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Singing Arc - Flyback

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Billybobjoe

Tue May 30 2006, 09:05PM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

Recently I constructed the flyback singing arc circuit pictured here -

. I guess the designer of that website is on this forum too (great site BTW)

Anyway, I go to hook everything up and all I get is one big short and lots of smoke from my power supply transformer. It was a 30 volt, MOT sized tranny that I rewound - I full wave rectified it and smoothed it. I have a 12 volt regulator on the board but I'm not sure if this is necessary. I didn't heat sink it but it cylces on and off from overheating. This seems odd because its a 3 amp one and it doesn't seem like it should get that hot?

Anyway, what could be some likely causes? I'm not too good with electronics yet but I'd really like to get it working. I checked for shorts in the circuit and didn't find any.

Heres a picture of my board - I know you won't really be able to see too much, but it may give you a good idea of the thing and you can see if theres any major mistakes. (Yes I know those pots are huge).

If theres any more info that would be helpful just ask - I can get more pics too. Thanks!

EDY19

Wed May 31 2006, 01:47AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

Is the voltage regulator supplying the flyback or just the control electronics? If it is powering the flyback, there is probably too much current for the regulator to handle. Also, dead shorts can indicate two things- a short on the mosfet itself due to a broken mosfet, or the mosfet being turned on from a gate that is always on, which is due to faulty control electronics. Is the 494 putting out a square wave pulse? you could check this by putting a small speaker from the output to ground. If it clicks once, something isn't connected right, if it makes a buzzing sound, theres a good chance it is working alright.

Billybobjoe

Fri Jun 02 2006, 01:42AM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

The regulator is just for the 12 volt driver - I had it laying around and figured it couldn't hurt.

I'll try to do the speaker test - I just need to find a 30+ volt power supply - but I suppose I could test the 494 with 12 volts. I'll let you know in a minute.

EDIT - Ok, the voltage reg. is getting really hot, but there is no ouput from the 494 (pin 9,10 if I'm correct). So I guess the problem is with the 494?

Desmogod

Fri Jun 02 2006, 02:04AM

Registered Member #139 Joined: Sat Feb 11 2006, 11:01AM
Location: Perth, Western Australia
Posts: 358

If the Vreg is getting really hot, it means that it's supplying too much current.
Check for shorts etc etc.

joshua_

Fri Jun 02 2006, 07:28PM

Registered Member #61 Joined: Thu Feb 09 2006, 05:50AM
Location: Mountain View, CA
Posts: 43

Hmm, I tried making a rudimentary "singing arc" awhile ago by using a PIC18 as my oscillation source. It'd count down to the new year with beeps, then it'd play happy birthday (hopefully), since we have a family friend whose birthday is January 1st. Unfortunately, in the last few hours of debugging it, I shorted out the output of the gate driver and killed it...

I did get it to play a tone on the arc by doing pulse-width modulation. That was sorta neat. Too bad I didn't finish debugging my table read code.

I'll post my source on request...

WRT your problem -- check kfor shorts. Check to make sure that Vout on the regulator isn't 0...

Billybobjoe

Fri Jun 02 2006, 09:19PM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

Alright, I got rid of the volt reg. because it was just causing trouble. The resistance between v+ and ground is something like 250k ohms, so there aren't any physical shorts. BUT, it now draws 7 amps from my 12 volt power supply and one of the MOSFETs (the one circled in the pic) gets really hot (probably shorted).

So I guess that one is bad - maybe I killed it when soldering (Forgot a heatsink)? Should I slap on a new one or does anyone think something caused it to fail and it will kill other MOSFETS?

Fullbridge

BTW, joshua, thats pretty sweet with the countdown - I was hoping for some Led Zeppelin with mine though

One more question - will this still give an output (for the flyback) without any audio input so as to make testing easier?

EDY19

Sat Jun 03 2006, 12:06AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

Im not sure if this might cause the problem, but Im thinking that you might need a pulldown resistor on the output pins of the 494- usually people put a 220 ohm resistor from those two pins to ground. If the mosfet gate didn't get pulled down, this would appear as a short when the thing ran. You might also check the mosfet driver- or replace it just to be sure.

cjk2

Sat Jun 03 2006, 04:11AM

Registered Member #51 Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263

I agree. In this circuit, the tl494 will simply charge the gate of the tranistor and leave it on. This explains the heating and short circuit condition. Check to be sure the transitor is not already dead from overheating, and put a resistor from pins 9 and 10 on the 494 to ground. The value is not critical, anything between 1000 and 200 ohms should work. Hope this helps.

Dr. Dark Current

Sat Jun 03 2006, 07:01AM

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

cjk2 wrote ...

I agree. In this circuit, the tl494 will simply charge the gate of the tranistor and leave it on. This explains the heating and short circuit condition. Check to be sure the transitor is not already dead from overheating, and put a resistor from pins 9 and 10 on the 494 to ground. The value is not critical, anything between 1000 and 200 ohms should work. Hope this helps.

I was driving a power mosfet (3nf gate cap.) using 494 without any pull resistors, and it worked really well and the fet was cold. I also thought that was strange but it worked well.

J.M.

cjk2

Sat Jun 03 2006, 07:23AM

Registered Member #51 Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263

Hmm, interesting. I would have thought that the output stage on the 494 could only source current in that circuit... looking at a data sheet, it does look like when both the emiters (pins 9,10) are tied together, the chip will not sink any current. perhaps it has an internal pull down resistor, or it varies transistor to transistor. I think it still would be a good idea to see if a pull down resistior helps.

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