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Mystery HV Transformer and Frequency Troubles

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dwarnecke11

Thu Apr 29 2010, 03:02AM

Registered Member #2833 Joined: Thu Apr 29 2010, 02:54AM
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Posts: 8

I just picked up this strange high voltage transformer:

Good price and unrectified output are positives, but it has a heavy core and seems to hate running at higher frequencies. I used this simple 555 MOSFET circuit:

Acfjlz

I can run it at very low frequencies (50 Hz or so) and get an impressive 20-25kV out with 12 V in, but this frequency makes thin white crackly arcs and seems to interfere with the 555/MOSFET and my variable power supply. I can turn the frequency up and get purple arcs but the sound is very shrill (several kHz) and the output voltage drops quite a bit. When I increase the frequency near 15 kHz the output dies off and the current draw spikes.

Is it basically impossible to get clean, thick high frequency arcs out of a big heavy transformer like this?

Turkey9

Thu Apr 29 2010, 04:25AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

It looks like an iron core transformer which means it is designed to work at 50 or 60hz ac. The losses in the core at higher frequencies will kill the output. Find a little AC step-down transformer that will give you about 12v and put that into your photocopy one and you'll get great fat arcs. The reason you're not with the 555 circuit is that it puts out a square wave and I doubt that transistor would like running at such a low frequency with that load.

ScotchTapeLord

Thu Apr 29 2010, 04:28AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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Posts: 635

I don't think laminated iron can be used for high frequency. High frequency should use a ferrite core. You should measure the primary inductance to get some sort of idea of what frequency it was designed for.

It seems to be a standard transformer, not a high frequency or air gapped one, so you're lucky that this topology (single switch) is giving you anything. Can you be more specific about this "interference"? It wouldn't be surprising if you're destroying the circuit, as your schematic doesn't indicate any protection at all, i.e. decoupling capacitors, freewheel diodes. Please look them up if you don't know what they are.

I would expect that this pulsed DC, random duty cycle might be saturating your ungapped core...

With a little protection this circuit may be fine for a flyback transformer, which depending on the level of power, may be capable of the thicker arcs you want. You can get a flyback transformer from any old CRT monitor.

dmg

Thu Apr 29 2010, 04:30AM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
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Posts: 627

hello,
If I understood correctly, the transformer you are describing (and the site is) is an iron cored transformer, which are specifically designed to run at lower frequencies, this circuit is more typicly efficient with flyback/kickback transformers, that sport a ferrite core.
this circuit is an inefficient driver for such an iron cored transformer, due to the fact that it was originally designed to drive ferrite transformers. and that the core is experiencing a great deal of losses due to the frequency,
as for the current spikes, that seems to be the core saturating due to the high frequency

about the circuit, you will get decent results if you use this to drive a CRT television transformer and stable arcs can be achived this way.
hope this helps.

cjk2

Thu Apr 29 2010, 04:32AM

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

Possibly stupid question: Won't a transformer tend to saturate if used with a driver like this? Perhaps the driver is set up to provide some time between each pulse to allow the core to reset?

dwarnecke11

Thu Apr 29 2010, 04:47AM

Registered Member #2833 Joined: Thu Apr 29 2010, 02:54AM
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Posts: 8

Thanks - yeah I figured it wouldn't be suited to this circuit. That's good news about it running on standard 60 Hz AC though - it should make for a really easy design for making a plasma globe or other HV AC setups. I've had amazing results using this circuit (with a few modifications) on standard flyback transformers as seen on my video here:

Patrick

Thu Apr 29 2010, 06:38AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

above 500Hz youll hit a wall, it was probly meant for 60hz rectified to several hundered volts to 1kv.

radhoo

Thu Apr 29 2010, 11:11AM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

If you use the 555 to drive a mosfet instead of that bipolar (BD243), then you need to add a mosfet driver, the simplest is a totem pole using two transistors. Without that there are chances (depending on your mosfet type) the mosfet is not fully opened (weak performancE)

You can use this as a reference:

Link:

ScotchTapeLord

Thu Apr 29 2010, 12:00PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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Posts: 635

The 555-timer has usually had enough current-sinking ability for my IRF740 gates. When its output goes low it discharges the gate. For frequencies in the low kHz, a 10k-ohm resistor would also usually suffice for draining a FET gate if the 555 ain't puttin out... or pulling low, in this case.

klugesmith

Thu Apr 29 2010, 02:17PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Laminated iron transformer cores aren't limited to mains frequencies.
They've been used for about 100 years between vacuum-tube amplifiers and loudspeakers.
And are found in galvanically-isolated microphone inputs on professional audio boards.
And in 56K modems (remember them?).
The laminations for audio transformers are commonly thinner than those for mains transformers.
Eddy current core loss is a linear process (does not introduce distortion).
Hysteresis loss is present in ferrites as well as steel cores.

As mentioned previously, the OP talks about a MOSFET but shows none in schematic.

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