Bleed Resistor - UK Vintage Radio Repair and Restoration Discussion Forum

Neil Purling · 29th Jul 2014, 8:23 am

On something home-built I want to install a resistor/s so that the smoothing & reservoir capacitors won't still have a charge hanging around to bite you when the equipment is switched off.
Is it practicable to choose a value which will slowly empty the capacitors but not significantly affect the operation of the PSU when in use?

petervk2mlg · 29th Jul 2014, 8:33 am

I just made a 32 volt supply for a farm radio. I put a red LED across the output as a dc power on indicator. It slowly bleeds the voltage away from the output terminals when the unit is turned off.
Peter

Station X · 29th Jul 2014, 8:47 am

A 1M resistor will be fine.

lesmw0sec · 29th Jul 2014, 8:47 am

T=CxR should give you a rough value e.g. 200 mfd/220K should get rid of most of it around 44 seconds. Say your HT in this case was 200V, the resistor would need to be rated at around 1/2 watt and be rated for the voltage. This would draw an insignificant current during operation.

Les.

vidjoman · 29th Jul 2014, 9:04 am

If the voltages are higher make sure the resistor has a voltage rating that is suitable.

G6Tanuki · 29th Jul 2014, 9:07 am

Be aware that a lot of the modern 0.5 and 1-watt metal film resistors are only rated to handle 250V. In a valve radio, particularly if you're using semiconductor diodes or a directly-heated rectifier, the HT line can 'over-volt' at switch-on before the rest of the valves start drawing current.

This could cause your bleeder resistor to fail - therefore lulling you into a false sense of security! So - make sure your 'safety' precaution is suitably rated.

In the past for 250-ish volts of HT I've used a standard 'mains' neon-indicator: these usually have a series resistor of 470K or so which won't load your HT excessively. Using a neon has several advantages:

1: It acts as a 'power-on' indicator.

2: the illumination of the neon serves as an indication that the bleeder resistor is still intact.

3: If the neon's still lit and the power's switched off - Do Not Handle until it goes out.

trobbins · 29th Jul 2014, 9:17 am

Just be aware that a neon-resistor only discharge path will stop acting as a discharge at about 80V, so unless there is some other discharge path then you could have caps at a hazardous voltage level. Of course any kind of valve amp will quickly discharge the HT supply, but if valves are removed then you may well have no discharge path.

GrimJosef · 29th Jul 2014, 9:45 am

Quote:

Originally Posted by trobbins

... any kind of valve amp will quickly discharge the HT supply, but if valves are removed then you may well have no discharge path.

I worked on one recently which had an HT fuse to protect the output transformer, among other things, if one of the output valves should go into current run-away. The fuse was between the psu and the valves so if it blew (which it was doing - that's why the amp was sent to me) it left 800uF at 580V with a 130k bleed resistor. With an RC of more than 100s I treated it with a very great deal of caution.

Cheers,

GJ

trobbins · 29th Jul 2014, 10:20 am

I recall some safety standards that require <60VDC (ie. non hazardous designation) after 60 secs from mains turn off.

Yet another good reason for NOT putting a fuse in the DC HT line!

Skywave · 29th Jul 2014, 11:23 am

Quote:

Originally Posted by G6Tanuki

This could cause your bleeder resistor to fail - therefore lulling you into a false sense of security! So - make sure your 'safety' precaution is suitably rated.

For HT bleeder resistors, I always use high-power wire-wound resistors. Choose the wattage-rating so that they run cool to warm.

Al.

julie_m · 29th Jul 2014, 2:34 pm

Actually, after (R * C) seconds, the voltage on the capacitor will have decayed to 1 / 2.71828 of its original value. Which might possibly still be dangerous.

Bearing GJ's experience in mind, make sure the bleeder resistor is situated upstream of any fuse in the HT line.

Tyso_Bl · 29th Jul 2014, 3:03 pm

I'm not having a go at anyone, but why? Is the equipment built in a fashion that leave HV points exposed? or does the OP have some unsafe workshop practices.

I mention this because bleeder resistors should not be used as a way of compensating for either of the above, enough of us are dying out by natural causes without adding accidental ones.

Station X · 29th Jul 2014, 3:37 pm

If people died as a result of receiving a shock from an HT line, I'm sure that I and many other forum members would be dead by now.

Adding a bleeder resistor reduces the risk of receiving an HT shock after a set is switched off.

If you're working on a set which is uncased and powered up, then inevitably points carrying mains, HT or EHT are going to be exposed.

I cannot see that a bleeder resistor in a domestic set is present other than for safety reasons.

