New HV stuff, questions
Posted: Sun Sep 19, 2010 1:43 pm
Bill, our master scrounger, just came up with the pictured stuff (among other cool things), and I have some questions for anyone (Cliff? John?) who might know more than diddly about this stuff.
Here's a pic of some of the new goodies.
Here's a detail of a stack disk:
And here's what my sleuthing says is inside one of the disks:
Now for the questions. The seller says each disk is rated at 20kv output -- does this sound reasonable, or can anyone find data on these? Seems Wallis High volt was absorbed by another company that has a not working web page. As these are full wave doublers, that implies I need on the close order of 20kv pk-pk on each drive phase.
The smaller transformer appears to be a Spellman, identical to the one CarlW has been talking about on fusor.net, and mine resonates at 11.5 khz, quite high Q.
The huge Wallis transformer is about 85::1 turns ratio each side, and resonates at 127khz, also high Q. It's all FAT litz wire, even the secondary is fatter than number 20 solid.
Ah, perhaps I got faked by a parastic (leakage L) resonance on the Wallis. It has another resonance at 28khz or so, which acts like a 50::1 turns ratio too.
Starting from here....and assuming I really need 20kv pk pk -- what would y'all do? I can pretty much make anything electronic, no troubles. I see Carl has used an inductor across his transformer to raise the resonant frequency some -- a good idea, but it would seem to have some limits due to increased circulating current. I'd guess you'd do the opposite on that Wallis transformer to lower the resonance some, I kind of doubt the stack diodes are all that fast. (or see above, maybe run it at the lower resonance)
What's everybody's favorite topology for this sort of thing? I can see where the circuit that Carl wound up with is good, and solves the problem of eating the harmonics on the drive waveform so these big resonances don't draw tons of current during switching harmonics, for example -- possibly an added feature of having that extra series resonant circuit in series with the drive?
Having looked inside a couple of Spellmans, I know they like the lm3525 kind of pwm chip (and so do I, I've been using them for other things for a lotta years).
I am not looking for super output power here -- just don't need it, my Spellman SL2KW is way more than enough watts, just not enough volts for some things I want to do here, and sadly, Cliff, I can't afford to buy a higher volt one new right now (maybe next year). I'd have to take my wife to Hawaii to calm her down after that and I can't afford both
It's interesting that these stack disks have a separate cap in them. The fiberglass board sitting on top seems to be a choke with 2 47k resistors and 2 spark gap arresters series-ed across it, which would seem to be intended as an output filter using that extra set of caps. (the spark arrestors appear to have taken some heavy abuse).
I'd prefer to use the Spellman transformer if possible -- it looks better-made, and is obviously a lot smaller, which will simplify packaging -- if I can run it fast enough to get to voltage before I get to saturation. From the size, it will do plenty of watts for my uses.
Here's a dirt simple driver that I like when I can live with its limits....I did up a PCB for this guy and have used it for testing things. Amazingly, the FETs rarely need heatsinking if things are right otherwise. Click the pix as usual to get a larger version you can actually see. I only add some proximate stiff power supply filters and an RC snubber to their example here, and it works great -- you can add an open collector to the RC timing to get current limit/shutdown and that works well too. Of course, you can't change duty cycle, and deadtime is a function of the fet and gate R (it works well if you get the R right), so to change output, you change the input volts. I run mine on a 7815 from a separate floating xfrmr. Hard to beat for simple, but not flexible.
I've used this one with great success to drive a Glassman stack and transformers with, just straight in, no fancy L's and C's outside their little TV flyback-appearing xfrmrs. Those have two primary windings, one on the opposite side of the core, I'm guessing to get some amount of leakage L. They used a half bridge with 6.8 uf in series in their (super overly complicated) circuit.
That thing must have had 40 trimpots! So I ditched it and just kept the "good parts".
Thoughts?
Here's a pic of some of the new goodies.
Here's a detail of a stack disk:
And here's what my sleuthing says is inside one of the disks:
Now for the questions. The seller says each disk is rated at 20kv output -- does this sound reasonable, or can anyone find data on these? Seems Wallis High volt was absorbed by another company that has a not working web page. As these are full wave doublers, that implies I need on the close order of 20kv pk-pk on each drive phase.
The smaller transformer appears to be a Spellman, identical to the one CarlW has been talking about on fusor.net, and mine resonates at 11.5 khz, quite high Q.
The huge Wallis transformer is about 85::1 turns ratio each side, and resonates at 127khz, also high Q. It's all FAT litz wire, even the secondary is fatter than number 20 solid.
Ah, perhaps I got faked by a parastic (leakage L) resonance on the Wallis. It has another resonance at 28khz or so, which acts like a 50::1 turns ratio too.
Starting from here....and assuming I really need 20kv pk pk -- what would y'all do? I can pretty much make anything electronic, no troubles. I see Carl has used an inductor across his transformer to raise the resonant frequency some -- a good idea, but it would seem to have some limits due to increased circulating current. I'd guess you'd do the opposite on that Wallis transformer to lower the resonance some, I kind of doubt the stack diodes are all that fast. (or see above, maybe run it at the lower resonance)
What's everybody's favorite topology for this sort of thing? I can see where the circuit that Carl wound up with is good, and solves the problem of eating the harmonics on the drive waveform so these big resonances don't draw tons of current during switching harmonics, for example -- possibly an added feature of having that extra series resonant circuit in series with the drive?
Having looked inside a couple of Spellmans, I know they like the lm3525 kind of pwm chip (and so do I, I've been using them for other things for a lotta years).
I am not looking for super output power here -- just don't need it, my Spellman SL2KW is way more than enough watts, just not enough volts for some things I want to do here, and sadly, Cliff, I can't afford to buy a higher volt one new right now (maybe next year). I'd have to take my wife to Hawaii to calm her down after that and I can't afford both
It's interesting that these stack disks have a separate cap in them. The fiberglass board sitting on top seems to be a choke with 2 47k resistors and 2 spark gap arresters series-ed across it, which would seem to be intended as an output filter using that extra set of caps. (the spark arrestors appear to have taken some heavy abuse).
I'd prefer to use the Spellman transformer if possible -- it looks better-made, and is obviously a lot smaller, which will simplify packaging -- if I can run it fast enough to get to voltage before I get to saturation. From the size, it will do plenty of watts for my uses.
Here's a dirt simple driver that I like when I can live with its limits....I did up a PCB for this guy and have used it for testing things. Amazingly, the FETs rarely need heatsinking if things are right otherwise. Click the pix as usual to get a larger version you can actually see. I only add some proximate stiff power supply filters and an RC snubber to their example here, and it works great -- you can add an open collector to the RC timing to get current limit/shutdown and that works well too. Of course, you can't change duty cycle, and deadtime is a function of the fet and gate R (it works well if you get the R right), so to change output, you change the input volts. I run mine on a 7815 from a separate floating xfrmr. Hard to beat for simple, but not flexible.
I've used this one with great success to drive a Glassman stack and transformers with, just straight in, no fancy L's and C's outside their little TV flyback-appearing xfrmrs. Those have two primary windings, one on the opposite side of the core, I'm guessing to get some amount of leakage L. They used a half bridge with 6.8 uf in series in their (super overly complicated) circuit.
That thing must have had 40 trimpots! So I ditched it and just kept the "good parts".
Thoughts?