A simple test with a multimeter is to check gate resistance. If it seems to be OK, then the bulk of the silicon usually is. If you find non-continuity or a short in the gate circuit, then you can conclude that there is a fault in the thyristor.

Do this: Use an ordinary multimeter in ohms range. Measure resistance between the two wires (that is where the gate is). If resistance is below one or two ohms (these data are for large disk thyristors) or more than 1000 ohms, then you have a faulty thyristor. If it is somewhere between 10 and 100 ohms, then the gate is OK. If between 1 and 10 ohms, you need to check further - it is a grey zone where also gating on needs to be checked.

To check gating on (the ability to be switched on by the gating pulse) do this:
1, Locate cathode side (usually indicated by the thyristor symbol on the ceramic) and connect the negative side of a DC source to it (a mains adapter or battery or whatever you have available).
2, Set the DC level at around 12 V (this is not very critical, just about any voltage will work, as long as it is more than five volts).
3, Find an incandescent lamp and connect between positive side of DC source to it. Wattage is, again, not critical. Use whatever lamp you find, but make sure that the voltage corresponds to your DC source.
4, Connect other side of lamp to thyristor's anode. You need to apply a pressure similar to the pressure in the actual thyristor stack in order to get a reliable contact between anode side and internal silicon.
5, If the lamp is dark, then the thyristor is blocking, which is good. If the lamp lights up, then the thyristor is shorted (or you have made a mistake). If no mistake, then mark the thyristor as defect.
6, Check gating on. To do this, you need to identify the gate wire. One of the wires is the gate wire and one is the auxiliary cathode wire. The auxiliary cathode wire can be located by measuring against cathode. The one that is shorted to cathode is the auxiliary cathode wire. The other one is the gate wire.
7, When you have identified the gating wire, connect momentarily to the anode (NOT to the DC source, you need the current limiting lamp in the circuit so as not to kill the gate). The lamp shall light up and then remain on. If it does, chances are that the thyristor is OK. If it does not, you may have to try with a higher voltage or a lamp that has a higher wattage. The reason is that both gating current and latching current is dependent on lamp wattage. Too little current will neither gate on nor latch the thyristor.
8, If you were sucessful, then open the circuit and close again. The lamp shall stay dark until you touch gating wire momentarily to anode, which shall light the lamp again.

Sounds awfully complicated. But can be done in a few minutes if you have a 12 V DC supply and a 5 - 50 W lamp available. For smaller thyristors, you shall use lesser wattage.

Please note that this does not check nominal blocking voltage and reverse voltage. For that, you need a full-blown thyristor checker.
 

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

The method Gunnar describes is very effective. I've used the same technique to check AVR thyristors using the 110V turbine battery as a source with a bank of tungsten lamps as a load. To test blocking capability I've used a Megger at a voltage close to, but less than, the rated blocking voltage. Results are a little open to interpretation because the leakage varies with die size and temperature, but a 'good' result should equate to a very high resistance value (low current).
  

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If we learn from our mistakes I'm getting a great education!
 

Gunnar's point about clamping the thyristor is very important. Without pressure being applied to the disk it will remain open circuit no matter what you do.

Hi Sibeen,  I know technically that is supposed to be true but I have succsessfully gated many hockey puck thyristors (2000A)without comnpression with no problem.  I started out compressing them but later found that it didn't make a difference.  Any idea why this might be the case?

Well, thyristors are not exacty built like beer cans. They are quite rigid and usually make contact internally, also without pressure. But it is good practice to avoid that pitfall. Scrapping thyristors that are good is expensive. A thyristor like that costs around 10 000 USD. So, it pays to be careful

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

Thanks Gunnar for lps deserving tip.

Muthu
www.edison.co.in

Gunnar, have you got an extra zero in there? I was paying roughly £800 for a Powerex 4kV 2kA rectifier grade thyristor a few years ago.
  

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If we learn from our mistakes I'm getting a great education!
 

Thanks, Gunnar.

And of course if I ever find a bad one I compress it and test it again.
 

No, the price was actually higher. I got it from a guy that had problems with a large ABB drive and he told me that it did cost around 90 000 SEK to change one of those disks. Can be that he included costs for stillstand and the ABB expert. That, I do not know. Still, even at a bargain price like yours, it is a little too much money to just throw away.

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

Ha-ha, didn't you know that 'ABB' is an abbreviation for Another Big Bill?

But I agree, at roughly £1000 each they are too expensive to throw away unless they are definitely dead. I was buying through Bowers Semiconductors in the UK, it must be three or four years ago since I last used them. I've just checked the type I was buying and it was 4kV 1.2kA rating, not 4kV 2kA as I thought. My memory is failing me... I need a whisky.
  

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If we learn from our mistakes I'm getting a great education!
 

That makes a lot of sense. Also, that wasn't buying from stock, but via ABB Service. Spare parts are a lot more expensive than commodities. What 'medicine' are you taking tonight?

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

I took a picture of that very expensive thyristor. See attached picture.

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

• http://files.engineering.com/getfile.aspx?folder=6b5775d0-5e35-482a-b2c9-22

A similar looking beastie to the ones I was working with - from your part number I guess 3000V / 1800A. No photo but here's the datasheet for the TAK7 series.

Powerex have a good range of big thyristors if you ever need any in future.
  

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If we learn from our mistakes I'm getting a great education!
 

As you can see, it is a GTO (gate symbol). That may explain the high cost of this particular one.

I should have added that a GTO also can be turned off via the gate. That part of the testing is a lot more demanding. There, you definitely need a specialized tester.

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

Sorry Gunnar, I missed that completely.
  

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If we learn from our mistakes I'm getting a great education!
 

dear friends,,
i have tested the three different thyristors through lamp circuit.. but not satisfied.. as per some sites information, i connected the 12 vdc source. i given positive to anode and gate. before gate there is switch.. negative given to 12 vdc lamp one side.. while another side lamp connected to cathode.. when i given pulse through switch means gate then lamp lights up, but when gate opening lamp goes OFF , why happening like this.. Thyristor should remains ON until circuit disconnected, it should hold but not holding.. just i want to know where is my circuit mistake. i have checked three different thyristors high voltage using. thank u very much

GTO thyristors can be turned back off.  What kind do you have?

Keith Cress
kcress - http://www.flaminsystems.com

Possibly your load is too small to cause the thyristor to latch into the 'on' state or your gate drive is too low to cause it to latch. At very low currents it is possible to get transistor-like behaviour - 'very low' is relative to the nominal rating and nominal gate drive currents.
  

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If we learn from our mistakes I'm getting a great education!
 

The description of your circuit is not very clear. Can you, please say again? The way I read your description, it sounds like you are killing your thyristors. A safer way for the neophyte is to include a resistor in the gate path. And, does it hurt much to use proper punctuation? Extremely difficult to be sure what you are saying the way you build your sentences.

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

Agree with Scotty...not enough current

My all thyristors are using in TCR ( Thyristor control reactor )high Voltage circuit and i dont know weather it is GTO or Not. but it should be latch as per my Knowledge. i used 12 VDC source as i mentioned through strong Battery charger and lamp is 12V, 21Watt. OR any other method to test the thyrsitor. can someone give me another circuit. i can give u thyristor type EGR03-05 , 250 v 1000 A
thank  u