Skywave · 29th Jul 2014, 4:21 pm

Well said, Graham! I shall also add that if a HV PSU uses a choke input filter and if that choke is a swinging choke, then the bleeder resistor is mandatory and should be of the correct value.

Al.

29th Jul 2014, 9:49 pm

I just hang an AVO (a proper moving coil jobbie) on the HT, sometimes it takes a while to discharge but a darn sight less hassle than getting a shock. And you know when it is discharged.

Herald1360 · 30th Jul 2014, 12:56 pm

Quote:

Originally Posted by ajs_derby;696251
Bearing GJ's experience in mind, make sure the bleeder resistor is situated [i

upstream[/i] of any fuse in the HT line.

That wouldn't help if the capacitor were downstream. Best place for a bleeder is directly across the capacitor it's bleeding at any rate electrically if not physically.

30th Jul 2014, 1:22 pm

Quote:

Best place for a bleeder is directly across the capacitor

Yes indeed, still check with a meter, last ditch also short out the cap just in case. With household electrics I short all the leads together, again, just in case. What's worse, a bang or a fatal shock?

GrimJosef · 30th Jul 2014, 1:26 pm

In my case the fuse didn't isolate the bleeder resistor. It just isolated the output valves. I mentioned it in response to trobbins advice that the output valves in a valve amp generally stay hot enough for long enough to act as much more effective bleeders than the bleeder resistors themselves. The problem in my case was that the reservoir capacitors were such high-value ones that they held a lot of energy for a very long time.

In response to Tyso Bl I'd say that it's common for a service engineer to find HV points exposed if he's working deep inside a high power valve amp. And as for 'unsafe workshop practices' I think that if we could rely on people never ever to make a mistake then we'd hardly have a need for any other safety systems at all

.

Cheers,

GJ

turretslug · 30th Jul 2014, 2:31 pm

Beware of the phenomenon of dielectric stress if you directly short a capacitor hoping to discharge it- having removed the short, the cap may bite you later by way of retaliation! Best arrange a suitable resistive leak whilst working on the kit- eg. the aforementioned meter.

Stylo N M · 30th Jul 2014, 10:29 pm

Hi,

If you are concerned at all about any subsequent charge, why not just have a resistor say about 5k6 at a reasonable wattage wired permanently to a couple of crocodile clips on the work bench and then you can connect it and disconect it to any capacitor if it's just simple valve radio work, leave for half an hour and all should be OK.

Chris is right the best place for a permenent bleed resistor is directly accross the capacitor (in most cases)

Paul.

29th Jul 2014, 8:23 am	#1
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Bleed Resistor On something home-built I want to install a resistor/s so that the smoothing & reservoir capacitors won't still have a charge hanging around to bite you when the equipment is switched off. Is it practicable to choose a value which will slowly empty the capacitors but not significantly affect the operation of the PSU when in use?

29th Jul 2014, 8:33 am	#2
petervk2mlg Heptode Join Date: Jan 2007 Location: Parkes, NSW Australia Posts: 877	Re: Bleed Resistor I just made a 32 volt supply for a farm radio. I put a red LED across the output as a dc power on indicator. It slowly bleeds the voltage away from the output terminals when the unit is turned off. Peter Last edited by petervk2mlg; 29th Jul 2014 at 8:34 am. Reason: Correct typo

29th Jul 2014, 8:47 am	#3
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Bleed Resistor A 1M resistor will be fine. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

29th Jul 2014, 8:47 am	#4
lesmw0sec Octode Join Date: Jul 2009 Location: Carmel, Llannerchymedd, Anglesey, UK. Posts: 1,507	Re: Bleed Resistor T=CxR should give you a rough value e.g. 200 mfd/220K should get rid of most of it around 44 seconds. Say your HT in this case was 200V, the resistor would need to be rated at around 1/2 watt and be rated for the voltage. This would draw an insignificant current during operation. Les.

29th Jul 2014, 9:04 am	#5
vidjoman Dekatron Join Date: Jun 2013 Location: East Sussex, UK. Posts: 3,326	Re: Bleed Resistor If the voltages are higher make sure the resistor has a voltage rating that is suitable.

29th Jul 2014, 9:07 am	#6
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: Bleed Resistor Be aware that a lot of the modern 0.5 and 1-watt metal film resistors are only rated to handle 250V. In a valve radio, particularly if you're using semiconductor diodes or a directly-heated rectifier, the HT line can 'over-volt' at switch-on before the rest of the valves start drawing current. This could cause your bleeder resistor to fail - therefore lulling you into a false sense of security! So - make sure your 'safety' precaution is suitably rated. In the past for 250-ish volts of HT I've used a standard 'mains' neon-indicator: these usually have a series resistor of 470K or so which won't load your HT excessively. Using a neon has several advantages: 1: It acts as a 'power-on' indicator. 2: the illumination of the neon serves as an indication that the bleeder resistor is still intact. 3: If the neon's still lit and the power's switched off - Do Not Handle until it goes out.

