Velonex 345 high-power pulse generator teardown

[D Straney]

Now here's an interesting one - I actually took these a while back when the piece of equipment in question was going to be scrapped soon (which then didn't happen).  Apologies for the worse-than-usual photography.

Seems this instrument is all about generating short high-power pulses. You can set an amplitude, width, and repetition frequency; installing different output modules will then adapt the pulses to different rise/fall times or special shapes through passive pulse-shaping networks.

This is useful for...plasmas? Radar? Lasers?

Undoing some screws lets the main chassis slide out of the housing.  Let's start with the section of the front panel behind the pulse width and repetition frequency knobs:

This board seems the most straightforward.  I'd guess it's just an oscillator with indepenedent frequency and on-time control, which generates a gating signal to be used by the rest of the instrument.  Check out the triple-ganged rotary switches though:


As for the rest of it, here's the top view:

...and the bottom view:

We've got a couple large transformers, a capacitor bank, some mystery stuff, and a PCB.  There's also a variac behind the amplitude control knob!

I think the capacitor bank visible from the bottom is the source of the output pulse energy, and the amplitude setting takes a 70's-style direct and simple approach by letting the amplitude control directly set a turns ratio to scale the incoming AC power.  Given that the voltages are high (the meter reads up to 1 kV), the two fixed transformers visible from the bottom probably post-scale the variac voltage before it gets rectified and used to charge the capacitor bank.

The cap bank has 6x Mallory 480uF 450V caps; I would've assumed they were connected in 2x parallel strings of 3-in-series for 320uF @ < 1350V, but the busbar/discharge resistor connections on top seem to show something like 1+1+((1+1)||(1+1)), which would make it a 119uF @ < 1800V string.

I guess the "abs max" position on the meter is more like 1.2 kV, and the +/- 20% tolerance swings on these caps really kill the voltage sharing worst-case.
There's also a smaller orange cap next to the cap bank that shares at least one terminal, so I'd assume this is a ceramic connected in parallel for some lower-ESR action:


The board on top has what may be charging control (for the cap bank, with the power relays) and gate triggering circuitry for what I assume is the troll SCR that lives under the bridge weird FR-4 overhang.


Even in person, I don't think I was able to get the part number off that heatsink.  Also note the box made of glued-together pieces of FR-4 just behind the probable-SCR.  Don't let EEs do your mechanical designs, kids, or it'll be fiberglass as far as the eye can see.  The high-voltage rectifiers probably live behind there but I don't remember being able to get it off non-destructively.

The output path, as seen in that last photo, runs from (I assume) the cap bank to the probable-SCR to the big red screw terminal for the output unit.  It also connects to that ladder of resistors that can be seen at far-right, which seem to connect to the chassis at the other end; these probably are there to do a safety-discharge of any capacitance in the output unit when idle.  Honestly, for all the large terminals and wide flat busbars that the output units have...

...I'm surprised at all the tiny little thin-gauge wires that carry the output pulse around inside the unit.

[capt bullshot]

Thanks, looks quite interesting.
Wonder if the "supposedly SCR" device that obviously does all the magic within this unit is a SCR or some other fancy high energy switching device?

[D Straney]

I actually opened it up again, since it's still sitting around waiting to be either sold or scrapped.  Turns out it's a triode!.  A powerful one too, it's rated for 2.2kV plate voltage, and 300W of dissipation.  There was "Varian 8875" printed on the hidden side, and a very blurry logo that I now recognize as Eimac's from that datasheet.

Also managed to remove the FR-4 box and there's a power diode under there...

...but it's actually just a 1N3008B 120V zener.  There's a bridge rectifier next to it too, but that's only rated for 600V.  Makes me wonder how it actually manages to get 1.2kVDC on that cap bank.

[T3sl4co1l]

Hah, nice.  That tube isn't good for too many kilowatts peak, so I'd expect it could do, say... EFT-like pulses up to 1kV or so, maybe some higher impedance surges (like the 20Ω telecom surge, 1.5/50us width, at up to mid-100s V), and various ringing waves.

Faster pulses like EFT, assumes adequate drive -- the problem with tubes is the large grid swing required, with fair grid capacitance to boot (~20pF), as well as the high plate impedance (for this case, low ~kV and peak currents in the low ~A).  Makes it difficult to couple the plate into a ~50Ω load, and high impedance transformers are very difficult to make.

What's cool about semiconductors today is, we can make pulses like EFT directly with SiC or GaN power transistors.  SiC is just fast enough, and GaN is dangerously fast.

I would've otherwise figured this thing to have SCRs (as you were expecting), for the slower surges, or a hydrogen thyratron or HV reed relay for EFT and ESD type surges!

Tim

[capt bullshot]

Looks more like this pulse generator is of the HP214B class, but providing somewhat higher voltage. But not surge / burst / EFT generator used for EMI testing. I've seen indeed Schaffner units that use spark gaps / SCR / HV reed contacts as the switching device, depending on the pulse specification.

The HP214B can do 2A into 50 Ohm (using a MOSFET / Pentode cascode circuit), from the triode spec sheet, I'd guess this one can do maybe 5 ... 10A.
Interesting are the different output modules that apparently can do some impedance matching.
https://alphaomegapt.com/pdf%20files/Velonex%20Output%20Units.pdf

[T3sl4co1l]

True, that would explain the timing dials.  The overall construction and modular output look very similar to EMI generators.

Tim

[Hellwin]

I’m looking for output of 4000Vdc with pulse of 0 to 100hz.  High voltage pulse generator.  Any suggestions?

[Hellwin]

Need an output of 4000vdc and power of 20 to 30Watt with pulse of 0 to 100hz.  Any suggestions?