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Old 01-24-2012, 01:26 PM   #1
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Default A 555 timer, a mosfet, some inductors and diodes = dead PC fans.

So, over the last couple days I have been again tinkering with my 555 timer based mosfet driver. This time I decided to see what would happen if I attached the drain pin of my mosfet in between a random ferrite donut connected to V+ and a SB560 Schottky diode hooked to a simple 120mm .46 amp rated computer box fan.

I set the frequency of the 555 timer's oscillator to around 25khz, turned down the duty cycle all the way via my voltage divider trimpot, set my resistance to around 18k going into pin 7 and hit the switch.

Amazingly the voltage on my output shot up from the ~9.35V out of my 2.5 amp rated SMPS wall wart to around 10.5v. Adjusting the duty cycle up resulted in a final output voltage of around 22v at which point the fan broke free from what I had propping it up from the force of the air it was moving. When the fan hit it got something on my desk caught up in the blades breaking one off. With the fan stopped the open circuit voltage shot up to around 68 volts.

I quickly proceeded to hit the oh shit cutoff button I rigged on my breadboard since my output caps were only rated at 50v. Luckily nothing blew up. A second attempt at driving another cheap recycled fan resulted in me achieving 30v with no signs of the toroid having reached its saturation point.

Still not satisfied, I fished around in my transformer pile and dug out a small 4 pin transformer maybe around the size of an ATX PSU 5v standby transfomer, just shorter and more rectangular. Realizing exactly the potential these things have from my donut experiences I decided to change my output filtering caps to a couple 200v caps also scavenged out of a PSU. Also, if the output of transformer went high enough I had to take precautions to protect myself from anything blowing up.

The idea was to use the transformer as a flyback. To do this I decided it would be best to rectify the output of the transformer into a capacitor and measure the voltage. I ended up using a BYV26C ultrafast sinterglass avalanche diode with a reverse breakdown voltage of 600v and reverse recovery time of 30ns as my rectifier, a .68uF nonpolarized 400v rated film capacitor as my tank, a 200v rated p6ke200 zener/transient diode connected to the positive pin in parallel with a beefy 180k resistor to safely bleed down the cap upon turning off the power or in the even the output of the transformer went overvoltage.

With my driver again turned all the way down I fired it up. ~750mv. Crap. Ok so I must have hooked it up to the primary... lets try it the other way around. Bingo... 100+ volts. So far so good... I remembered somewhere that SMPS transformers can be like wound and polarity biased so I reversed the output leads at which time I found out exactly what the little beast was capable of... imagine that 200+ volts out of a 9v PSU!
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Old 01-24-2012, 01:37 PM   #2
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Default Re: A 555 timer, a mosfet, some inductors and diodes = dead PC fans.

Been thru that phase myself, but you'll quickly find out that you can't get any significant current out of such a setup, and it won't be as fun anymore. Sure it's good for charging caps but that's about all it can do.

12v fans will work fine up to 26-28 volts.
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Old 01-29-2012, 08:26 AM   #3
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Default Re: A 555 timer, a mosfet, some inductors and diodes = dead PC fans.

With a transformer with enough windings, the right rectifiers, the right filter caps and enough insulation in the right places you can transform volts into kilovolts pretty "easily". Of course, a smoke-generator/light-dimmer is even easier.
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Old 01-29-2012, 10:20 AM   #4
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Default Re: A 555 timer, a mosfet, some inductors and diodes = dead PC fans.

Heheh one step ahead of you. Took an old monitor flyback, wrapped 10 turns around the core for a new primary, turned the juice on and... not much of anything. Reversed wires, getting a nice inch and a half plasma stream coming out the end of the HV lead.

Mosfet is heating up like a sumbitch... had to put a bigger heatsink on it and add a fan.

Gimme less plasma, bigger spark... dunno what I should do though.

Anyways, I love the smell of ozone in the morning.
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Old 01-29-2012, 11:53 AM   #5
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Default Re: A 555 timer, a mosfet, some inductors and diodes = dead PC fans.

To make sparks instead of arcs you would need a voltage multiplier. Unfortunately all modern flybacks already include HV rectifier diodes and it's impossible to take them out, and you need AC to feed a multiplier, so you'd have to wind your own transformer.
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