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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: I feel I must warn you posted by Russ57 on October 20, 2005 at 12:12:00:
This response falls under the dreaded "Just say no, but if you don't say no, at least use some protection" type lecture.You can make it a little safer with a faster discharge, but the circuit would still be dangerous to have around. Here's the untested concept:
Place a >5 amp >600V diode bridge across the capacitor bank side of the main fuse. The AC markings go from hot to neutral.
The + side of the diode bridge feeds a 75 ohm 250W power resistor in series with a 600V FET or IGBT.
The - side of the bridge goes to the source of the FET.
The gate of the FET is biased on by a 220K 1/2W resistor going to the + side of the bridge. A 15V zener gate-source on the FET keeps the FET from popping. A small ceramic cap is also placed across the gate-source of the FET.
In series with the "15A" fuse, you place a current transformer with a center tapped secondary (200:1 CT). Across the ends of the secondary is a 7V bi-directional >1400W pk transorb. Across the transorb is a 510 ohm resistor. The transorb and resistor limit the voltage out of the current sense transformer. The current sense CT goes to the source of the FET. Each leg of the two secondary leads feeds the base of a 2n2222 through a 330 ohm resistor in series with 0.5 A diode. The emitter of the 2n2222 goes to the source of the FET and the collector to the gate of the FET.
The purpose of the 2n2222 is to discharge the gate of the FET twice each line cycle in order to keep the FET off while power is applied. Once the ac current that feeds the 180 uF cap stops, the cap on the gate of the FET charges up, the FET turns on and the 75 ohm 250W resistor bleeds the cap down from 187V peak to <40V in less than 22 msec not counting the 8.33 msec variation of where in the line cycle the power got pulled.
Tweak the size of the cap on the gate of the FET so that at low line, the FET stays off. I'd add a little hysteresis in the gate drive. The hysteresis circuit is a bear to describe and I don't want to draw and draw a circuit for it.
I'd use two of these circuits across the cap bank for redundancy.
A fuse in series with each cap and two bleeders across each cap would help a bit more with safety; however, it is still a dangerous circuit to have around the house.
Play safe and play longer! Don't be an "OUCH!" casualty.
Unplug it, discharge it and measure it (twice) before you touch it.. . .Oh!. . .Remember: Modifying things voids their warrantee.
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Follow Ups
- You can make it a little safer - VoltSecond 22:03:21 10/22/05 (0)
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