[Amps] Bleeder Resistor Inquiry

[Will Matney]

See below;

*********** REPLY SEPARATOR  ***********

On 4/27/05 at 2:26 PM Bill Fuqua wrote:

>Believe or not Resistors have a voltage limit and rating. The standard 2 
>watt carbon film resistor is only rated for 300 or 400 volts DC. They can 
>be pushed higher but not by a lot. The same is true for Carbon Comp. 
>resistors. If you put several thousand volts across a standard carbon film 
>or comp resistor you may get some exciting fireworks even thought you have 
>not exceeded their power ratings. A series string is the best choice for 
>high voltage applications. 

It's the same method used to determine the breakdown voltage in the windings of a transformer. As, there's so many volts per layer of winding. The difference between any two layers in volts is what the thickness of the fish paper is determined by, and not by how many total volts the whole winding is.

They do make special high voltage resistors. 
>They are much longer than ordinary resistors. We have some carbon comp HV 
>resistors that are almost a foot long.

I remember the old focus resistors in TV's. You could see that spiral where they put on the resistant coating down its length. Kind of reminds you of a barber pole. They looked to me like they just covered them with a varnish or something. They were about three inches long or so, and the turns had a decent air gap between each. I think the air gap on any resistor is what determines its voltage rating. The only type that wouldn't apply to would be a carbon comp as the just had a slug of resistance material with leads attached to each end.

>
>73
>Bill wa4lav
>
>
>At 11:11 AM 4/27/2005 -0700, aborg wrote:
>>What about using 10 2 Meg 2 Watt resistors in
>>parallel ?  The math indicates it should work.
>>****************************************************
>>--- Phil Clements <philc at texascellnet.com> wrote:
>> > Your bleeder is too small. All you need a bleeder
>> > for on a C input filter
>> > P.S. is to drain the HV after shut-down. Your
>> > 100,000 ohm resistor is
>> > flowing 160 ma of current, and just heating itself,
>> > your components, and
>> > your ham shack. I would use at least 200 k to 400k
>> > ohms. In all my 4 kv+
>> > power supplies here, I use 4 X 100k ohm @ 100 watt
>> > units in series. (225
>> > watt units will be better) Even these get really
>> > warm, but do the job just
>> > fine. I always double the wattage on the bleeder
>> > resistors as a safety
>> > factor. You don't want one of these babies to go
>> > open on you!
>> >
>> > (((73)))
>> > Phil Clements, K5PC
>> >
>> >
>> > -----Original Message-----
>> > From: amps-bounces at contesting.com
>> > [mailto:amps-bounces at contesting.com] On
>> > Behalf Of aborg
>> > Sent: Sunday, April 24, 2005 7:44 AM
>> > To: Amps Amps
>> > Subject: [Amps] Bleeder Resistor Inquiry
>> >
>> > I am currenty building a HV Power Supply using a
>> > 4800vac pole pig, 40uF 5kv oil filled cap , 10kv PIV
>> > rectifier board, 20 amp variac and a 100k 225watt
>> > bleeder.
>> >
>> > The initial power up started well without the
>> > bleeder
>> > connected to the circuit after 30 min burn-in no
>> > problems. Discharged the circuit and added the 100k
>> > bleeder and 5 mins the bleeder starts to smell and
>> > the
>> > exterior paint starts to bubble. Shut it down and
>> > examined the circuit, all seem accurate.
>> >
>> > Question : Is my bleeder to small ?
>> >
>> > P.S. : The variac was set at 4KV at all times.
>> >
>> > Mike-KK4MS
>> > a_borg at yahoo.com
>> >
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>> >
>>
>>Mike-KK4MS
>>a_borg at yahoo.com
>>
>>
>>
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Best,

Will