how do I figure out which IGBT I need for my camera flash?

Thread Starter

cvangordon

Joined Mar 28, 2016
31
I built a xenon tube flash for my camera that uses a 320v, 220 micro farrad capacitor. Currently it's a simple manual flash that just dumps all the cap energy into the flash tube when it is triggered and then I wait for it to charge back up before I can use the flash again. I want to have options now where I can flick a switch making it only use 1/3rd of the cap's full energy so I can use 3 small flashes before needing to recharge. I know I need an IGBT that pulses the energy from the cap to the tube.

So when I look for an IGBT what info am I looking for from the data sheets? I'm unfamiliar with IGBTs so I know how they work and what they do but how do I find out if it can output the energy level I need and handle the input current from the cap? I know IGBTs can usually have a wide range of output currents so is that achieved from some kind of input current to tell the IGBT how much to output?

Like I said, I'm using a 320v, 220 micro farrad capacitor and want to use 1/3rd of the full energy from that at a time.
 

Sensacell

Joined Jun 19, 2012
2,449
Micro-controller, monstable multivibrator, 555 timer, to name a few.

You need a variable width pulse, from about 50 us to 2000 us.
 

Thread Starter

cvangordon

Joined Mar 28, 2016
31
Micro-controller, monstable multivibrator, 555 timer, to name a few.

You need a variable width pulse, from about 50 us to 2000 us.
So I have since first posting this figured out a few things in regards to my original question. You were right I do need a monostable multivibrator. I know most people are unfamiliar with how flash circuits work so I'll just give a quick explanation as a guide just in case.

So after the capacitor is full the only thing keeping it from discharging into the circuit is the fact that it is an open circuit until the xenon in the flash tube is made conductive by being ionized. There is a small capacitance high voltage capacitor that is charged at the same time as the main cap next to the trigger transformer. When you push the shutter button on your camera it sends a 5v pulse that discharges the trigger cap into the trigger transformer that then takes the trigger capacitor's voltage and bumps it up to about 6000v which ionizes the xenon in the flash tube. As soon as the xenon is conductive the main cap empties into the tube causing the flash. The more capacitance or micro farrads the main cap has, the brighter the flash. This type of circuit always uses all of the energy in the main cap.

So my question is, can a monostable multivibrator circuit be added to the line from the main cap to the flash tube that would only allow part of the energy to be used? Can ICs like the LM555 handle the current from the main cap or are they meant only to drive other parts like an IGBT which CAN handle the current from the cap. Basically I just need to stop the cap from fully discharging. I know it can be done because I've discharged a cap before by shorting the leads and then shorted them again and a little was still left.

I saw a cicuit that was meant for a power LED flash that involved a monostable multivibrator sending a drive pulse to two MOSFETs. Would this work with a xenon tube circuit as well?
 

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Sensacell

Joined Jun 19, 2012
2,449
The idea is to open the IGBT switch in the middle of the flash.

Without the IGBT, the tube gets ionized and remains in conduction until the capacitor voltage drops to some minimum voltage where the tube ceases to conduct.

This process is not instantaneous, by turning off the IGBT mid-flash, you can shorten the discharge by interrupting the current flow.
The flash can be fast enough so that a large portion of the capacitor energy is conserved, to be used for the next flash.

The design trick is to make the IGBT pulse the trigger transformer to start the flash, then rapidly turn it off to quench.
 

Thread Starter

cvangordon

Joined Mar 28, 2016
31
The idea is to open the IGBT switch in the middle of the flash.

Without the IGBT, the tube gets ionized and remains in conduction until the capacitor voltage drops to some minimum voltage where the tube ceases to conduct.

This process is not instantaneous, by turning off the IGBT mid-flash, you can shorten the discharge by interrupting the current flow.
The flash can be fast enough so that a large portion of the capacitor energy is conserved, to be used for the next flash.

The design trick is to make the IGBT pulse the trigger transformer to start the flash, then rapidly turn it off to quench.
OK good to know. Thanks this helps. This is actually what I was beginning to think too.
 
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