Slow cap charging - are crappy diodes to blame?

Alright, I have a high-voltage DC-DC converter circuit made up, and it works sort-of ok... the problem is, it doesn't want to charge photo-flash caps nearly as fast as a proper camera flash circuit actually would.

I don't have a schematic drawn up (although I could probably scribble one out if needed), but basically my setup is this:

An oscillator using a camera flash transformer, which feeds into a voltage doubler, giving me about 580V-ish from a 1.95V input. If I push the input voltage much higher I get very odd things happening so that's as high as I dare to crank it. Current draw from my PSU is just under 40mA. This output is fed into a pair of photo-flash caps in series, which will eventually, after a few minutes, get up to around 500V... but I want to charge them faster, under a minute would be good. And no, I'm not making a coilgun ;)

I'm using 1N4007 diodes all over the place as they're the only ones I have rated for that kind of voltage, but I have read they suck for high-frequency switching... so I'm wondering if I'd see an improvement in performance by using UF4007 diodes instead? Also, what's the best/safest way to dump the cap charge? I have some scavenged horizontal output transistors that seem like they'd be up to the task.

It sounds like your DC-DC voltage converter is lacking in current drive. This is much more likely to be due to not getting sufficient current in the primary coil, or saturation of the transformer core than anything to do with the diodes. Have you measured the voltage output while it is charging? Put in a series resistor to see what current you are drawing during the charge.

what's the best/safest way to dump the cap charge?

Discharge through a resistor, have a time constant of about 5 seconds.

I have some scavenged horizontal output transistors that seem like they'd be up to the task.

These are designed to give a voltage output at not much current so it is unlikely they would be suitable.

Thanks for the reply, Mike - I'll hook it up in a bit and do a couple of measurements. :)

As for the H.O.Ts, I thought that might be the case... I DID mean "safe" for the component doing the discharge though, not safe for me! I need some way to reliably dump that charge into a resistive load (I'm actually making a blasting box, for igniting pyros)... so I want it to be as sudden and violent as possible without obliterating whatever's doing the switching. Do I need to be looking at some sort of gas-discharge tube? Is there anything solid-state and affordable that can do it? I'm quite happy to go and buy a thyratron or something if that's what I need.

I'm also wondering, are these circuits SUPPOSED to make a noise when working? Mine usually doesn't... but still seems to function albeit slowly.

I would look at an SCR for switching the discharge.

are these circuits SUPPOSED to make a noise when working

Most do, a combination of sparks and the electro magnetic pulse which tends to move things slightly.

Ok, well goodness knows what I've done, but after a couple of weeks of the circuit - on a breadboard - sitting around doing sod-all in the Man Drawer, I've just fired it up and not only does it make "the noise" now, it also charges the caps to around 350V (where I turned it off with a "close enough" nod) in maybe 90 seconds or so. Current draw on charging was 85-87mA according to the panel meter on my PSU.

Would it make sense to blame previous crappy performance on horrendously untidy breadboarding and connections of questionable quality? As it seems to more than do the job now and I've done nothing but ignore it for a while!

Would it make sense to blame previous crappy performance on horrendously untidy breadboarding and connections of questionable quality?

Well it would explain it.

I've cracked it! :) I caved in to temptation and bought some UF4007 diodes, installing one in my circuit and stripping the voltage doubler out. Shitty breadboard construction or not, those caps will hit 400V in a matter of seconds now and I get the lovely PWWWeeeeee charging noise (which, because I am a huge manchild, makes me quite happy). The circuit now pulls 120-130mA at 2.5V input, and I think we're ready for some veroboard builds! Thanks for letting me throw ideas around... now all I need is a suitable switch and I'm in business. ]:D

So, how do you now reach 400V after removing the voltage doubler? Pieces are missing in my mental picture.

Well, before, my oscillator/transformer stage was behaving a bit wimpy and only dishing out the juice at around 230-250V. So, I put the doubler in there to get it up nice and high. But now, it seems that I don't need it to do the job. You can see the sort of setup I was using before in this short and slightly childish (but fun to make) video:

http://www.youtube.com/watch?v=Gp60F-GHQnw

It took an ungodly amount of time to reach full charge, only going at a few volts per second ¬___¬