Test A Thyristor

What is a Thyristor?

A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. It acts exclusively as a bistable switch, conducting when the gate receives a current trigger, and continuing to conduct while the voltage across the device is not reversed (forward-biased).

thyristor1

How to test a Thyristor

This post came about when I was testing a BRC/Thorn PSU module. It uses a Thyristor, should the chopper output voltage rise above 72V (68V in some receivers) caused by a short-circuit chopper transistor or faulty control circuit, then zener W617 conducts and fires the ‘crowbar’ Thyristor W621, thus shorting the 240V line and operating the Cut-Out or blowing the main fuse.

So how to Test?

Using a multimeter is one method as detailed below.

  1. Remove the Thyristor from the chassis
  2. Select low ohms range on your test meter
  3. Connect the negative lead of your meter to the Anode
  4. Connect the positive lead of your meter to the Cathode
  5. The meter should read o/c
  6. Short the Gate to the Anode on the Thyristor
  7. The meter should now read in the region of 20R for a good Thyristor
Alternative Test

Using a home-brew Thyristor tester using a few resistors, a couple of micro switches, an LED and a 9V battery. I picked up such a device at a local Rally for a few pence and you could build one for the same.

You connect the Thyristor up to the terminals then operate the switches, this allows you to operate the latching of the Thyristor (on & off) via the switches.

thytesterThe same can be achieved using a bench PSU with current limit on. In my opinion this can give better results for higher voltage Thyristors.

thyris3

Example

I had a tripping PSU on a 3000 It was W621 a Thyristor at fault.

The Thyristor in the BRC/Thorn is a 72V package when removed and tested, the thyristor latched and unlatched quite happily at 9V or under. However when the Bench PSU was employed and taken to 30V it would latch for a second then fail to remain so. Intermittently I could get it to latch and stay latched. Whereas when I was just feeding it lower voltages it would latch every single time, so it was breaking down on higher loads.

2 thoughts on “Test A Thyristor

  1. Hello Chris, sorry if i am mistaken, but i am sure that the longer *leg* on a BT106 is the cathode, not the short one as shown in the above picture. Thank you for the excellent site, i really enjoy reading the articles.
    Best wishes David

  2. Hi David,

    You’re not mistaken and I have amended the graphic accordingly. It helps to have people spotting mistakes, proof reading on your own often allows errors to slip through no matter how many times one reads it.

    Nice to see you here and hopefully see you on the e-ctv forum

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