Testing a SCR with a multi meter

[chrismac]

I am trying to replace some SCR's on a printed circuit board but am not sure if what I am doing is correct, the first in the list tests as expected using my mutli meter, negative probe to cathode, positive probe to gate get a reading then roll the positive probe onto the anode and the reading holds, also try in reverse and no reading .

I have upped the ratings on the SCR's but cant get the 2nd and 3rd on the list to test as expected, what am I doing wrong?

1st MCR72-4 VINTAGE MOTOROLA REVERSE BLOCKING TRIODE THYRISTORS 8AMP 200V
2nd S4025L 400V 25A Thyristor SCR
3rd 4 x TIC126D Thyristor SCR 400V 12A TO-220 through hole

Thanks in advance

 

[Chippie]

I would test with your meter across the A-K on Ohms, then with a small battery, apply a gate trigger voltage between G-K….this would rule out any issue with your meter not being able to trigger the dut….

 

[Misterg]

The MCR72 has a (very) low gate current requirement: 30 - 200µA

TIC126 is 5-20 mA
S4025L is 1 - 35mA

I'm guessing your meter is unable to supply enough gate current for the last two.

If you've swapped them around in an existing circuit, check that there's enough drive capacity to trigger the higher Ig ones.

(For testing lower power devices, I put the meter between A & K on and dab a licked finger between A+G to see if it triggers.)

 

[chrismac]

Thanks both, I'll have a check over the next couple of days and report back.

I should have added I had the meter set to diode test, on the 2nd & 3rd there was a reading in both directions, A to K and K to A, positive and negative leads being reversed, where on the first I only get a reading in one direction.

I ended up heading out to the garage, got nothing on the 2nd and 3rd with the damp finger, worked on the first, rigged up a 1.5v AA battery - gate, + to anode, - probe to cathode, + probe to anode, got a reading in both directions when - to gate from battery was touched, meter set to diode test, should I be getting a reading in both directions when the symbol is like a diode?

I will check tomorrow on ohms to see what I get.