29th Jul 2014, 9:17 am	#7
trobbins Heptode Join Date: Oct 2009 Location: Melbourne Australia Posts: 901	Re: Bleed Resistor Just be aware that a neon-resistor only discharge path will stop acting as a discharge at about 80V, so unless there is some other discharge path then you could have caps at a hazardous voltage level. Of course any kind of valve amp will quickly discharge the HT supply, but if valves are removed then you may well have no discharge path.

29th Jul 2014, 10:20 am	#9
trobbins Heptode Join Date: Oct 2009 Location: Melbourne Australia Posts: 901	Re: Bleed Resistor I recall some safety standards that require <60VDC (ie. non hazardous designation) after 60 secs from mains turn off. Yet another good reason for NOT putting a fuse in the DC HT line!

29th Jul 2014, 2:34 pm	#11
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Bleed Resistor Actually, after (R * C) seconds, the voltage on the capacitor will have decayed to 1 / 2.71828 of its original value. Which might possibly still be dangerous. Bearing GJ's experience in mind, make sure the bleeder resistor is situated upstream of any fuse in the HT line. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

29th Jul 2014, 3:03 pm	#12
Tyso_Bl Hexode Join Date: Jul 2014 Location: Magor, Monmouthshire, Wales, UK. Posts: 436	Re: Bleed Resistor I'm not having a go at anyone, but why? Is the equipment built in a fashion that leave HV points exposed? or does the OP have some unsafe workshop practices. I mention this because bleeder resistors should not be used as a way of compensating for either of the above, enough of us are dying out by natural causes without adding accidental ones. __________________ Adapt, Improvise, Oh Bother.....

29th Jul 2014, 3:37 pm	#13
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Bleed Resistor If people died as a result of receiving a shock from an HT line, I'm sure that I and many other forum members would be dead by now. Adding a bleeder resistor reduces the risk of receiving an HT shock after a set is switched off. If you're working on a set which is uncased and powered up, then inevitably points carrying mains, HT or EHT are going to be exposed. I cannot see that a bleeder resistor in a domestic set is present other than for safety reasons. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

29th Jul 2014, 4:21 pm	#14
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Bleed Resistor Well said, Graham! I shall also add that if a HV PSU uses a choke input filter and if that choke is a swinging choke, then the bleeder resistor is mandatory and should be of the correct value. Al.

29th Jul 2014, 9:49 pm	#15
Guest Guest Posts: n/a	Re: Bleed Resistor I just hang an AVO (a proper moving coil jobbie) on the HT, sometimes it takes a while to discharge but a darn sight less hassle than getting a shock. And you know when it is discharged.

30th Jul 2014, 1:26 pm	#18
GrimJosef Dekatron Join Date: Sep 2007 Location: Oxfordshire, UK. Posts: 4,311	Re: Bleed Resistor In my case the fuse didn't isolate the bleeder resistor. It just isolated the output valves. I mentioned it in response to trobbins advice that the output valves in a valve amp generally stay hot enough for long enough to act as much more effective bleeders than the bleeder resistors themselves. The problem in my case was that the reservoir capacitors were such high-value ones that they held a lot of energy for a very long time. In response to Tyso Bl I'd say that it's common for a service engineer to find HV points exposed if he's working deep inside a high power valve amp. And as for 'unsafe workshop practices' I think that if we could rely on people never ever to make a mistake then we'd hardly have a need for any other safety systems at all . Cheers, GJ __________________ http://www.ampregen.com

30th Jul 2014, 2:31 pm	#19
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,395	Re: Bleed Resistor Beware of the phenomenon of dielectric stress if you directly short a capacitor hoping to discharge it- having removed the short, the cap may bite you later by way of retaliation! Best arrange a suitable resistive leak whilst working on the kit- eg. the aforementioned meter.

30th Jul 2014, 10:29 pm	#20
Stylo N M Heptode Join Date: Jul 2012 Location: Derby DE1, Derbyshire, UK. Posts: 626	Re: Bleed Resistor Hi, If you are concerned at all about any subsequent charge, why not just have a resistor say about 5k6 at a reasonable wattage wired permanently to a couple of crocodile clips on the work bench and then you can connect it and disconect it to any capacitor if it's just simple valve radio work, leave for half an hour and all should be OK. Chris is right the best place for a permenent bleed resistor is directly accross the capacitor (in most cases) Paul.