Thryistor controlling capacitor discharge

Thread Starter

rjjenkins

Joined Apr 16, 2011
214
Hello

I'm trying to use a thyristor to discharge a capacitor into a coil. This is like a coil gun (but isn't a coil gun). I have 350DV coming from a disposable camera circuit, which charges up a capacitor. I want to be able to press a button and have the capacitor discharge through the coil. Because I'm not too sure about the maximum gate voltage, I've put a voltage divider so that it should not get above about 3V.

The 350V in works OK, and without the thryistor circuit in place, the capacitor will charge. However, when the thyristor is in the circuit, the capacitor will not charge, and the voltage does not go above about 5VView attachment 306815. This is true even the gate is floating, ie. no R1 and R2

What am I doing wrong please?


View attachment 306815
 

Thread Starter

rjjenkins

Joined Apr 16, 2011
214
My bad - I drew the circuit wrong. This is what I have. But even with no resistors in place, the capacitor will not charge

1699205423323.png
 

Ian0

Joined Aug 7, 2020
9,519
1) 1N4001 won't stand 350V. It will break down at 50V.
2) A S4065 needs 50mA gate current 350V across 100k is only 3.5mA.
However, neither of the above would prevent the capacitor from charging, so I concur with @sghioto that the SCR is faulty.
 

Thread Starter

rjjenkins

Joined Apr 16, 2011
214
I don't think the SCR is defective. I can get it to turn on a relay using a 12V PSU.
Appreciate the advice about the diode but since the voltage never goes above 5V, I don't think that can be the issue
 

Thread Starter

rjjenkins

Joined Apr 16, 2011
214
The "switch" is at the moment two pieces of bare wire ...
However maybe there is something wrong with the SCR. I tried this below - with the PSU at about 8V the relay switches on and latches as expected when the gate is triggered. However, at about 12V, the relay comes on spontaneously even with nothing at the gate.

1699208356898.png
 

Ian0

Joined Aug 7, 2020
9,519
The "switch" is at the moment two pieces of bare wire ...
However maybe there is something wrong with the SCR. I tried this below - with the PSU at about 8V the relay switches on and latches as expected when the gate is triggered. However, at about 12V, the relay comes on spontaneously even with nothing at the gate.

View attachment 306824
If you have wired it like that and pressed the button, you will have killed it (due to excessive gate current)
 

Thread Starter

rjjenkins

Joined Apr 16, 2011
214
2k resistor makes it come on at about 13V !
I have to admit I don't really understand SCRs - thought they were triggered by gate current (not voltage like mosfet) so it wouldn't matter if the gate was left to float
 

Ian0

Joined Aug 7, 2020
9,519
2k resistor makes it come on at about 13V !
I have to admit I don't really understand SCRs - thought they were triggered by gate current (not voltage like mosfet) so it wouldn't matter if the gate was left to float
The gate terminal looks like a NPN transistor base. Larger ones have built-in gate-cathode resistors, so they don't float.
Small thyristors have much more sensitive gates, so do need a pull-down resistor.
 

Thread Starter

rjjenkins

Joined Apr 16, 2011
214
OK thanks. This is a pretty hefty SCR (and the only one I have). Think I've reached the end of the road with this - am going to try it with a mosfet next
 

Dodgydave

Joined Jun 22, 2012
11,253
You can test the thyristor using a multimeter on diode function, .
If you measure across the Anode and Cathode ( Anode positive lead Cathode negative lead) it should read open circuit, then temporarily join the gate to the anode and it will read about 700mV, .
 

Thread Starter

rjjenkins

Joined Apr 16, 2011
214
I think the problem was that the diode failed, and effectively shorted, which would explain the capacitor not charging. I tried again, with a new SCR (having ditched the old one), and R1 now at 6k (so gate current should be 60mA),
This time I used 1N4004 rated at 400V
Charged the capacitor up to about 200V, at which point the SCR triggered OK.
However, after this the capacitors would not charge - tested the diode and discovered it had failed. Not sure whether the SCR is intact or not.
Assuming that the diode is the problem, can I wire two or more 1N4004s in series (or parallel?) to make them better able to withstand the back EMF?
 
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