Bleeder Resistor Question

[KE6DF]

I'm working on a 2KV power supply.

For a bleeder resistor, I'm figuring about 100K Ohms which would require about a 40 watt unit minimum.

I have available some 100K 100 Watt resistors.

The problem is, they are the Vishay Dale style of finned metal brass colored units that bolt to a heat sink.

I figure I could bolt them to the aluminum chassis and that would work at the 40W level easily.

But my question is, can they stand 2KV. They look kind of small.

One would think so, because what good would a 100 watt rating be on a 100K resistor if it couldn't stand up to 3KV or so.

Somehow the long, tubular, vitreous enamel Ohmites seem better, but I don't have any of them.

Dave

[WQ9E]

Dave,

If your resistor is a RH or NH series 100 watt then it is rated to withstand 4,500 volts RMS short term so 2,000 volts DC long term should not be a problem. 

Here is the PDF data sheet from Mouser:  http://www.mouser.com/catalog/specsheets/rhnh.pdf

[WA1GFZ]

Go the Vishay web site and look up the part number. You should be able to find the voltage rating.

[KM1H]

Id be careful of 100K, just about all the ones Ive checked at hamfests are open....even the 200W.

And if it opens in use you could get a nasty surprise

Carl

[KE6DF]

I
And if it opens in use you could get a nasty surprise

I plan to have a volt meter on the panel of the supply.

So, I figure the string of resistors for the volt meter works as a backup to the main bleeder.

Also, if the 100K bleeder opens, the voltage will drop more slowly as indicated on the volt meter so I'm hoping I notice that.

You have a good point, though.

Dave

[Opcom]

I got to wonder how they open. Some have very thin coatings, so handling? Burned open? I always check them before paying that's for sure..

[KE6DF]

I got to wonder how they open. Some have very thin coatings, so handling? Burned open? I always check them before paying that's for sure..

Perhaps the Vishay Dale style resistors are more reliable after all -- seems like they would be less likely to damaged by handling, anyway.

[Bill, KD0HG]

You need a secondary backup.

Use a couple of series connected 2-watt carbon jobs. Maybe 470K each. Just remember the max voltage per resistor is 500 volts. Five in series for a 2 KV supply.

[k4kyv]

Id be careful of 100K, just about all the ones Ive checked at hamfests are open....even the 200W.

And if it opens in use you could get a nasty surprise

I had the bleeder in my +2000v plate supply open.  Just noticed it the other day, no telling how long it had been opened.  I decided  to fire up on CW for the first time in many weeks, and noticed in key-up condition the plate voltage jumped up to 2500.  Normally it jumps up to about 2150.  The first thing I suspected was an open bleeder.  Turned the rig off, shorted out the filter cap and felt of the resistors, and sure enough that one was cold to the touch.  Checked it with a meter and it was open.  30K @ 200 watts wirewound.  I have a box of 200w ww resistors.  Checked them, and several, of various resistance values, were open.  I tossed those out. Didn't have another 30k one, but did find a 25k.  It will do the job, and I have another one for a spare, plus I  could parallel a 100k with a 75k to get 30k, and I have several each of those.  The 25K holds the voltage regulation slightly better than the 30k, and I calculated it just draws a few mills more than the 30k, not enough to cause any concern,since everything in that rig runs with plenty of safety margin.

The bleeder resistance in ohms should be no more than 1000 times the idling choke inductance in Henries.  For example, you have a 5/25 Hy swinging choke in the power supply.  The maximum bleeder resistance should be no more than 25k, to prevent a voltage surge under light load.  This is especially important if you use the final on CW, or operate it in slopbucket leenyar service, or use the power supply to run a  class-B modulator with separate supply for the RF final.  In class-B linear or modulator service, the idling current provides some bleeder action as well.

I'm not sure they still make the large 200w wirewound resistors in the 25k-100k range any more.  I couldn't find any listed anywhere a few years ago when I needed one not in my junk collection.  And they tend to go open over time.  I suspect corrosion where the wire connects to the terminals.  Sometimes the wire opens in the middle of the resistor too.

Looks like large wirewound resistors are gradually going the way of high voltage electrolytics and many of the common transmitting tubes, and other "vintage" parts - unobtanium. Something to accumulate and hoard away when you run across them at hamfests and estate sales, hoping enough of them will stay good that you will have one as needed.  Perhaps keeping them stored away in a dry environment will help preserve the shelf life.

[KE6DF]

"I'm not sure they still make the large 200w wirewound resistors in the 25k-100k range any more.  I couldn't find any listed anywhere a few years ago when I needed one not in my junk collection"

My 2012 Allied catalog still lists 225W Ohmite resistors up to 100K Ohms.

[KA2DZT]

I always use two bleeders in series.  Two 50K 100W would work for 2KV.  Add a volt meter on the center between the two bleeders.  It should always read half of the HV.  If the first bleeder opens the meter will read zero.  If the second bleeder opens the meter will read near the full HV.  This is one way to check your bleeders.

Fred

[KE6DF]

"I'm not sure they still make the large 200w wirewound resistors in the 25k-100k range any more.  I couldn't find any listed anywhere a few years ago when I needed one not in my junk collection"

My 2012 Allied catalog still lists 225W Ohmite resistors up to 100K Ohms.

Also, there are new 100K 100W resistors listed on ebay by Chinese sellers. Not sure of the quality, but at least they are still being made.

I agree with the comment about having a backup bleeder. In addition to the voltmeter dropping resistors, I may order a 1Meg or 2Meg high voltage resistor and put it right across the capacitor's terminals.

