Developing HV DC Pulses

Sorry about that! Mikek

Corona will be very nonlinear. Crank your Variac down 50% and measure again.

Mikek

Milliamps of corona should be pretty obvious, I'd have thought. Dunno then.

Cheers

Phil Hobbs

And with 1 KV per resistor, 8KV total, there shouldn't be much corona.

It is interesting how the 1 KV increments aren't.

I just ran tests at 8000V, 2000V and 200V output into the divider string. There are 8 resistors, starting at the low end the calculated numbers are 12.5%, 25%, 37.5% of the input voltage. Those three position have the largest errors. The other position are under 2% error (low).

The first position, 32% low (8kV), 32% low (2kV) and 66% low (200V).

The second position, 8.1% low (8kV), 8.9% low (2kV), 24.9% low, (200V).

And the third position, 2.8% low (8kV), 2.1% low (2kV), 6.2% low (200V).

I can't explain the odd numbers AT 200v, but, the others make me think there is a a resistor tolerance adds up to cause the error. If I get a chance, I'll do resistor measurements. Mikek

BITD I did some work on corona point air ionizers for semiconductor clean rooms. Each point ran at about 2 kV, and put out a few microamps.

Should I have a concern about radiation from the 6BK4C tube running at 8kV?

The C is a leaded glass tube, but I still wonder if it should have a cage around it. As I recall the old TVs had a perforated iron cage around them for x-ray protection. I'm thinking about a thin wall steel tube as both x-ray and physical protection of the tube. I'd leave an open area at the bottom for heat flow up through the tube. Mikek P.S. There is some old RCA literature that said during service, replace the 6BK4 with a 6BK4C to reduce radiation exposure.

The resistors are fine , very tight tolerance to each other, the group is 1.6% low. I went through calculating how much the 100MΩ meter loading affected each reading, it's not responsible for much of the error. I'm at a loss. Wasted to much time yesterday trying to figure it out. I will probably revisit it. I have enough parts to start building the pulse amp. Mikek

If the tube acts as a switch, it either has low anode bias (when ON) or zero anode current (when OFF) so no bremstrahllung radiation is expected. Not in the X-ray energy region, at any rate.

Did they have deliberately super-sharp points and small gaps to create high field gradients? A resistor soldered to a pair of banana jacks shouldn't do that.

No.

At 25 KV, maybe so.

They’re pointy, but are kept away from other conductors

Corona is a complicated process, with quite different physics depending on whether the point is positive or negative.

I worked with an outfit that made tomographic atom probe microscopes; we did the electronics. At high fields, like 1e8 v/cm, the field rips atoms out of a sharp metal tip.

Electron emission from a sharp tip or nanotube has been tried too, as a tube cold cathode and as an electron microscope source. But the rip-the-atoms effect dulls the tip pretty soon. Ions are bad news too.

A good field-emission cathode could make a dynamite microwave tube, sort of a PIN structure but the I part being vacuum.

I wonder if the ionizer tips deteriorate.

Focused ion beam mills use liquid gallium tips. The field pulls the metal out into an atomically-sharp tip that renews itself continuously.

Arc lamps rely on the same mechanism. The tungsten and molybdenum electrodes don't melt, but the emitting surface does get hot enough to shape itself into sharp tips under the influences of the local electric field, which is high. The electrodes don't last all that long.

Field emission electron microscope sources don't last all that long either - ones that run warm do last longer.They do need a very high vacuum at the emitter - as Phil says positive ion bombardment, doesn't do them any good at all.

I have now hit a point where I think I have a problem where I am putting one side of the neon sign transformer secondary at ground potential. The scenario is using a sig gen to drive the tube grid, this ties the HV PS at ground (near) potential though the circuit breaker box (sure would be nice to know if the neon transformer could handle that). I have made a drawing showing in red how the neutral/ground bond causes this to happen.

Is there another way around this?

Thanks, Mikek

Does adding an Isolation transformer on the neon sign transformer primary solve the issue? That would be easy and I have several isolation transformers, that will fit in the HV PS enclosure. Thanks, Mikek

The neon sign transformer generates rather high voltages across it's secondaries. Your isolating transformers may not have been constructed to stand off this kind of voltage.

If you were using the neon sign transformer in the job it was designed for, only the ends of it's secondaries would be at a high voltage with respect to ground, but once you start improvising you need a clear picture of which bits are running a long way from ground.

"An isolation transformer" is an unlikely phrase to pull up a safe solution.

The only transformer you are likely to find that has isolation up to the secondary voltage of your neon sign transformer is... the neon sign transformer. Since you WANT DC, and high voltage but low-ish current, the AC HV solution of that neon sign unit is just not a good fit. Consider instead a modest (1 or 2 kV) transformer with a voltage-multiplier output rectifier circuit, diodes and capacitors.

Most laser printers (used to) have something of that sort built in, for drum charging. CCFL circuits used switchmode power with tiny such transformers to make kilovolts of AC, which you can voltage-multiplier rectify.