Print

[W0BTU]

I'm looking for a bleeder resistor for a 3500 volt DC power supply / 35 uf. Something in the area of 150k 100 watts.

P= E²/R

What do you have?   TIA.

73 Mike

[AD4U]

FWIW - You probably already know this, but as somebody who has built a number of amps over the years, you really need several bleeder resistors instead of just one.  Putting 3500 volts across just one resistor MAY cause problems.  I would suggest 4 or 5 resistors that when added together will make up the OHMS and WATTAGE you desire.  This will distribute the HV across each resistor so no more than 700 volts or so appears across each.  This may reduce the tendancy of the PS to arc.

But then as I said in the beginning, you probably already know this.

Dick  AD4U

[KH6AQ]

Ohmite L225J100KE, 100k ohms, 225 watts, 4600 volts, $18.64 in stock at DIGIKEY

[KH6AQ]

Ohmite 270 series resistor mounting hardware: http://search.digikey.com/scripts/DkSearch/dksus.dll

Or go to DIGIKEY.COM and search under resistors then accessories, then 270 hardware

[WB2WIK]

Ohmite L225J100KE, 100k ohms, 225 watts, 4600 volts, $18.64 in stock at DIGIKEY

Not a bad choice, although -- ouch! -- that's ~123W of dissipation and heat.  Even though the resistor might take it, seems like a lot to waste and you'd need good ventilation to let all that heat escape efficiently and not start burning up other stuff nearby.

I'd stick with 150K if possible, it's "only" 82W and will still almost completely discharge in 21 seconds (four time constants, t = 5.25s).  In fact, if the user doesn't mind waiting longer for the discharge, I might even go with 200K or so (61W dissipation, will still discharge in ~30s).

[KH6AQ]

100k ohms is the highest value I have found for wirewound power resistors. The 120 watts makes for a nice preload.

These Ohmite seem to be nearly indestructable. I've seen them get so hot that the vitreous enamel formed drips but the resistor kept on resisting.

[KF6QEX]

If you are using a number of caps in series, you don't need one high wattage resistor but one across each cap.
Unless you are using a single 35uF 3-4KV cap

[KH6AQ]

Right you are. I had assumed there was one capacitor in the form of an oil filled paper cap.

If these are electrolytics in series the bleeder should be in the form of a bleeder across each cap. What value? I have never seen a method that everyone agrees on. Based on the capacitor specified leakage current after X hours of use working with caps that have zero leakage current and calculating to keep all caps within the maximum voltage rating one can calculate a fairly stiff and power hungry bleeder. So, most designers use a higher resistance value based on nothing but hope.

If it common to run bleeder resistors at no more than 1/2 their power rating. We want to avoid bleeder resistors opening.

[W0BTU]

If you are using a number of caps in series, you don't need one high wattage resistor but one across each cap.
Unless you are using a single 35uF 3-4KV cap

Using a single 35 uF 5 kV oil-filled capacitor, yes.

Thanks for all the helpful replies.

73 Mike

[W8JI]

If you are using a number of caps in series, you don't need one high wattage resistor but one across each cap.
Unless you are using a single 35uF 3-4KV cap

Using a single 35 uF 5 kV oil-filled capacitor, yes.

Thanks for all the helpful replies.

73 Mike

I stock brand new 50K 100 watt ohmites for a project I do, Mike.

If you can't get some elsewhere then I can let a few go, but I think you can find what you need direcly form a supplier.

[W8JI]

Right you are. I had assumed there was one capacitor in the form of an oil filled paper cap.

If these are electrolytics in series the bleeder should be in the form of a bleeder across each cap. What value? I have never seen a method that everyone agrees on. Based on the capacitor specified leakage current after X hours of use working with caps that have zero leakage current and calculating to keep all caps within the maximum voltage rating one can calculate a fairly stiff and power hungry bleeder. So, most designers use a higher resistance value based on nothing but hope.

If it common to run bleeder resistors at no more than 1/2 their power rating. We want to avoid bleeder resistors opening.

Dave,

I ran those calculation on capacitors we used in an Ameritron amp based on barely making ends meet for stability with varying leakage, and some strange guy out in California started telling people to remove the 50K resistors and "always use a 100K resistors to make capacitors last longer".

[KH6AQ]

Tom, I remember something about that resistor change. All of us where I work never agree on the value of bleeder resistors for series connected Aluminum Electrolytics and Tantalum caps.

[WB2WIK]

Is there some "magic number" for bleeder or equalizing/bleeder resistors?

Damn, I missed a few days in engineering school and must have missed that one.

I thought we calculated this by how long we wanted the discharge to take.  For a "ham" amplifier, I always figured 20-30 seconds to "almost" complete discharge (4-5 time constants) was about right.

[W0BTU]

Ohmite 270 series resistor ... DIGIKEY.COM ...

I've looked at DigiKey over the years, and they never seemed to have what I want. But your suggestion prompted me to look there, and also at Mouser and Newark. I see that products that I need have been added to their inventory. :-)

Mouser had the best selection and lowest prices of bleeder resistors, which were added since the last time I looked. And Newark has some nice, inexpensive panel meters.

Who would have thought? Can't find parts for my amplifier project on eBay or the forums, but I can buy some of what I need both brand new and cheap.

Thanks, WX7G and everyone else for their help and offers.

73 Mike

EDIT: Mouser has 150K resistors, both 100 watts and 225 watts.

[W8JI]

Is there some "magic number" for bleeder or equalizing/bleeder resistors?

Damn, I missed a few days in engineering school and must have missed that one.

I thought we calculated this by how long we wanted the discharge to take.  For a "ham" amplifier, I always figured 20-30 seconds to "almost" complete discharge (4-5 time constants) was about right.

I did it two ways at Ameritron. First I looked at how much swamping current would typically handle imbalance between capacitors. Then I looked at how long it would take to discharge the caps, and made sure that time was reasonable.