SC26502 semiconductor

It's to cold out for me to look in my parts interchange books. But the place where I've got parts for old welders before is Galco.

Give them a call. They where very helpful to me in the past.

This is a SIGNETICS NPN power resistor - try Digikey for parts.

I will send you the data sheet from ISC - Chineese IC/Semi house.

They make the Signetics generic version.

Mart> I was helping a friend troubleshoot an Airco Tig Welder. And I am

I sent the data sheet but Dan your email bounced.

So send me an email and I'll send you the data sheet.

Mart> This is a SIGNETICS NPN power resistor - try Digikey for parts.

Sorry about that. My email address is snipped-for-privacy@yahoo.com The one that is on my posts is a very old address that I have not updated. I do not know if that reduces the amount of spam I receive, but it might.

Thanks a lot. Knowing what kind of semiconductor it is will make a big difference. Typical of welding manufacturers the schematic in the manual just shows the connections to a circuit board and no info on the actual circuit. But the board has all discrete components, so should not be hard to get it working correctly. The symptom is that the current is at the max. It uses a saturable reactor and the control voltage stays at the max regardless of what the current pot is set at.

=20 Dan

Sounds like a short between emitter and collector. - likely from a voltage spike from the reactor.

Good luck - would seem to be easy to buy another at Digikey.com and be aware it might have taken out a resistor or other part as well.

Mart> >> I sent the data sheet but Dan your email bounced.

Gee, I didn't know that power resistors came in NPN -- do they come in PNP as well? How does Ohm's law apply to an NPN resistor?

Good grief Tim - they use them stacked on top of each other to make a totem pole output power buffer in amplifiers and power supplies.

Common today are not the bipolar ones but FET ones. Generally easier to design with in DC circuits as they work on voltages while bi-polar are current driven. Different designs by a little here and there.

A little more flexible than tubes. Mart> >

Last time I checked they called those things transistors, not resistors.

I don't know if the spell checker got it or my flu was still killing me a bit.

Yes NPN and PNP parts are transistors and the field effect ones that are used extensively now and many in motor speed controls in remote control cars/airplanes and such are FET's N-channel and P-channel.

Mart> Last time I checked they called those things transistors, not resistors. >

FWIW: Transistor comes from "transfer resistor"

Chuck P.

trans(fer) (res)istor trans-istor or transistor.

It was a chunk - Wedge of germanium with a plug inserted. The wedge conducted current and when current was injected onto the plug, the current either increased or decreased - depending on the vector or magnitude and direction (polarity). AT&T patent.

Mart> >

My friend read the manual. It lists a 40 amp 400 volt triac as part of the PC board assembly. The schematic does not show said triac, but it does show up on the parts list. The parts list shows the PC board, the triac, and the PC board assembly. We did check that the triac did not block the voltage with the circuit open to the control pin. Still could be a bad snubber circuit on the PC board so I recommended some checks he could do on the triac to verify it is really bad.

Dan

Need a transistor / SCR waveform display.

They are 4 layers and a little tough to test. The bad news is it might work with low voltages/currents but arc over or punch through if high voltage or current.

Usually they are high impedance ans won't conduct from the anode to cathode if plus flow. MT1 to MT2 might be terminal names now.

A 40 amp triac is thumb size and has a large heat sink. It isn't on the pcb unless there is a large heat sink on it. Likely it is on the chassis or the sink is mounted top, center, bottom..... somewhere.

I have a curve tracer, but can't run those currents. Nominal functional is possible.

Mart> >> I was helping a friend troubleshoot an Airco Tig Welder. And I am

The triac is wired to the PC board, but mounted to an aluminum plate next to the PC board. That is probably why they had the triac and PC board listed as part of the PC board assembly. The original triac is a stud mount. Smaller than my thumb but definitely a power semiconductor.

Dan

Can you tell if trigger pulses are present on the gate, Dan?

--Winston

We did not try to verify pulses. The problem is that the triac is full on all the time even when the wire to the triac control is clipped.

Dan

Have a look at the last two schematics if you would please:

Is the gate completely floating now or is there a resistor connected between MT2 and the gate?

"It can be triggered by either a positive or a negative voltage being applied to its gate electrode (with respect to A1, otherwise known as MT1)"

Does the triac turn off when you short the gate to MT1?

--Winston

I had a prof that referred to tubes as "glassFETs".

I suppose I should feel bad for pulling your leg if you were down sick.

I'll think about that and get back to you.