K5GW wrote:
>
>Those big wirewound resistors have a voltage rating that should be
>followed. An over voltage will lead to premature death by way of an
>open turn. It takes time but will happen eventually when you least
>expect it. Since they are in series, only one resistor has to go open
>to create a death dealing scenario.
>
>Over the years, I have seen this happen enough that I will not use wire
>wound resistors for bleeders. Now 10 to 15 series connected 3 to 5 watt
>resistors are used as needed to limit the voltage to 300v or less. This
>also serves well as a hv meter multiplier.
>
The more independent bleeder paths you have, the better - so if one
resistor chain fails, you still have at least one backup to discharge
the capacitors. For that reason I would always build a separate HV meter
multiplier chain.
The main bleeder resistor should be designed to discharge the
capacitor(s) within a reasonable time, but that involves high power
dissipation and high operating temperatures, which affect component
reliability and may significantly change the resistance value. In short:
your main bleeder chain may NOT be outstandingly reliable.
On the other hand, the HV meter multiplier will be more accurate and
reliable if it is designed using higher-value resistors, several in
series and generously rated for both power and voltage. And then, if the
main bleeder fails, you'll still be able to measure the voltage and see
that there has been a fault... and eventually the HV will be discharged
through the multipler chain.
The important thing is NEVER to be stuck with a fully charged capacitor
and no bleeder! (You can always rig up some way to discharge the
capacitor, but those are also the most likely times to make mistakes.)
A couple more "gotchas" about multiplier chains:
1. Don't trust a meter reading of exactly zero - it could mean that the
multiplier chain itself has failed. If you are planning to work on the
HV, then ALWAYS watch the meter reading as you switch off, to confirm
that the HV meter is still telling the truth. Then always use a shorting
stick anyway.
2. Moving-coil meters are made from extremely thin wire. If that coil
fails open-circuit, the "+" terminal of the meter will fly up to your
full HV! Unless of course you prevent that, by using a permanent shunt
resistor and maybe a zener diode.
I always use a complete multiplier chain with the bottom end hard-wired
to chassis. The resistor at the bottom end of the chain is selected to
give a voltage drop of roughly 10V, and then the HV meter is configured
as a 0-10V voltmeter looking at that final resistor.
To cut through all the calculations about resistance values and
component tolerances, use a multi-turn trimpot for the 0-10V multiplier
resistor and calibrate the whole thing accurately against your DMM. If
your DMM doesn't have an accurate high-voltage probe, use a variac to
reduce the HV to a safe value for the DMM (usually about 750V) and then
adjust the trimpot so that the HV meter reads exactly the same as the
DMM.
Accidents mostly don't happen... but these precautions are so simple,
they aren't worth betting your life against.
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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