I was trying to replace the contact switch on a camera flash (from a single use camera) with a mosfet (so I could trigger the flash with an electrical signal). The mosfet is rated to about 600V 30A for the drain-source.

The drain is attached to the top bit of the switch (higher voltage), the source to the bottom, which is also attached to the ground of the power supply used to trigger the gate. When I measured there was 200V across the open switch.

For some reason this doesn't want to work, am I missing something basic here?

 

Do you have the gate connected to the source when you make the measurement? You can't leave a MOSFET gate floating.

 

Do you have the gate connected to the source when you make the measurement? You can't leave a MOSFET gate floating.

Yeah the gate is connected to the source across a 10K resistor.

 

Yeah the gate is connected to the source across a 10K resistor.

An image incase it helps

 

Hello,

Wich mosfet do you intend to use?
In your schematic, it looks like a logic level one.

Bertus

 

Hello,

Wich mosfet do you intend to use?
In your schematic, it looks like a logic level one.

Bertus

I'm using an FCP11N60F N channel mosfet. The one in the schematic was just the 1st one I found.

 

Hello,

Looking at the datasheet, the mosfet would need 10 volts to conduct.

Bertus

 

Hello,

Looking at the datasheet, the mosfet would need 10 volts to conduct.

Bertus

Oh ok. I was looking at the gate threshold voltage where it says 3-5V, my bad. I did manage to get it to run an LED with 5V though, I'm guessing it wasn't putting through enough current to run the flash though. I'll try it out tomorrow with 10V and let you know how it goes.

 

Hello,

Looking at the datasheet, the mosfet would need 10 volts to conduct.

Bertus

No luck I'm afraid. First I tried the mosfet across the switch like before (with 10V at the gate) and nothing happened.

Then I manually tied the contact switch together and put the mosfet in series with the flash itself (-ve terminal of the flash into the drain, and the source connected to the point where the -ve flash terminal went back into the board) but this didn't work either.

In both cases the capacitor still had all it's juice left for when I manually discharged the switch. Any ideas?

 

I use the NTE3046 SCR Optoisolator to fire a flash with high voltage trigger circuits. Don't have a schematic handy but it works well and the input is an LED which can be triggered with the voltage you were first trying to use. It also isolates the high voltage from whatever the circuit is you are triggering it with.

This may help, from a previous project..

 

I use the NTE3046 SCR Optoisolator to fire a flash with high voltage trigger circuits. Don't have a schematic handy but it works well and the input is an LED which can be triggered with the voltage you were first trying to use. It also isolates the high voltage from whatever the circuit is you are triggering it with.

This may help, from a previous project..

Thanks allot for the schematic. Just checking are the square waves used both +5V? Also I'm a bit confused what the second square wave (from the ttl strobe trigger) is for if the first one triggers the switch.

 

I did manage to get it to run an LED with 5V though, I'm guessing it wasn't putting through enough current to run the flash though.

Something is goofy here. I can't believe the switch needs to pass more current than the ~10mA required to light the LED. With the MOSFET in place, does the flash fire if you short across the drain and source pins?

 

Something is goofy here. I can't believe the switch needs to pass more current than the ~10mA required to light the LED. With the MOSFET in place, does the flash fire if you short across the drain and source pins?

Yes.

 

And yet, when you pulse 10V (between source and gate) onto the gate pin (which is otherwise pulled down to source by a 10K), it does not fire?

 

Thanks allot for the schematic. Just checking are the square waves used both +5V? Also I'm a bit confused what the second square wave (from the ttl strobe trigger) is for if the first one triggers the switch.

Just showing the polarity of the trigger pulse to help someone understand how it worked back when I drew it.

 

It's alright guys, I was only using the flash to test if I could get the circuit's capacitor to discharge, I gave up on using the contact switch and just directly connected to the capacitor (very stupid of me I should have thought of it earlier) and that worked.