Thanks to all that replied, on and off the list.
It seems that the sum of popular opinion is:
(1) With a C-input filter, bleeders are a safety issue only, otherwise
just generate heat in the room, rather than fire in the wire.
(2) Wire-wounds can be less reliable than film types, even when run
within ratings.
... and based on my own observations after perusing the Mouser website,
(3) Film resistors are commonly available with voltage ratings up to 250V.
So:
(1) If I go with the traditional large, vitreous-enamel wirewounds, I
need to step the power rating up the 50-watt range in order to get a
1625 working volt rating, (per unit) even though 50 watt units would be
gross overkill, physically large and somewhat fragile.
(2) A PC board with a suitably large number of smaller-valued film
resistors might be another way to go. (spaced out for insulation and in
deference to Admiral Hopper)
Thanks again,
Jim N7CXI
On 2/28/2012 2:04 PM, TexasRF@aol.com wrote:
> Jim, T=RC is the equation to discharge 65% of the energy. T is seconds, R
> is ohms and C is farads.
>
> 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.
>
> Yes, it takes a while to bleed the voltage. If in a hurry, the tube idling
> current will bring the voltage down to a few hundred volts in a few
> seconds. Then the shorting stick only causes a modest pop instead a giant
> KAPOW.
>
> Do be careful: there's danger lurking out there!
>
> 73,
> Gerald K5GW
>
>
>
>
>
> In a message dated 2/28/2012 3:49:50 P.M. Central Standard Time,
> audioguy@q.com writes:
>
> [forgot to CC this back to the list]
>
> Thanks, Jim.
>
> I believe your comments about heavier bleeder current being a carry-over
> are probably true.
>
> I'm going to pencil in four 100k, 20-watt resistors with a working
> voltage of something greater than 1KV. I don't know if that would bleed
> 4KV in 32uF off in 2 seconds or less, but it would get there. If I trip
> over any energy storage equations in the next few days I may try to work
> it out just for fun.
>
> Thanks and 73,
> Jim, N7CXI
>
> On 2/28/2012 1:34 PM, Jim Hargrave wrote:
>> Jim,
>>
>> The ARRL handbook has two recommendations.
>>
>> Bleeder resistance that will bleed the HV to 30 volts in 2 seconds after
>> shutdown.
>>
>> Another place it recommends a bleeder resistance that will equate to
> approx
>> 30% of the anticipated max load.
>> The later seems to be a carry over from Tube rectifiers, where the
> bleeder
>> assisted in parallel current regulation.
>>
>> My Clipperton-L has 800k which is distributed across the series filter
>> capacitors. (HV is around 2800)
>>
>> That will give you a range that should fit most "Treasure Box" of parts.
>>
>> 73s de Jim
>> W5IFP
>>
>>
>>
>> > -----Original Message-----
>> > From: amps-bounces@contesting.com
> [mailto:amps-bounces@contesting.com]On
>> > Behalf Of Jim Barber
>> > Sent: Tuesday, February 28, 2012 2:43 PM
>> > To: Amps@contesting.com
>> > Subject: [Amps] Current thinking on HV supply bleeder current?
>> >
>> >
>> >
>> > I have a 3KV, 2A CCS transformer, and 32uf of filter. What's
>> > the current
>> > state of argument for appropriate bleeder current? The last time
> this
>> > came up I seem to remember seeing everything from "HV safety bleed
> off
>> > only" up to a rather high number.
>> >
>> > So... 100 ohms per DC volt? That would work out to a convenient
> 400K,
>> > which could be four 100K in series, each with 1KV or so across
> them.
>> >
>> > Comments?
>> >
>> > Thanks,
>> > Jim N7CXI
>> > _______________________________________________
>> > Amps mailing list
>> > Amps@contesting.com
>> > http://lists.contesting.com/mailman/listinfo/amps
>>
>>
>
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