Author Topic: How to measure SCR in AC circuit, one shot?  (Read 1275 times)

Offline PekkaNF

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How to measure SCR in AC circuit, one shot?
« on: December 28, 2015, 08:11:47 AM »
Relating to spot welder I'm working with....

There is a change that I will blow up SCR:s first on 230 VAC and later on 400 VAC.

When I was at the university we hooked up scope on gate/cathode, fairly close to "neutral" if possible (othervice scope body was near mains voltage......). Then worked attenuators and current shuts and the works, to get usefull signal without too much differential voltage.

That was long time ago and my memories are a bit sketchy, but I remember sparks and bangs, when something went wrong.....

So, how nowadays is measured thyristor firing (or not firing) and possibly blowing up due to too little gate current or something.

There might be only few mains cycles...sort of memory scope/logger would be really nice. Something cheap and even easy triggering.

Suggestions on measuremet setup and scope kind of tool?

I only have 20 Mhz analog scope I'm going to take out from the top self and see if capacitors will blow up after 10 years of storage.

Pekka

Offline John Rudd

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Re: How to measure SCR in AC circuit, one shot?
« Reply #1 on: December 28, 2015, 10:07:45 AM »
Pekka,
Typical failure mode for an scr is either open circuit or short circuit....Ideal test would be dynamic using a couple of passive components....

Connect a 12v supply to the device, with a small,lamp inseries with the anode connected to +ve, connect cathode to -ve, then with a resistor connected between gate and anode..device should remain on once triggered. Switch off supply to delatch.
Resistor needs to be sized for gate current of device under test...try something like 330 ohms...
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Offline PekkaNF

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Re: How to measure SCR in AC circuit, one shot?
« Reply #2 on: December 28, 2015, 01:28:10 PM »
Thanks. that would tell me that thyristor is dud....

I want to measure the thyristor in the circuit, while it it pounded...and if it spits internals out I want to have glue why, and then adjust the circuit. Basically, I want to see that it triggers well, stays on whole cycle and current does not fry it on first few cycles.

Pekka

Offline PekkaNF

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Re: How to measure SCR in AC circuit, one shot?
« Reply #3 on: January 04, 2016, 03:42:08 PM »
Now I have a circuit and transformer that seems to work (not tested under load yet).

Tomorow I'll dig up the scope and see if it works and if I can get any decent waveforms out of it.

Pekka