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Could a phototransistor be used to trigger a somewhat high voltage capacitor discharge? Answered

Or would it require a secondary switching mechanism?  I am trying to build a light trigger circuit that is as simple as possible.  I have an idea to use an ir led across from an ir phototransistor  and have it trigger when the light is blocked.  I need it to run 450v through.  is there a phototransistor that can do this and if not what might be a better idea?

Edit:  This is to trigger a secondary acceleration coil in a coilgun.  I am leery of using a relay for it because i don't know if that will cause too much of a delay.

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lemonieBest Answer (author)2010-04-25

Don't think about running power through these devices.
Hook the electronics to a transitor, have that operate a relay that switches the 450V supply.

L

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finfan7 (author)lemonie2010-04-25

Thanks.  I considered using a relay but i was worried that it might cause too much of a delay.  Also, this is for a coilgun.

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finfan7 (author)lemonie2010-04-25

so would something like this image but with a phototransistor in the place of the switch work? 

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lemonie (author)finfan72010-04-26

Use the light-device as the trigger, discharging the capacitor through the silicon.
Like this image, but S1 is normally closed / not there, the bell is the coil, and the power-supply is the capacitor.


L

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steveastrouk (author)lemonie2010-04-26

Yes, a good basic circuit.

Unfortunately thyristors are tricky to turn OFF on DC supplies, here you have to release the switch to switch off the alarm.

In a flash trigger, the phototransistor would still see 450V BTW

Like I said, Xenon tubes are usually struck with an ignition electrode, which you inject a current from a little pulse transformer into.

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lemonie (author)steveastrouk2010-04-26

He said it was for a coil-gun. If the supply discharges fully it would turn off? Or would you need to momentary-reset before recharging?

L

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steveastrouk (author)lemonie2010-04-26

Yes, it would turn off then. A flash tube would make an awesome high speed trigger though, if the tube current could be low enough.

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lemonie (author)steveastrouk2010-04-26

I can see that, what's the typical tube current (yest I could look but you'll already know)

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steveastrouk (author)lemonie2010-04-26

Its certainly amps. We have just independently reinvented the trigatron which is handy if you have an implosion design for yer war'ed

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lemonie (author)steveastrouk2010-04-26

I like the chain-maille on that... sounds "beefy".

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steveastrouk (author)lemonie2010-04-27

Kilovolts and MegaAmps. Interesting 'ible there.

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finfan7 (author)lemonie2010-04-26

Sorry, I'm back again.  What keeps the coil from firing in the time before the projectile blocks the beam?

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lemonie (author)finfan72010-04-26

You're after a 2-stage acceleration then?
If the light-device triggers the coil discharge, it's down to your design when the beam is blocked or not.
How much research have you done on coil guns so far? People usually construct two coils with an in-built delay I think.

L

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finfan7 (author)lemonie2010-04-26

Thanks.  I'll be trying it out ASAP.

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lemonie (author)finfan72010-04-26

Check the spec' of the device carefully before buying though.

L

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jeff-o (author)2010-04-26

A phototransistor could be used to switch a high voltage MOSFET or SCR.

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steveastrouk (author)2010-04-25

Its not a good idea. The flash tube itself is used as the trigger  - inject a current pulse into the trigger pin and the tube goes off. There are plenty of free circuits for that out there.

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lemonie (author)steveastrouk2010-04-25

That's assuming this is for a flash... it'd help if people gave more details.

L

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