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Trigger problems with Triac and Fet

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OiD

Mon Jun 08 2009, 03:18AM

Registered Member #2150 Joined: Tue Jun 02 2009, 08:33PM
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Posts: 29

Hi all. New account, my fault for not visiting in a loooooooong time :p

I am currently building the trigger for a ringflash I've built, I was hoping to trigger it like I do with all my other flashes, with a BTA10 Triac.
Unfortunately triggering the circuit needs a very low resistance path and the only type of Triac I have doesn't cut it, and neither does a 2SK1511 MosFet (on resistance of 3Ohms).

A relay would work, but unfortunately they would cause an unwanted delay.

Any ideas? Transistor? IGBT?

Thanks for reading ;)

Steve Conner

Mon Jun 08 2009, 09:55AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

What's the difference between this circuit and your other flashes that makes it harder to trigger? Let's work on that instead of just looking for a bigger hammer.

Linear Technology, Fairchild and others make photoflash IGBTs that are capable of switching fast enough to trigger the transformer, as well as carrying and interrupting the full tube current. This means the flash can be turned both on and off by a logic signal. That's a pretty big hammer.

OiD

Mon Jun 08 2009, 01:21PM

Registered Member #2150 Joined: Tue Jun 02 2009, 08:33PM
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Posts: 29

Hi Steve, thanks for replying.

The main difference is that the trigger circuits in this design are tiny.
Most of my other circuits use a larger trigger transformer and a larger trigger cap (about 100nF)

I'm thinking that the problem is that the caps are so small that any losses while switching doesn't allow for a good energy pulse to conduct through the transformer, in turn not exciting the xenon gas in the flash tube.

The circuit is rebuilt from disposable camera parts, so I think that increasing the capacitance could drastically shorten the transformers life.

OiD

Mon Jun 08 2009, 09:57PM

Registered Member #2150 Joined: Tue Jun 02 2009, 08:33PM
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Posts: 29

I've tried a 12v relay at 30v but it is still far to slow as the delay doesn't let me use a shutter speed faster than 1/80sec opposed to 1/320sec when using silicon.

And even the relay has a few miss fires.

I think I'll dig around and see if I can find some larger caps for the trigger circuit.

Steve Conner

Tue Jun 09 2009, 10:32AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Are you using one trigger transformer for each flashtube? If so, try a single larger transformer and connect it to all the trigger electrodes in parallel. You can now hit this bigger transformer with more energy.

Oh and watch out for the double posting rule: no bumping and no double posting inside of 48 hours.

OiD

Tue Jun 09 2009, 12:33PM

Registered Member #2150 Joined: Tue Jun 02 2009, 08:33PM
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Posts: 29

Ah! Sorry! I Simply posted an update without thinking.

Yes, I am using a single transformer for each tube. I haven't thought about doing that. But I guess it should fire them reliably.

I'll give that a try as well.

They seemed to trigger individually quite well, another thought would be individual Triacs or Fets.

Thanks for the idea!

OiD

Fri Jun 12 2009, 04:36AM

Registered Member #2150 Joined: Tue Jun 02 2009, 08:33PM
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Posts: 29

Hmmm. Making a bit of progress...

I'm now using a FET with a gate voltage at 29.8v (max is 30v). Changed out some of the capacitors from 63nF to 150nF but it still doesn't fire them all. Well now at least some are firing.

Could it be that one flash is firing before the other and affecting it some how?

Times like these make me wish for a scope! ;)

I guess I'll change out every single cap, maybe when i trigger the circuit the caps charge each other up instead of going trough the transformer.... Equalizing resistors maybe?

Sorry if I'm not making sense. Off to get some rest.

Dr. Dark Current

Fri Jun 12 2009, 02:11PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

What do you use to pulse the gate? The circuit doing this should have a very fast rise time (few hundreds of ns max.) and should be able to supply a few amps of peak current.

OiD

Sat Jun 13 2009, 03:50AM

Registered Member #2150 Joined: Tue Jun 02 2009, 08:33PM
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Posts: 29

Hi Dr. Kilovolt.

I'm simply switching the power to the gate. Should i try with a gate driver? I have a few from some IGBT.

I have attached a complete circuit drawing of the current state of the circuit.

Ah... I've finally found a datasheet with the FET RDS on value; 3Ohms... pretty high for a 1000v 5A Fet?. Turn on time 120nS.
Sounds like the problem... The circuit wont trigger with a 3Ohm resistor. So much for "Low on resistance". The rest of the family has 0.9Ohms.

I'll keep on trying :)

1244864631 2150 FT70978 Complete Circuit

OiD

Thu Jun 18 2009, 02:18PM

Registered Member #2150 Joined: Tue Jun 02 2009, 08:33PM
Location:
Posts: 29

I got some more FETs from another photocopier machine, and they have a RDS on value of max 1.4 Ohms. I have paralleled the TWO FETs and it works a bit better.

I have a hand full of RDS on FETs with 20mOhms or so but none of them will handle more than 50V.

I think I'll search for a potentiometer and see with what resistance the circuit will fire.

Maybe I'll get started on the inverter.

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