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| Faster capacitor bank bleeder |
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| grythumn:
I have built a 400V nominal, 500V maximum, 4800uF capacitor bank for a linear accelerator experiment. (Capacitor bank and charger are in a containment vessel, rails and feed wires are shielded; standard precautions.) Right now I'm using a pair of 1M Ohm half-watt resistors as bleeders, but it takes about 3 hours to reach safe handling voltages without shorting the rails, which will really slow down the tuning process. I've thought about using switches to short out sections of a series resistor discharge string as the bank voltage falls to approximate constant power. A string of 4 incandescent light bulbs would also work, I think, but are fragile and fairly bulky. A constant power electronic load would be ideal, but most of the examples I've found are designed for less than 200V, or are expensive desktop instruments. Any suggestions? Thanks! -Bob |
| DBecker:
Have you considered using a standard non-isolated power supply chip, something like a LinkSwitch, to run an incandescent indicator lamp? This will give a constant power discharge -- increasing current as the voltage drops -- to under 50V. Many are rated at 700-800V, since margin is needed for reliability in the face of line voltage surges and inductive kick during plug-in. You'll be a bit close to the margin for production-level reliability, but it should be reliable enough for a one-off build. Edit: There are now industrial-rated LinkSwitch chips with 900V MOSFETs. The examples are all for isolated supplies using transformers, but look in the datasheets for the cheaper chips to see how you can use them in the "white goods" non-isolated configuration with an inexpensive stock inductor. |
| nctnico:
It shouldn't be difficult to use a standard DC load schematic and replace the MOSFET with one which can handle 600V. |
| Sylvi:
Hi Why would use 1M as a bleeder? In tube amps it is more typical to use about 220k to 330k. Use four Rs in series-parallel to have sufficient power rating without violating the voltage rating of standard 1W metal-oxide resistors. If you have caps in series, use the bleeders to assure voltage sharing of the caps. The bleeder Rs should be selected to carry more than the leakage current of the caps - which is what you rely on when no bleeders are present, so a total crap shoot :) You do not need active discharge if the power source is straight-forward. It is easy to come with active discharge circuits using an n-channel mosfet and a pulse circuit for the gate, BUT... It will damage the caps if you try to discharge them too quickly - even poly caps have a maximum current rise/fall rate. |
| coromonadalix:
Same as Sylvi, just use a good resistor value and proper wattage, no need of complicated gimmick To be safe maybe add 25% more wattage than needed. Done that, for an 450v 36000uf capacitor bank, 220k ohms at 20 watts, thats what i had on hand. |
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