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Registered Member #3447
Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97
I've about finished building a current regulated (2 x 0-80mA) 3kW, 20kV power supply to drive a yet-unbuilt CO2 laser head, and I have a quick question about the possibility of devising an active filter capacitor (25kV, 2uF poly) discharging circuit.
I was going to just use a 100M, 10W resistive bleeder, but I'd like to not have that continuous power drain (few watts) during operation.
Does anyone know of a method to begin draining the cap only at switch-off? This is a hobby project, but it isn't "hobby-quality" and all the parts are built into a 16U rack cabinet, so I'm not interested in damp wooden dowels and chicken-stick lever action stuff.
Also, the capacitor is a monster poly (scroll down to big red capacitor picture) and the top connector is just a copper plate with what looks like a connector nub, but the only way I can think to connect to it is with a battery terminal clamp. Has anyone dealt with mounting and connecting one of these big boys?
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837
Switch into a LR series circuit (critically or over-damped, if for some reason you want to go faster put a flyby diode in parallel) so your switch doesn't arc so bad.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
sngecko wrote ...
This is a hobby project, but it isn't "hobby-quality" and all the parts are built into a 16U rack cabinet, so I'm not interested in damp wooden dowels and chicken-stick lever action stuff.
Registered Member #3447
Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97
Once again, thanks Proud Mary. I guess I just needed someone to say that it's not a huge deal if it takes an hour to come does to safe voltage, as long I know that. I will do exactly as you suggest.
As for the form factor question, did anyone look at the picture of the capacitor? It stands pretty tall, has a stainless steel bottom (ground) with some tapped holes and a copper top with that nub. It was found in a dye laser, but I didn't get the mounting hardware... Has anyone seen one of these?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
sngecko wrote ...
Once again, thanks Proud Mary. I guess I just needed someone to say that it's not a huge deal if it takes an hour to come does to safe voltage, as long I know that. I will do exactly as you suggest.
As for the form factor question, did anyone look at the picture of the capacitor? It stands pretty tall, has a stainless steel bottom (ground) with some tapped holes and a copper top with that nub. It was found in a dye laser, but I didn't get the mounting hardware... Has anyone seen one of these?
Hot switching at 20kV would involve really costly vacuum or SF6 relays, and even if you had one I'd still advise strapping the hard-wired bleeder across it, lest it fail.
I'd suggest sticking an orange hazard sign on the back of the case saying that it must be disconnected from the supply at the wall for 60 minutes before opening.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Might I suggest here, that, as usual, PM's advice is excellent as a failsafe, but also having a solenoid switched resistor of a lower value that 'fails' in 'safe' mode would significantly decrease discharge time.
If properly designed it would be incapable of 'jamming open'. (and I was answering your original question).
I'm sure everyone here would agree that, when it comes to safety, a backup is 'always' required.
Even with two safety systems, you still want the 'don't open for an hour' sticker, and you want to check that both systems still function on a regular basis.
(Leave it for an hour, then open it and check the function of both systems with the 'main circuit' isolated. This is probably the most sensible post I've made today )
Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
What I have done for my 4KV power supply (used for magnetic impulse experiments) is I placed a standard neon bulb (the ubiquitous NE2) in series with my bleeder resitor. That way, I can visually see that there is lethal voltage in my caps. The lamp will go out when the voltage gets below abour 85 volts (varies slightly with each neon bulb). If you decide to use this scheme, make sure that the bleeder resistor value, and the peak DC voltage on your caps, doesn't excede the current capabilities of the lamp.
Registered Member #3447
Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97
That's a good idea. Maybe I can work that into the instrument panel on the rack. Along with a sign that reads, "When this lamp is on, disassembly will result in a painful death."
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Active Capacitor Discharge Bleeder Circuits
