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RE: Bleeder Resistors
Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>
At 07:37 AM 1/4/2006, you wrote:
Original poster: "Lau, Gary" <Gary.Lau@xxxxxx>
I hope your wife hasn't recently taken out a substantial life insurance
policy on you...
1) Let's do the math. Assuming you have a 15/60 NST, peak voltage is
about 21kV. 21kV across 1KOhm will draw 21 Amps. But you have only a
.06A transformer. Nope, no good. I suggest something between 50 and
100 MegOhms. A 50Meg bleeder will draw 0.42mA peak.
2) A dead short will discharge any cap to zero in a matter of
microseconds. All you need to do is ensure that you've made proper
contact with both terminals, and you're done. I figure that I have two
discharge mechanisms in place - the normal path through the NST
secondary and primary coil, and my bleeder resistor. The chances of
either one failing is small but not zero. But the chances of BOTH
mechanisms failing are so remote that I don't use a hot stick.
There's two ways in which hot sticks (or grounding hooks) are used:
1) To discharge the capacitor (in which case a 50-100 ohm power
resistor in series is nice, to limit the peak current. less of a
loud noise, not so hard on the capacitor.. important if you're
discharging a big bank of electrolytics)
2) To keep something at ground potential, even if someone turns on
the power. You attach the grounding hook AND leave it connected
while you're working on the gear. If power gets applied somehow,
hopefully a fuse blows, or something else happens, but at least, the
juice doesn't go through YOU.
3) Industrial equipment often had safety lockout switches that turn
power off during servicing. This prevents accidental turn on by others
without the key. But for our purposes, just pulling the plug out of the
wall is probably the best thing. It's clear when the plug is lying on
the floor, but not so clear which position a lockout switch is in.
That's why the lockout has a key. You take that key and put it in
YOUR pocket, and then YOU know.
Bear in mind it's not unheard of for some idiot to bolt cutter the
lock off the switch, so always check. I was working around one of
those big overhead cranes that run on tracks along a warehouse, and
had locked out the 480V power to the crane (provided through 3
exposed rails running along the walls). Imagine my surprise when the
crane at the other end of the building moved (same rails).
4) I suppose you could, although the hazard of a cap charged to 120-160V
pales in comparison to the tank cap hazard. Also, PFC caps are normally
permanently wired in parallel with the NST primary, which would
immediately discharge the cap when power is off. But then I'm the guy
that disconnected his primary tap and got zapped...
A lot of caps intended for PFC use have built in bleeder resistors.