Triggering a thyristor with P channel mosfet

Thread Starter

rjjenkins

Joined Apr 16, 2011
233
Hello

Let me start by saying that despite what this circuit may look like, I am not building a coil gun, or any kind of projectile system. It's for a magic trick, where a magnet, or a ferrous coin, is made to jump "magically".

The schematic is below. There is also a module (not shown) that steps up 12V to 250V DC. A N channel mosfet, U1 is used to charge the capacitor, when 5V is applied to the "trigger input". So long as the trigger is held high, the capacitor charges. When the trigger is taken low, the P channel mosfet, U2, then conducts, and applies 5V to the thyristor gate, and the capacitor discharges through the coil.

It has worked, but now it has stopped working. I find that when the U2 is connected to the SCR gate, the voltage coming from U3 drops to about 2.5V instead of 5V, which suggests that a lot of current is flowing through U2 and the SCR. I don't know why.

The U1 side of things seems to be working, i.e. long as the U2 drain is disconnected, the capacitor will charge and discharge through the SCR.

1714740228846.png
 

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Last edited:

ericgibbs

Joined Jan 29, 2010
19,091
Hi rj,
Have you considered the forward biassed internal diode of U2.?
And the polarisation of U2 wrt the supplies?

E
EG57_ 1699.png
 
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Thread Starter

rjjenkins

Joined Apr 16, 2011
233
No, but I don't quite understand the implication of what you are saying, sorry
At the moment I'm thinking of using the U2 drain to trigger another N channel mosfet which will in turn trigger the SCR, but that seems a bit clunky, and since I don't really know what's going wrong, it would be a bit of a stab in the dark
 

Thread Starter

rjjenkins

Joined Apr 16, 2011
233
Hi rj,
Look at this image.
E
View attachment 321520
I still don't get it. When the gate of U2 is low, the drain is at 5V as I would expect. When the gate is high, the drain is at 0V. (If the drain is not connected to anything) When the drain is connected to the SCR gate, where is the current flowing when the U2 gate is low? Not through the SCR, surely?
 

Thread Starter

rjjenkins

Joined Apr 16, 2011
233
hi rj,
This is an alternative.
E
BTW: Where and how is the 250Vdc supply created.?????
Thanks but how will this work? When the trigger is low, won't the SCR gate be pulled low by the 10k resistor? I need the SCR gate to be high when the trigger is low.

The 250V comes from an off the shelf module that takes a 12V input (from AA batteries in this case) and outputs 250V DC
 

Thread Starter

rjjenkins

Joined Apr 16, 2011
233
OK, so the idea was to have 5V on the source
Then there would be 5V on the drain when the gate is low (and 0V on the drain when the gate is high). Which is what I have - even though the diagram was drawn wrong, the wiring was as described.
Does this make sense?
 

Thread Starter

rjjenkins

Joined Apr 16, 2011
233
Ultimately, the trigger input will be from a remote control unit, which produces 5V on an output pin when a keyfob button is pressed. For testing purposes, I am just using a slide switch to take a raw 5V from U3
 

Thread Starter

rjjenkins

Joined Apr 16, 2011
233
hi,
Pulling the Gate low to 0V, will turn On the PFET, pulling high to +5V will turn it OFF.

E
That's what I want. I basically want U2 to conduct when the trigger input is low, hence causing the SCR to conduct and discharge the capacitor. (The trigger input has two functions - to turn on U1 when the trigger input is high, and allow the capacitor to charge; and then when it trigger input goes low, to turn on U2, and put 5V on the SCR gate and make the capacitor discharge). U2 is meant to act as a simple inverter.

So I've tried something simpler


1714748247988.png
This does what you would expect - when the trigger is high, the output is low (and vice versa). EXCEPT when I connect the output to the SCR gate, then the output goes low even when the trigger is high. So it doesn't cause the SCR to conduct. (The SCR DOES conduct when I just attach its gate to 5V).
 

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sarahMCML

Joined May 11, 2019
417
What happens if you remove U1. Because if the two gates are normally tied together, when the trigger input goes low, U1 turns off, and the lower end of C1 is disconnected from ground, meaning no current through the coil and SCR! It may well complete the circuit by going - 0.6V below ground due to the MOSFET's internal diode, but I'm unsure?
 

Thread Starter

rjjenkins

Joined Apr 16, 2011
233
I tried removing U1 altogether, and the same thing happens (using my new transistor inverter) - as soon as the collector of the transistor is connected to the gate of the SCR, it goes low, regardless of what happens at the transistor gate. If feels as if the SCR is gate is sucking current, which is surely not right at all.
 
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