Bleeder resistor power ratings - UK Vintage Radio Repair and Restoration Discussion Forum

Waler97 · 27th Jul 2015, 9:01 pm

Hi all.

I'm currently in the process of restoring a pair of KW2000s, one being an original and the other being a 2000A. I've decided to start at the power supplies and besides the transformers, diodes and bridge rectifiers, the whole lot needs to come out. Each is packed with bulging caps and high resistors.
Some of these resistors are of course bleeders for the 3 large smoothing caps. Each is the same size and all are quite large, however I'm unsure of the exact power rating. If anybody knows how to work out the power rating required for a bleed resistor I'd be very appreciative so that I can source and order suitable replacements.

Many thanks and 73

Liam Wale

G8HQP Dave · 27th Jul 2015, 9:32 pm

You work out the power in the same way as for any resistor. V^2 /R, and leave some spare to avoid overheating.

GrimJosef · 27th Jul 2015, 10:26 pm

I'm not familiar with the 'KW2000' but it's worth remembering that modern resistors may not be rated to withstand very high voltages. If these are high-voltage supplies then it may be worth checking before you buy.

Cheers,

GJ

Herald1360 · 27th Jul 2015, 10:38 pm

Looking at the PSU, they're not really bleeders, just voltage balancers for the electrolytics. Dissipation is less than half a watt each. 500V rated 2W modern types should be fine.

Vishay PRO2 look the biz at 500V or even PRO3 at 750V for suitable overkill.

G6Tanuki · 28th Jul 2015, 9:25 am

As well as balancing the voltages across the capacitors the resistors do serve as discharge bleeders and should be considered safety-critical components (as anyone who's ever been bitten by a still-charged stack of electrolytics in an apparently 'powered off' piece of equipment can testify).

I like to over-rate these resistors significantly - say 2.5 or 5 Watts. Run them well within their thermal and voltage-ratings. One popular trick was to use one of the bleeder/balancer resistors with a neon in series with its bottom end to act as a "HT On" lamp - you can then watch the neon dim and go out when you switch off the PSU, and wait until it's out before making any adjustments or connecting/disconnecting things.

G8HQP Dave · 28th Jul 2015, 10:32 am

A neon goes out at a voltage which might still make you jump a little. Once the neon goes out you no longer have bleeder action, either. These days an LED might be better.

Skywave · 29th Jul 2015, 1:23 am

If you want a resistor to act as a bleeder to discharge the capacitors when you switch off the mains supply, but at the same time you do not want - or require - that resistor to contribute to the PSU's regulation, you could configure a relay with its contacts connected between the HT line and ground / 0v. and with its coil and contacts so arranged that when the mains goes off, the contacts close and only then is the resistor in circuit.
One advantage of that arrangement is that the power rating of that resistor can be substantially reduced compared to what would be necessary if the resistor was 'in circuit' permanently.

Al.

Skywave · 29th Jul 2015, 1:32 am

Quote:

Originally Posted by GrimJosef

. . . but it's worth remembering that modern resistors may not be rated to withstand very high voltages.

A valid observation and one that is often over-looked.

Connecting several lower-ohmic value resistors in series helps to solve that problem. Also carbon composition Rs are better for that type of task compared to modern helical-cut ones. However, I've often used the old Welwyn green vitreous-coated enamel wire-wound Rs for bleeder resistor duty in HV PSU's without problems arising.

Al.

Radio Wrangler · 29th Jul 2015, 8:09 am

The problem with neons or LEDs in a bleeder chain is that if a resistor fails, or the light emitting component fails, it goes dark, which makes the system look safe under exactly those circumstances when there is no bleed current to bring the voltage down.

I've always fitted bleeders, but then I've always given things a poke with a voltmeter and I eventually learned the value of hooking on a wandering ground lead. I got bitten by some capacitors which came back up enough from dielectric absorption.

David

mole42uk · 29th Jul 2015, 9:08 am

Quote:

Originally Posted by Radio Wrangler

the value of hooking on a wandering ground lead.