[W7TFO]

That red job looks more suited to duty as a multiplier for your plate Volts meter.

73DG

[KE6DF]

That red job looks more suited to duty as a multiplier for your plate Volts meter.

73DG

I have a somewhat unusual meter I plan to use. It's a Weston 0 - 3KV round meter. What is unusual is it has a 100 ohms per volt movement. Most high voltage meters are 1000 ohms per volt.

It's a real nice NOS meter.

I have 6 x 50K Ohm 5W precision (1%) resistors I plan to use with this meter.

I may also put a current meter on the supply, but I'm not sure what good it will do as you usually want to know the current of individual components of the TX (e.g., final plate, screen, modulator) not the total output of the supply.

But I have plenty of meters sitting around collecting dust and might as well use them.

Besides, I just like lots of meters -- they look cool. :-)

Dave

[W7TFO]

But I have plenty of meters sitting around collecting dust and might as well use them.

Besides, I just like lots of meters -- they look cool. :-)

Dave

Me too....

[KM1H]

You need a secondary backup.

Good idea

Use a couple of series connected 2-watt carbon jobs.

Bad idea as carbons drift high fairly rapidly under voltage.
Id use 3-5W MOX

Maybe 470K each. Just remember the max voltage per resistor is 500 volts. Five in series for a 2 KV supply.

Way too high, more like 470K total.

Carl

[Opcom]

But I have plenty of meters sitting around collecting dust and might as well use them.

Besides, I just like lots of meters -- they look cool. :-)

Dave

Me too....

I'm going to stay out of this one.

[W2PFY]

I hope your going to use some mercury vapor tubes in there

[KE6DF]

I hope your going to use some mercury vapor tubes in there

I'm toying with using a pair of thyratrons set up so I can switch the HV on and off by biasing the grid.

I probably won't do the full blown phase shift variable voltage circuit -- just providing a negative DC bias to turn the tubes off.

I have a ton of 5557's which are essentially gridded 866a's. And they are MV tubes. 

Since 5557's use the same 4 pin socket as 866s, I can just plug in an 866 (which I don't have many of) or a 3b28 (I have several of those).

Dave

[k4kyv]

I have a somewhat unusual meter I plan to use. It's a Weston 0 - 3KV round meter. What is unusual is it has a 100 ohms per volt movement. Most high voltage meters are 1000 ohms per volt.

I have 6 x 50K Ohm 5W precision (1%) resistors I plan to use with this meter.

Same with my Jewell kilovoltmeters in the HF-300 rig.  The multiplier resistors are brown vitreous enamelled wirewound, 50K each, look like 50w size, installed in a custom shielded metal case made by Jewell. Full scale current for each meter is 10 ma, so the multiplier resistor string serves as a back-up bleeder, and the one on the +HV to the final measures the voltage at the modulated side of the transformer secondary. I use the first resistor on the grounded end to pick off the audio voltage for the trapezoid pattern on the scope. The multiplier string does triple duty.

The open bleeder reminded how essential it is to short the +HV supply before changing coils or otherwise poking round in the high voltage circuitry.

[W2PFY]

I'm toying with using a pair of thyratrons set up so I can switch the HV on and off by biasing the grid.

I have a pair of thyratrons real old buzzard types. They look like the old style 866 yubes. I know they were used in some BC rigs but I don't know what they were doing with them. Some looked just like a 4B32 xenon tube but it had a grid and glowed a  violet color.

Quote from Don:

The bleeder resistance in ohms should be no more than 1000 times the idling choke inductance in Henries.  For example, you have a 5/25 Hy swinging choke in the power supply.  The maximum bleeder resistance should be no more than 25k, to prevent a voltage surge under light load.

Is this a rule of thumb thing Don? Would it apply to three phase? An example would be my Westinghouse MW-2 where it uses just one 4 Henry choke at 4 KV operating voltage. This choke is rated at 4 amps as well. Would that rule mean the resistor would be 4000 ohms? Seems like it would need a high wattage resistor for that voltage. If I remember correctly, the bleeder that's in there are four parallel resistors to give me 35K ohms of resistance. The nice thing about resistors in parallel are obviously, redundancy. 

[k4kyv]

That rule is a calculation for the critical inductance, the borderline inductance that makes the filter act like choke input and not capacitor input.

But this only assures acceptable static regulation; the dynamic regulation may still suck.  Put a scope on the output of the power supply and you may be surprised at how much the voltage bounces around under a varying load, such as with a keyed CW amplifier, class B modulator or slopbucket leenyar.  Using as much capacitance as you can without the need for soft-start (or else adding the complication of soft-start), and as low a bleeder resistance as you can get away with without exceeding the current rating of the transformer, rectifiers and choke, will work to improve the dynamic regulation.

This is why most AM broadcast transmitters use a common power supply for modulator and final.  The final acts like a low resistance bleeder to maintain good regulation for the modulator.  In 24/7 operation, the power wasted in a heavy bleeder resistance costs money; it is better to maintain the load on the power supply with something that serves a useful function, rather than just generating heat.  OTOH, amateur radio operation is inherently low duty cycle (even with the most old-buzzardly transmissions) compared to a BC station, so the cost of a low resistance bleeder with separate modulator/final plate supplies isn't so much a factor.  Besides, in winter, the whole transmitter is 100% efficient, since the waste heat from the rig helps heat the shack and keeps the thermostat from kicking in quite so much.