are there pros and cons to using high vs low capacitor discharge/bleeder resistor?

[mrjayviper]

as above.

I've searched the net but came up empty.

Digikey has a calculator to select the required resistor value for the selected time or resistance (https://www.digikey.com.au/en/resou...version-calculator-capacitor-safety-discharge)

Thanks

EDIT: permanently soldered to the caps in the power supply

 

[Mooly]

Do you mean a permanently connector bleeder or one you apply to manually to discharge the cap?

 

[mrjayviper]

Do you mean a permanently connector bleeder or one you apply to manually to discharge the cap?

permanently connected to the cap leads

 

[JMFahey]

I would design for a time constant of 2 or 3 minutes, so by that time after turning OFF you already lost 66% of original voltage or 2/3, after another 2 or 3 minutes 2/3 of the remainder, which should be "safe" , and so on.

Much faster will unduly heat resistors, much slower it will take "forever" to discharge and dangerous if you in tend to mess with amp innards.
jm2c

 

[Mooly]

permanently connected to the cap leads

It's as JMFahey suggests then. To low in value generates heat which wastes power. Depending on any reservoir cap size a higher current will also increase the ripple component on the rail.

Be aware that high value resistors and high voltages need careful component selection. Resistors have voltage as well as power ratings. Choose something unsuitable and it will go open circuit or worse even though the power dissipation is seemingly well within limits.

 

[NickKUK]

It doesn’t take much to add a LED to indicate power (also providing 10mA or so of additional bleed).
In safety - test, bleed and tie down before work. Personally I like the LED which drops off at about 1-2V, I have a perm separate bleed resistor. The bleed runs for 30 seconds or so.

 

[kodabmx]

I design with a power LED connected to the B+ as a visual indication, and use resistors sized in such a way that by the time you turn off the amp, unhook it, get it on the bench, and open it up, the caps are empty.
For me, that means 47k across 320V and 200k across 640V.
I don't use bleed resistors in my preamps because the tubes will empty the caps at turn off.

 

[NanoFarad]

I second NickKUKs recommendation, use LEDs.

 

[mrjayviper]

It doesn’t take much to add a LED to indicate power (also providing 10mA or so of additional bleed).
In safety - test, bleed and tie down before work. Personally I like the LED which drops off at about 1-2V, I have a perm separate bleed resistor. The bleed runs for 30 seconds or so.

That's a large drop. What sort of LED and resistance value? Thanks

(I'm already using a red LED paired with 15k ohm)

 

[NickKUK]

That's a large drop. What sort of LED and resistance value? Thanks

(I'm already using a red LED paired with 15k ohm)

I’ll dig out the details, i have both HV and LV psus. Same LED but different resistors.

 

[mrjayviper]

I’ll dig out the details, i have both HV and LV psus. Same LED but different resistors.

Thank you very much

 

[NickKUK]

HV 117Vac input I used a 11.5Kohm in series with the LED, 100Kohm bleed.
LV 22Vac input I used a 1.5Kohm in series with the LED, plus a 39Kohm bleed
Heater runs a 475ohm in series with the LED with a 10Kohm bleeder.
LEDs were vishay two colour ones - however just work out the current required.
IIRC the resistors were yeago higher voltage (iirc 350 or 500V).

 

[kodabmx]

For 320V, I use 1M for the blue LED, and 47k for bleed.
I don't bleed anything under 300V

 

[sesebe]

I also do not bleed anything under 300V.
For relatively low voltage, a LED powered with a current source is a good idea. Relatively low power dissipation combined with a fast/linear dischearge.

 

[NickKUK]

Just for completeness the LED I was using: https://www.mouser.co.uk/datasheet/2/427/tluv5300-1767302.pdf

 

[mrjayviper]

I also do not bleed anything under 300V.
For relatively low voltage, a LED powered with a current source is a good idea. Relatively low power dissipation combined with a fast/linear dischearge.

hello, would you be able to point me to a sample of a current source schematic? Thanks

 

[sesebe]

simple: https://www.nexperia.com/applications/sub-systems/constant-current-source.html
precise: https://www.analog.com/media/en/technical-documentation/data-sheets/lt3092.pdf
variants: https://www.merghart.com/p/27/Two-Transistor-laser-current-source
https://www.researchgate.net/figure/A-simple-but-effective-constant-current-source_fig1_230975767

 

[krca45]

I don't use bleed resistors in my preamps because the tubes will empty the caps at turn off.

This is not always so - I have built a phono+line preamp with simple C-RC-RC-RC psu filter and without bleeder resistor.

The capacitors remained charged after turn-off for hours ie until the next turn-on, the last PSU cap with potential of almost 290 Vdc.

After measuring this with multimeter I installed 220K / 2W bleeder resistor.

Regards, Krca

 

[kodabmx]

If you find the caps don't bleed out, use a bleeder of course... You could put an LED in series with that 220k resistor to make a visual "voltage is present" indicator, too.
My Preamps use CRC only - anything more than that was a waste of money and time in testing since they run on regulated power.
The only caps that don't drain out are the coupling caps - they stay charged at ~140V since they come from the mid point of two triodes...

 

[captJackSparrow]

Hi Everyone,

What would be safe value for as bleeder resistor so it doesn't waste power while in operation and quickly discharges capacitors when turned off. Currently it takes lot of time to discharge caps. I can use Chassi mounted resistors(50W), but I do not want to waste power during operation for what is not necessary.

F5 Turbo
Rail voltage - 45
Capacitors - 22000uF * 8 (Eight capacitors)

Aleph2
Rail voltage - 57 V
Capacitors - 22000uF * 8 (Eight capacitors)