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Bleeder Resistor Value & Wattage

D

Dave.H

Jan 1, 1970
0
I would like to know a general value & wattage rating for bleeder
resistors. I do occasionally recapping of vintage radios and would
like to build a unit with a resistor and alligator clips to attach to
the electrolytic leads to discharge them. The electrolytics I come
across mostly are rated for 200 to 600 volts.

Thanks
 
B

Baron

Jan 1, 1970
0
Dave.H said:
I would like to know a general value & wattage rating for bleeder
resistors. I do occasionally recapping of vintage radios and would
like to build a unit with a resistor and alligator clips to attach to
the electrolytic leads to discharge them. The electrolytics I come
across mostly are rated for 200 to 600 volts.

Thanks

In the old days I had a stick (Chinese Chopstick would do) with a copper
nail pressed through the end, with a 10K resistor taped to it. One end
went to the nail the other had a wire with a crock clip on it. But
that was in the days of big solid carbon resistors. I don't know what
power rating it was, but I never managed to damage it even putting it
across a live circuit occasionally.

Any way I=E/R So 600/10,000=0.06 E*I=P So 600*0.06=36 The peak power
would be 36 Watts. Since the voltage is falling, I reckon that the
resistor would have been about a 5 Watt rating.
 
E

Ecnerwal

Jan 1, 1970
0
Dave.H said:
I would like to know a general value & wattage rating for bleeder
resistors. I do occasionally recapping of vintage radios and would
like to build a unit with a resistor and alligator clips to attach to
the electrolytic leads to discharge them. The electrolytics I come
across mostly are rated for 200 to 600 volts.

A bleeder is generally always in place, low wattage, high resistance,
may take quite a while to bleed down (RC of a minute or more), so that
it won't load the circuit too much when running.

A shorting stick (applied to caps without a bleeder) is generally not
"alligator clipped on both ends" since it is generally a poor idea to be
quite that close, and is usually a lower value, high-wattage resistor.
Exact figures depend on the caps in question - figure a reasonable time
constant (ie, you'll hold the shorting stick on the capacitor for 5 or
10 seconds, and want it to be down to, say, 6 V - about 5 RC time
constants, so a time constant of 1-2 seconds) for 600V on the largest
typical capacitor you work with. One side has a clip and lead, the other
side has some sort of stiff electrode, and an insulating stick handle
leads back to the operator.

Remember that an easy method to up the power is to use multiple
lower-resistance resistors in series - it may be much simpler to find 10
2 watt resistors than 1 20 watt resistor. You can also use a light bulb,
(nice low resistance 100W [or whatever you buy] resistor, easily found)
but in that case you need to check that it has not broken, and/or use a
resistor in parallel with it so that you have some discharge even if the
filament does break.

Once the cap is shorted, if there's some chance of stray charge pickup,
a two-alligator clip lead (no need for a resistor, but there can be one
if it makes you happy) can be installed.
 
D

Dave.H

Jan 1, 1970
0
Remember that an easy method to up the power is to use multiple
lower-resistance resistors in series - it may be much simpler to find 10
2 watt resistors than 1 20 watt resistor.

I prefer to parallel higher resistance ones for more wattage ratings,
like if I wanted a 10k 2 watt, I parallel 2 20k 1 watt resistors
together. Which makes it easier as Dick Smith only sells 1/4,1 watt
and 5 wa
 
E

Ecnerwal

Jan 1, 1970
0
Dave.H said:
I prefer to parallel higher resistance ones for more wattage ratings,
like if I wanted a 10k 2 watt, I parallel 2 20k 1 watt resistors
together. Which makes it easier as Dick Smith only sells 1/4,1 watt
and 5 wa

I come from a high voltage background - in that context, the reduction
of voltage across each resistor (when in series) is beneficial for
inexpensive parts. At the voltages the OP is working with, makes no
difference, either series or parallel would be fine.
 
B

Baron

Jan 1, 1970
0
Dave.H said:
I prefer to parallel higher resistance ones for more wattage ratings,
like if I wanted a 10k 2 watt, I parallel 2 20k 1 watt resistors
together. Which makes it easier as Dick Smith only sells 1/4,1 watt
and 5 wa

For the job that you are wanting to do wire wound resistors would be
just as good. Remember that the peak dissipation is only going to last
for a fraction of a second. I just used something that came to hand.
 
J

John Larkin

Jan 1, 1970
0
I would like to know a general value & wattage rating for bleeder
resistors. I do occasionally recapping of vintage radios and would
like to build a unit with a resistor and alligator clips to attach to
the electrolytic leads to discharge them. The electrolytics I come
across mostly are rated for 200 to 600 volts.

Thanks

Something like a 10 watt, roughly 100 ohm wirewound resistor works
fine.

John
 
D

Dave.H

Jan 1, 1970
0
Remember that an easy method to up the power is to use multiple
lower-resistance resistors in series - it may be much simpler to find 10
2 watt resistors than 1 20 watt resistor. You can also use a light bulb,
(nice low resistance 100W [or whatever you buy] resistor, easily found)
but in that case you need to check that it has not broken, and/or use a
resistor in parallel with it so that you have some discharge even if the
filament does break.

I will do this, I have a couple of old bakelite bayonet lamp sockets,
I can use one of them, what value resistor would I put in parallel? I
regular 240 volt 100 watt light bulb tests at around 42 ohms.
 
E

Ecnerwal

Jan 1, 1970
0
Dave.H said:
I will do this, I have a couple of old bakelite bayonet lamp sockets,
I can use one of them, what value resistor would I put in parallel? I
regular 240 volt 100 watt light bulb tests at around 42 ohms.

Whatever's handy - probably a couple of hundred ohms. If you simply
check (with an ohmmeter) that the filament is sound after you've
discharged things, and before you reach in and grab stuff, you can do
without it. Unless you drop it, the bulb should hold up for years in
this application, as it never should get hot.
 
D

Dave.H

Jan 1, 1970
0
Whatever's handy - probably a couple of hundred ohms. If you simply
check (with an ohmmeter) that the filament is sound after you've
discharged things, and before you reach in and grab stuff, you can do
without it. Unless you drop it, the bulb should hold up for years in
this application, as it never should get hot.

I'll just use a clear bulb, so the filament is easily visible. Think
I may have one lying around somewhere, probably with those lamp
sockets.
 
B

Ben Jackson

Jan 1, 1970
0
I prefer to parallel higher resistance ones for more wattage ratings,
like if I wanted a 10k 2 watt, I parallel 2 20k 1 watt resistors
together. Which makes it easier as Dick Smith only sells 1/4,1 watt
and 5 wa

There's a "learn to solder SMT" QRP dummy load kit out there that's
around 44 2k2 SMT resistors to make one 5W 50R load.
 
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