Weird results from air-core transformer

[Chancy99]

I'm seeing some strange results from an air-core transformer - basically two coils of 30ga wire. Perhaps someone can shed some light ?

Primary coil is 170 turns of 30ga wire that was wound on a 2" dowel. The secondary is 100 turns the same way. The two coils are sitting on the desk, the secondary on top of the primary with about a 1/4" gap.

The primary is driven by an IRF540 - the gate goes via 200ohm resistor to a micro generating a 15kHz 50% duty cycle PWM signal. The Source goes to GND. The coil is across the Drain and VCC. The secondary goes through a bridge rectifier.

I'm seeing about 23Vdc across the secondary. I was expecting around 8Vdc or so. 100/170 * 14 right ? Hrmm. The 23Vdc is what works out from 170/100 * 14, so it's as if the primary and secondary are backwards.

But if I reverse them, put the 100 turns as primary and the 170 turns as secondary, it puts out around 90Vdc.

The other oddity is that I can increase the 14V up to around 30V, and the output only rises from 23V to about 27V or so.

Dean.

 

[Chancy99]

Hey all -

I found another version of the air-core transformer here :

http://www.geocities.com/tjacodesign/propclock/propclock.html

My version looks like this :

http://seawall.areyes.com/Air-core-xformer.pdf

He's got only 20 turns for the primary, I have 170. He has 25 for the secondary, I have 100. Since he's actually stepping up slightly, I'm assuming that the 470uF cap will be soaking up most of the spikes before the 7805 regulator sees them ?

The 4.7nF cap across the primary coil is the same as the one is the propclock project. This is actually the best value, as determined with a capacitor decade box.

Well, been playing with this for a bit. It does work which is cool. But I can't seem to get any decent power through it, which isn't.

For instance, the secondary coil feeds a bridge rectifier, which then feeds an LM1086-3.3 regulator. There's a 220uF cap across the regulator input, and a 1uF cap across the output. I see a nice clean 3.3V output, while the secondary coil sees around 18V AC p-p.

This is unloaded though. As soon as I put a load (several LEDs) across the LM1086, everything drops. The secondary drops to around 5V or so. If I add on a bunch of LEDs, the clean 3.3V drops down to around 1.8V or so - each additional one dims it a bit more.

The bench power supply feeding the PWM circuit at 14V only shows a current draw of around 55mA unleaded, and about 40mA with any number of LEDs on.

Q1 is turning on nice and hard, and the drain waveform looks pretty good. Nothing is getting hot, not the mosfet nor the primary coil.

If I change C1 from 4.7nF up to 1uF, the current drawn according to the bench PS jumps to about 1A. Up to 10uF and it jumps to 1.5A. Things start to get hot.

OK, wait a minute here. Something just went weird. CUrrent draw is now 2A with the 4.7nF cap across the primary. The drain is now showing no waveform at all. The gate has anice clean 14V square-wave from the Q1 driver just fine, but the drain which used to show a nice ramp waveform, now just sits ... Hmmm, just tried another IRF540 - same thing.

What ??