Wirewound Resistors OK?

[W7SOE]

Hi all,
    I hope the experts here can help with this one.  I am building a PS for an ART-13.  I have a bag full of 220uF 250V caps that I want to use on the HV supply.  I plan on stringing nine i series for 24uF at 2250VDC.  I will put a bleed resistor across each.  For ~100k at ~100W I need ~11k and ~10W resistors on each. 

It is hard to find 10W resistors that are not WW.  Is it OK to use WW resistors in the PS?

Thanks

Rich

[AB2EZ]

Rich

Yes... since the resistors are not being asked to pass RF, the inductance of wirewound resistors is not an issue for this application.

Stu

[KB2WIG]

Yes, I do all the time ( actually not ALL the time).

You may want to parallel  some carbons accross the caps.... One of the failure modes is for the ww to open, thereby loosing the bleeder function. What I do is place some 1 meg, 2w across each of the caps as a backup. For your voltage, they dont dissipate much power ( 20 mW?), and are well within the voltage breakdown raeing..

Do what I did and join the search for Oil....  

I found some 21.4 uF / 5kV oil caps..  no problem

klc

[WA1GFZ]

The failure mode of a ww resistor is open while all others I have tested drop in resistance in the transition between emitting smoke to light.

[AB2EZ]

If one of the resistors opens up, most of the B+ will appear across the associated capacitor... and that will cause that capacitor to explode. Putting high resistance safety resistors in parallel with the bleeder/balancing resistors won't prevent this.

As an aside, make sure that the wirewound resistors are all within a few percent of each other. By using only nine (9) 250 volt capacitors to make a 2250 volt stack... you are assuming that the balancing resistors will keep the voltage across each capacitor to 1/9 of the total.

Example

Suppose you have nine (9) resistors, of which eight are 10 k ohms, and one is 12 k ohms. Then the voltage across the capacitor with the 12k ohm resistor will be 12/ (80 + 12) x 2250 volts = 293 volts.

I would suggest that you build more margin into your plans.

Stu

[W7SOE]

Stu, that is a good point about the extra resistor.  I am not planning of running higher than 1200V so the 2250V already contains the safety margin, unless I am missing something....

Rich

[WQ9E]

Stu is correct, fortunately modern wirewound resistors are quite reliable when operated within their ratings.

If the wire wound resistors are the only bleeder you could use a high resistance metal film or similar resistor in parallel with each wire wound so that the HV will still be bled off if one of the main resistors opens.  If the capacitors are pretty well matched and operating well below the rating of the entire series string the capacitor may not blow immediately and you could easily receive a painful or fatal shock from a supposedly discharged capacitor bank.

It is a good habit to become familiar with the normal rate at which your HV decays which will alert you to a bleeder problem but most importantly ALWAYS pull the plug and ground the HV before going inside.  If you don't want to dedicate a meter to always monitor the HV a neon lamp with the proper dropping resistors will at least let you know that HV is dropping at the usual rate and if the lamp stays illuminated longer after power off than usual then you have a problem.

My wife brought home a book on the Darwin Awards for me to read and one case describes how an individual was working beneath his car but the jack didn't lift it high enough for clearance.  So he lowered the jack and put a car battery under the jack to gain more lift.  He was found smashed under his car lying in battery acid since batteries are not designed to support much weight.  The individual in question was a safety officer for a meat processing plant and he had suffered a broken leg several years previously while working under another car.  Let's not have any AM'ers listed in the book!

Rodger WQ9E

[W7SOE]

The cap PCB.  The power resistors are on the back, spaced out for cooling. 

Rich