and they don't sell replacement elements for it. I can't find any element that will fit the handle anywhere online in the US. I bought a 936 clone handle to rig up to it leaving the thermocouple wires not connected and no matter what the setting of the temp on the station, it will slowly ramp up by itself until the power is on to the element nonstop.

This Stahl TCSS station is a disposable pile of dung. Everyone out there should avoid it.

Feliciano · « **Reply #41 on:** October 09, 2018, 02:25:29 am »

Been a long time reader, now I want to give back a little.

If somebody's still insterested in this piece of cheap hardware as I do, I can tell in fact it's cheap, and hence it's electronics it's also cheap, with several corners cutted.

I studied the schematics (which has a few errors like a diode reversed, a couple of missing components, and two capacitors interchanged, and I've also seen others with other Q's). Besides that, it can be reinforced adding a snubber to the triac and optionally other protection to the transformer, to avoid the transients, like a possible connection/disconnection of the handle during operation (not recommended), an eventual burnout of it, etc. I'll do those modifications myself one of these days.

On the other hand it's feedback is rather minimal/indirect, sensing the voltage over R3, which is influenced by the presence of the handle, and up to some extent, its temperature.

Feliciano · « **Reply #42 on:** January 05, 2019, 04:38:36 am »

I designed/added the snubbers, an additional filter for V+ and started to use it.

For my dissapointment, after half an hour of continous use at low temp, R3 burnt out, taking the transistor with it (at least my modifications seems to salvaged the triac and ICs). So I replaced the damaged components with more resiliant ones, improved the heat disipation, and used my oscilloscope to find out the Q of the main components. I confirmed what my simulations showed, that this design seems clever in principle, but it's too sensitive to component's values.

I'll use it again, and update this post if find something relevant to add.

NGY · « **Reply #43 on:** July 17, 2022, 10:09:23 am »

I understand this is a VERY old topic. Nevertheless, may I post my finding here, as I feel it relevant, and documenting the technical side here could probably be interesting for future readers too. These soldering stations are still being manufactured and sold.

I recently bought a disfunctional Fahrenheit 28003 soldering station (with the same ZD-919-2 board shown above), with its belonging iron, told as the defective part. The station looked neat and clean, inside out. The iron indeed shown an open circuit. Before purchasing a new heater, I wanted to understand how this control circuit worked - this is how I got here.

I opened the faulty heater element to get an idea, what is going on with this simple two-wire iron, that has to do with some kind of reasonable temperature control. Then I found the "trick": the iron's heater element consists of the actual heater coil, and a sort of thermocouple, in series with the coil (see my photos below).

So then yes, there is a kind of temperature control: the control circuit is watching the current flowing through the heater element, via the voltage across the 0R33 resistor amplified by opamp-1, then opamp-2 compares it to the voltage set by the pot for the required iron temperature, and triggers the triac via the 555 timer. Simple as it gets.

In my case, this thermocouple was broken. I redid the thermocouple joint - just a makeshift thing to get the iron working for the time of some quick experiments. Based on my very rough measurements, the resistance of the heater element (coil+thermocouple) varied like this: at room temperature (25 °C) ~3 Ohms, then at 100/200/300/400 °C approx. 20/27/35/42 Ohms respectively - in this practical range looked actually fairly linear.

Regarding the burnt 0R33 resistor (seen this issue also on other forums): I guess it happens when that "hidden" thermocouple fails and the control circuit keeps triggering the triac.

I truly can't believe that this resistor could burn when the soldering station is switched on without the iron attached. More important though is to use it with its own iron exclusively (with this special heater element), to keep the control circuit working in its appropriate range.

(One last commment: to improve the stability of the temperature control, I added a zener diode in the positive voltage rail circuit too, by simply soldering it in parallel to the elco's leads.)


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Author Topic: Stahl Soldering Station (Read 21881 times)

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