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Active Contributor

Join Date: Jul 2017
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Testing SCR

10/08/2017 5:47 PM

Hello

I have an old dual thyristor module like the one in the picture, it's been sitting in our workshop for ages and now I need to use it.

So i used a DMM to test it, connected the red lead to anode (3), black lead to cathode AK2 (1) and shorted the gate terminal (8) to anode, before shorting, DMM was reading about 700kohm and after shorting it was about 12ohm, so I suppose the scr turned on just fine. But when I remove the short, the scr turns off immediately, shouldn't it stay on until the current passing through it is removed? Repeated this for both thyristors in the module with the same results.

then, I used the circuit shown here to test the module again.

I connected anode A to terminal (3), K to terminal (1) and gate to terminal (8). However, neither thyristors in the module turned on. Although a voltage of 1v was present between G and ground (K) and a current of about 70mA was flowing in this path. This current and voltage value should turn the scr on (min trigger 0.25v 10ma, max trigger 2v 150ma according to datasheet). Note that, i didn't connect the gate voltage between G (8) and (7) which are the trigger circuit terminals because (7) is basically the same as terminal (1) which is already connected to ground.

So, in addition to the question above,

can the gate trigger circuit be separate from the main circuit, for example, can the main circuit be 240v load and the gate circuit is supplied from a 12v dc supply with a limiting resistor like the diagram above? Dc supply with resistor connected between terminal (8) and (7). Note that terminal (7) is already connected to a different voltage reference (240v ac or dc with a separate ground).

according to all this, do you thing my module is functional?

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#1

Re: Testing SCR

10/08/2017 7:06 PM

Should find anything you need to know in here....

http://www.ixys.com/Documents/Selectorguide.pdf

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#2
In reply to #1

Re: Testing SCR

10/08/2017 7:25 PM

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#6
In reply to #2

Re: Testing SCR

10/08/2017 8:16 PM

Nice neatly done project.

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#3

Re: Testing SCR

10/08/2017 7:38 PM

But when I remove the short, the scr turns off immediately, shouldn't it stay on until the current passing through it is removed?

Not necessarily, there is a minimum holding current (150ma) ... maybe more than is provided by your ohmmeter.

Here is the datasheet:

https://www.verical.com/datasheet/ixys-scr-modules-mcc200-18io1-1952510.pdf

I haven't been able to follow the rest of your description, there are 7 terminals, what is terminal 8?

Circuit diagram from datasheet

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Active Contributor

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#4
In reply to #3

Re: Testing SCR

10/08/2017 8:07 PM

Sorry, I meant terminal 6, gate terminal for the first thyristor. Can I edit my post here?

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#7
In reply to #4

Re: Testing SCR

10/08/2017 8:23 PM

Well, it should be fairly straight forward. From the schematic, you basically have 2 SCRs in series.

Your high current terminals are obviously 3 (pos) to 1 (neg) for the first SCR and 1 (pos) to 2 (neg) for the second SCR.

A gate current from 6 (pos) to 7 (neg) should turn on the first SCR allowing current to flow from 3 to 1.

A gate current from 5 (pos) to 4 (neg) should turn on the second SCR allowing current to flow from 1 to 2.

https://www.allaboutcircuits.com/textbook/semiconductors/chpt-7/silicon-controlled-rectifier-scr/

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#5

Re: Testing SCR

10/08/2017 8:08 PM

Sorry guys, I meant terminal 6 not 8. There is no terminal 8.

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#8

Re: Testing SCR

10/10/2017 7:34 AM

I believe you did the test correctly except the use of a DMM.....

You should have used a regular multi-tester like the old "Simpson 260 series" model...

The DMM cannot not provide enough SCR holding current unlike the old Simpson..

that maybe the reason why the SCR did not stay latch to an ON state...

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#9

Re: Testing SCR

10/10/2017 9:13 AM

Reduce R1, use a bigger bulb, use a heavier source with no ripple (car battery) and try the test again after confirming proper hookup.

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#10

Re: Testing SCR

10/11/2017 9:59 AM

On the datasheet [kindly provided by Rixter] Figure 1 & IGT in table....

Line 2 is 150 mA, same as IGT number at 25 degrees.

It is not clear if this is a minimum or a typical value and top of line 2 is 2 volts.

So your 70 mA may not be enough to fire SCR, especially if the 150 mA is a minimum or you are below 25 degrees, or that 2V applies.

Reduce your gate resistor to (12 - 2) volts/ 0.15 amp = 66 ohms.

But nearest standard resistor is 62 ohm - allowing 5% tolerance that is 65 0hmsmax.

If you are using wirewound 3W resistor (you might get 2W if SCR does not fire and button is held down), nearest value is probably 50 ohm.

If your 12V is a variable supply, you can vary trigger current by adjusting supply - it is above the 6V Vd in data sheet.

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#11

Re: Testing SCR

10/12/2017 8:07 AM

In reply to your question about how the SCR may be triggered...

It is usual for SCR of the size you have there, to be triggered via a pulse transformer - the gate/cathode is often at supply voltage rather than common/neutral/earth.

The pulse at the gate may require to be 1 microsecond rise time with 1 amp peak at 3 volts if the SCR is required to work at maximum peak current and rate of rise of current.

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