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Help needed with simple HV power supply

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philstesla

Tue Sept 18 2007, 08:24PM

Registered Member #990 Joined: Sat Sept 08 2007, 05:44PM
Location: North Staffordshire, UK
Posts: 7

Hi. I'm trying to build a simple HV power supply based on the flyback transformer from an old monitor. I've looked at a number of circuit designs on the web and cobbled this together from bits out of my parts drawers. Rather than take a chance on finding an appropriate primary, I wound my own with 14 turns of fairly thick wire. The circuit runs (no smoke!) and the transformer makes the usual whistling sound that I'd expect to hear. I've adjusted the 555 to run at about 15kHz according to my 'scope, but there's not even the slightest hint of a spark from the HT lead, even with the room lights turned down low.

I realise using a power darlington isn't exactly conventional and most people seem to be using FETs, but I can't see why it wouldn't work. Is this at the root of the problem though? Maybe I've just missed something really obvious, but I've spent a few evenings fiddling with it, without any joy. Any comments gratefully received.

Dr. Dark Current

Tue Sept 18 2007, 08:41PM

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

I'd replace your 18k resistor with a 1k one (and use bigger timing cap), because now you are running too high duty cycle.

One obvious fault in your circuit: Remove the diode D1!! It blocks the HV inductive spike which is actually what creates the high voltage in the secondary! (your switching transistor must be rated accordingly, for 12V I'd suggest at least 100V c-e or d-s rating).
If you still get nearly nothing out, switch polarity of primary winding.

Darlington should work but even a cheap FET will work much better.

Bauerb2

Tue Sept 18 2007, 08:55PM

Registered Member #973 Joined: Tue Aug 28 2007, 07:32PM
Location: Pittsburgh, PA
Posts: 66

i agree, i started out with this almost exact schem (555 driven flyback). i ran in to problems with the diode as well and noticed a great performance increase when i removed it. also, as jamrtis said, a fet will perform soooo much better than a darlington or single transistor.

anyway.. i found using a lower number of primary turns helped a great deal with the output. (ive gone down to about 3 turns once). try decreasing your turns one-by-one. be careful though, cause if your secondary voltage gets high enough, it'll want to arc to the primary side and into you 555 circuitry (ive lost so many 555 years ago when i worked on this). but i think youll be fine.

Spedy

Tue Sept 18 2007, 11:24PM

Registered Member #964 Joined: Wed Aug 22 2007, 12:39AM
Location: Stockton, CA
Posts: 134

I have absolutely nothing to say to help with this, But I'm a little confused about the schematic. Shouldn't the secondary be bigger/have more turns? It seems to me the way it has it in the schematic will still make the HV pulses, but shouldn't the output have more windings?

CT2

Tue Sept 18 2007, 11:29PM

Registered Member #180 Joined: Thu Feb 16 2006, 02:12AM
Location: Ontario, Canada
Posts: 187

The schematic representation of the transformer doesn't really have to show the secondary being bigger, it's known and implied that when using a flyback the secondary has more turns.

philstesla

Wed Sept 19 2007, 08:38PM

Registered Member #990 Joined: Sat Sept 08 2007, 05:44PM
Location: North Staffordshire, UK
Posts: 7

Thanks everyone for the feedback. I'll have another go and let you know what happens.

(transformer looks 'wrong' in the schematic because I couldn't find a better symbol in Eagle)

philstesla

Thu Sept 20 2007, 08:02AM

Registered Member #990 Joined: Sat Sept 08 2007, 05:44PM
Location: North Staffordshire, UK
Posts: 7

Okay, I removed the diode straight away, replaced R1 with a 1.2K resistor and finally switched the polarity of the primary and behold...a nice purple arc. At best, it's a little over 1cm long, so roughly 10kV. I'm going to try swapping the darlington for a FET, as experimenting with the number of turns on the primary and hopefully get the output up to 20kV.

Many thanks again for the help. Brilliant forum!

Myke

Thu Sept 20 2007, 10:08PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Was that the starting length? The starting length is determined by the voltage. The voltage is needed to sustain the arc but really it's the current that allows you to stretch it out. I could get about 10 inch (25 or so cm) arcs at 1KV but at a couple amps (this was with two MOTs). Nice to hear you got it working.

Bauerb2

Thu Sept 20 2007, 11:09PM

Registered Member #973 Joined: Tue Aug 28 2007, 07:32PM
Location: Pittsburgh, PA
Posts: 66

yes, it may arc at about 2mm but with enough current the heat and ionized air will create a 'low' resistance path for the arc to travel through causing it to stretch out. When i started with flyback drivers all i got was a series of fast clicking sparks, which was due to the low current (and lower freq) so there was no solid 'arc'.

im glad to see your making progress, id like to hear how the new fet arrangement will perform.

-Andrew

philstesla

Sat Sept 22 2007, 04:13PM

Registered Member #990 Joined: Sat Sept 08 2007, 05:44PM
Location: North Staffordshire, UK
Posts: 7

The arc will start at about 1cm maximum. Once it's running though, I can extend it to about 2cm before it stops. I've tried connecting it up to various components, including a xenon flash tube and a 18w compact fluorescent tube, both of which lit up nicely.

I'll post an update when I've built the FET driver version.

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