[Amps] Bleeder Resistor Inquiry

[Will Matney]

Mike,

The resistors model number shows a 225-watt core, 4595 volts. Joe showed that the watt rating was too small as the voltage and current you have will multiply to more than 225, or 314 watts. You'll notcie too that the voltage has been exceeded on this resistor as you should have 5600 Vdc open plate voltage at 56 mA, and maybe average 4950 Vdc under load. Both together would really heat that up if not blow it. What you might look at doing is use two 50K resistors of this size in series so as to divide the voltage and current up across two. You would effectively double the rating this way and have the same value in ohms for the discharge time of 4 seconds.

Saftey here is important as NEVER bet on those resistors to discharge those caps. I made a dischargeer here that has a big power resistor in it (actually 5, 100 watts in paralell). Plus, a switch will cause a relay to short out the bank. On the B+ side is a voltmeter with it's own ground to show if the bank is discharged. What I do is first place the probe on the B+ line and wait a couple of seconds. Then, I hit the pushbutton which shorts the rest to ground. Sometimes, you have to do this 2-3 times to get rid of any residual charge. You could just short them too but I swear I hate that big bang everytime on top of being hard on the components. The thing I made gets you buy with this and those caps are safely discharged before service.

In you case, I'd get two 50k-225 watt resistors and series them, or two 200K and paralell them.

Best,

Will


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

On 4/24/05 at 7:28 AM aborg wrote:

>Joe,
>
>  The bleeder is a Ohmite L225J100K. 
>Thanks too all who have replied, keep the info comming
>I love the info.
>73's
>****************************************************
>--- "Joe Subich, K4IK" <k4ik at subich.com> wrote:
>> 
>> Mike, 
>> 
>> Your bleeder resistor is too small.  
>> 
>> 4000 volt transformer with fullwave bridge should 
>> produce at least 5600 volts no load. 
>> 
>> 5600 volts across 100K -> 56 ma 
>> 
>> 56 ma * 56 ma * 100K = 313.6 watts 
>> 313.6 watts >> 225 watts 
>> 
>> At a minimum, you need either: (1) a pair of those
>> resistors 
>> in series, (2) a 200 K 225 watt bleeder or (3) a
>> 100K 500 watt 
>> bleeder.  Even then, you will be not have a lot of
>> safety 
>> factor and probably need to provide cooling for the
>> bleeder. 
>> Safer yet would be a (series) string of four 50 K
>> 225 watt 
>> bleeders.  
>> 
>> Finally, is the resistor rated for > 5 KV?  Most are
>> not.  
>> 
>> 73, 
>> 
>>    ... Joe, K4IK 
>>  
>> 
>> 
>> 
>> > -----Original Message-----
>> > From: amps-bounces at contesting.com 
>> > [mailto:amps-bounces at contesting.com] On Behalf Of
>> aborg
>> > Sent: Sunday, April 24, 2005 9:44 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
>> > 
>> 
>> 
>
>Mike-KK4MS
>a_borg at yahoo.com
>
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