Sound enough advice. When I was a working electrician, the second job to do in a 3-phase cabinet is clip the ground leads to the bus bars.

sentinel040 · 29th Jul 2015, 8:48 pm

S I D E

Switch Off
Isolate
Discharge / Dump
Earth

Ian

27th Jul 2015, 9:01 pm	#1
Waler97 Retired Dormant Member Join Date: Apr 2015 Location: Leicestershire, UK. Posts: 14	Bleeder resistor power ratings Hi all. I'm currently in the process of restoring a pair of KW2000s, one being an original and the other being a 2000A. I've decided to start at the power supplies and besides the transformers, diodes and bridge rectifiers, the whole lot needs to come out. Each is packed with bulging caps and high resistors. Some of these resistors are of course bleeders for the 3 large smoothing caps. Each is the same size and all are quite large, however I'm unsure of the exact power rating. If anybody knows how to work out the power rating required for a bleed resistor I'd be very appreciative so that I can source and order suitable replacements. Many thanks and 73 Liam Wale

27th Jul 2015, 9:32 pm	#2
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Bleeder resistor power ratings You work out the power in the same way as for any resistor. V^2 /R, and leave some spare to avoid overheating.

27th Jul 2015, 10:26 pm	#3
GrimJosef Dekatron Join Date: Sep 2007 Location: Oxfordshire, UK. Posts: 4,311	Re: Bleeder resistor power ratings I'm not familiar with the 'KW2000' but it's worth remembering that modern resistors may not be rated to withstand very high voltages. If these are high-voltage supplies then it may be worth checking before you buy. Cheers, GJ __________________ http://www.ampregen.com

27th Jul 2015, 10:38 pm	#4
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: Bleeder resistor power ratings Looking at the PSU, they're not really bleeders, just voltage balancers for the electrolytics. Dissipation is less than half a watt each. 500V rated 2W modern types should be fine. Vishay PRO2 look the biz at 500V or even PRO3 at 750V for suitable overkill. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

28th Jul 2015, 9:25 am	#5
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: Bleeder resistor power ratings As well as balancing the voltages across the capacitors the resistors do serve as discharge bleeders and should be considered safety-critical components (as anyone who's ever been bitten by a still-charged stack of electrolytics in an apparently 'powered off' piece of equipment can testify). I like to over-rate these resistors significantly - say 2.5 or 5 Watts. Run them well within their thermal and voltage-ratings. One popular trick was to use one of the bleeder/balancer resistors with a neon in series with its bottom end to act as a "HT On" lamp - you can then watch the neon dim and go out when you switch off the PSU, and wait until it's out before making any adjustments or connecting/disconnecting things.

28th Jul 2015, 10:32 am	#6
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Bleeder resistor power ratings A neon goes out at a voltage which might still make you jump a little. Once the neon goes out you no longer have bleeder action, either. These days an LED might be better.

29th Jul 2015, 1:23 am	#7
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Bleeder resistor power ratings If you want a resistor to act as a bleeder to discharge the capacitors when you switch off the mains supply, but at the same time you do not want - or require - that resistor to contribute to the PSU's regulation, you could configure a relay with its contacts connected between the HT line and ground / 0v. and with its coil and contacts so arranged that when the mains goes off, the contacts close and only then is the resistor in circuit. One advantage of that arrangement is that the power rating of that resistor can be substantially reduced compared to what would be necessary if the resistor was 'in circuit' permanently. Al.

29th Jul 2015, 8:09 am	#9
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,902	Re: Bleeder resistor power ratings The problem with neons or LEDs in a bleeder chain is that if a resistor fails, or the light emitting component fails, it goes dark, which makes the system look safe under exactly those circumstances when there is no bleed current to bring the voltage down. I've always fitted bleeders, but then I've always given things a poke with a voltmeter and I eventually learned the value of hooking on a wandering ground lead. I got bitten by some capacitors which came back up enough from dielectric absorption. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

29th Jul 2015, 8:48 pm	#11
sentinel040 Hexode Join Date: Feb 2007 Location: Near Lichfield, Staffordshire, UK, most of the time and Crystal Palace, S London, some of the time.. Posts: 330	Re: Bleeder resistor power ratings S I D E Switch Off Isolate Discharge / Dump Earth Ian