Camera Flash Circuit Questions & Comments

[TurboSuper]

Actually I was thinking of something along the lines of a big mofo of a capacitor (You can get real high values at 12V), followed by an SCR.

[mark.f]

Another thing I thought of last night: why not use a a solid state, (transistor based), switching mechanism after the coil? If you're worried about the contacts on your switch not handling the arcing due to the voltage spike upon disconnecting. Of course, you'd have to find a transistor rated high enough for your purposes. It would also give you the ability to use a radio transmitter to control it. You would (somehow) have the receiver remove input current from the transistor to open the circuit to the primary coil.

Just more brainstorming.

[Fnord]

Another thing I thought of last night: why not use a a solid state, (transistor based), switching mechanism after the coil?

I thought that's what people used auto relays for?
Correct me if I'm wrong (again).

[jimmy101]

Markfh11q: Good points. It is the rate of change of current versus time (dV/dt). A car battery can supply hundreds of amps, AA batteries perhaps 10 amps when brand new. The coil will limit the current to whatever the coil needs. One problem with AAs is that you eat'm up pretty fast and 8 of'm is kind of expensive.

You do want the fastest switch possible. A standard mechanical switch is actually fairly fast, especially when opening.

I'm pretty sure that the best spak is obtained when the switch is opened, even though you get a large dV/dt for both opening and closing of the switch. IIRC, in an old style (pre ~1990) car igntion system the spark occurs when the points open, not when they close.

[jimmy101]

You can use a solid state switch, google "coil gun" for a zillion ways to do it. To get it to work you need a pretty beefy solid state switch, a generic transistor or SCR won't handle much current. But it can certainly be done.

One problem with the SCR approach is that you can't turn an SCR off. Once turned on it stays on until both the trigger is removed and the voltage across the it drops to zero. SCRs work great for AC systems but don't work at all for purely DC systems, no way to turn'm back off once they have been turned on.

A standard 120V wall switch or an automotive relay is probably the simplest and cheapest way to do it. There are certainly better ways (SCRs, solid state relays, power transistors...) but they are more complex and require parts that are a bit harder to get and require more knowledge to use.

[mark.f]

The best spark is obtained when the contacts open. Our outboard coil doesn't spark when the contacts are closed at all. Only when they open.

[randas]

ive got this to work pretty good using a coil from a festiva.
the only question is, have any of you gotten the 'charged' led to work on your camera boards? ive experimented with 3 different cameras and not one of them can i get the led to light once the cap is charged.

[mark.f]

The one on my circuit lights up when it's charged. Take note that I didn't have to modify any of the internal circuitry of the camera. I just crimped on Q.D. connectors to the three leads going to the flash bulb and ionization plate.

BTW, jimmy, I found a good solution for the switching system, but if you're using this, you might as well just look for a beefy standard switch, as the contacts in the relay would wear about as fast as the ones in the normal switch, (says they last for about 10,000,000 cycles). DC high Inrush relays, (automotive relay). About 15 bucks. I might have to order one anyway, because if we're replacing the computer in my truck, I'm gonna have to replace the power relay with a newer one. McMaster-Carr page number 865. I searched "solid-state relays" and found some medium-amp rated ones, but I only found these expensive ones on pg. 861.

I guess a relay would be good if you didn't want to wire a high-voltage line outside of the internals of your ignition box, but if I'm building one for myself I'll just use an insulated switch.

[jimmy101]

One solution to a solid state switching setup is to use a "solid state relay". Bascially a big-ass SCR. New ones can be pretty pricey so get one from an overstock place.

For example, Silicon Power Cube, 1000V, 16 Amp (400 Amp surge) SCR and Diode, Part M25/16-1K, $4.50 from Surplus Sales of Nebraska (linky)