Precipitator HV Supply Hacking

[Mark Rowley]

Rex,
I like the 2106 idea. Sounds like a cool project and may give it a try after the first of the year. By then I'll have two 60's operating on different machines.

Mark Rowley

[Alan Sailer]

First I'd like to thank Rex for his terrific write-up on the presipitator supply. It allowed me to repair a blown up 30kV unit.

I would like to say that the "mystery" half bridge driver chip can be replaced with an IRS2153 with no obvious problems. I have not
yet re-attached the unit to the fusor but the voltage does go through it's paces.

Cheers.

[Rex Allers]

Alan,

Thanks for the good words on my shared results from hacking these supplies.

The two mystery chips (markings sanded off) were only mysteries for a while after the hacking began.

In one of my zips for the 60 KV unit there is a file with this:

------
Two chips used on the 60 kV supply

1) 8-pin driver for the switching transistors is an
IR 2153
it is a combined half-bridge driver and is switched by its own
internal oscillator, similar to a 555 timer.

2) 16-pin chip is a microprocessor, seems to be an
STC15W408S (STC = chinese chip mfgr)
a member of the 8051 family
----

So, yep, IR 2153 is a good replacement.

I found there were newer versions of this chip. I'd have to dig into my notes to get the numbers. My thought was that at least one other PN should work but I never tried to prove that.

[Rex Allers]

Oops, I see you used IRS... vs original IR...

Good to know that worked. As I said, I found a few chips that seemed they might work. I'd have to go look at the datasheets again to remember what the differences were.

I'm no power supply design engineer but the function of these supplies seems unusual to me. If I got the circuit right, the voltage adjust pot is controlling the frequency of the 50/50 switching pulses. There is no PWM. If I remember right, lower frequency gives higher voltage output.

That's a key reason why I wanted to try replacing the sw driver chip with one that has direct inputs rather than the internal oscillator. That replaced chip could be driven by an external (off board), more typical, switcher chip with adjustable PWM and separately frequency. -- Another thing on my list of -- not done it yet.

[Alan Sailer]

Rex,

I don't have a complete explanation but I did work a bit with flybacks for an earlier project.
All the flybacks I messed with* had a resonance somewhere in the 10's of kilohertz. The
resonance was due to the parasitic capacitance resonating with the secondary inductance.
At that point the impedance of the transformer is at a minimum and large currents and
voltages are created.

The voltage at this resonace is at a maximum but it is not a good place to operate the transformer.
I killed two by doing this.

The area you want to operate is below the resonance in the area where the transformer is
acting as an inductor ( above resonance it acts capacitive). You can vary the output voltage by
changing frequency which moves you along the slope, changing the output voltage as you do.

I did one circuit that used PWM at a fixed frequency to change the voltage but the range was
not as high as the frequency shifting technique. Maybe 9-12kV.

As a final note none of this operation is true flyback operation (which I don't understand). It apparently
uses a sawtooth drive to ramp up the magnetic energy in the transformer core and then
abruptly release it, causing an output current spike. Which results in a large voltage spike V=L(di/dt).

When I was a kid my Dad showed me how to make a self-oscillating circuit using a relay. When the relay
contacts opened, causing a collapse of the current in the relay coil, you would get a painful shock if you touched
the coil.

Cheers.

* Vintage off Ebay and new from China.