I've been told that to change the larger caps
on a Vector monitor chassis, it's better to discharge them with a resistor
rather than just shorting them to ground.

I'm told a high resistance value ( 1M-ohm + ) is best,
but I would think a high-wattage tolerance is also needed.

I've got 2.2M-ohm 1/4W resistors, but I'm afraid that's too small.

I've also got 1.1K-ohm 3W resistors,
which are physically 4 times bigger than the 2.2M ones,
but I don't know if that resistance is high enough.

I'm sure there must be some formula to determine
resistance Vs. Wattage rating, but I'm at a loss to figure it out.

Any advice from the experts ?

Thanks,
Steph

Don't bother with a high resistance value, that will cause the cap to take FOREVER to discharge.  Just use a low(er) value high wattage resistor to do the job. (I'd say 100ohm or less... even a 10R, and go with at least a 1W)

If you don't want to burn your hand on the resistor (yes, it should get hot depending on the value) use one of the ceramic ones.

D

Quoted from Menace

Don't bother with a high resistance value, that will cause the cap to take FOREVER to discharge.  Just use a low(er) value high wattage resistor to do the job.

So the 1.1K-ohm  3W resistor should more than adequate... ?

Thanks,
Steph

I assume you are talking about discharging the filter caps in the power-supply,
and not the picture tube's 2nd anode.
Obviously, discharging the caps need to be done with the monitor unplugged.

Which cap value and voltage ?

One example: cap 100uF operating at 400 VDC.
400 V / 1100 ohms = 0.36A peak
(will come down quick, as the energy/voltage stored in the cap
dissipates as heat thru the resistor).

3W is probably sufficient for the discharge time.
(if the 400V was permanent, the resistor would dissipate:
(400)(400)/1100 = 145 W).

To be on the safe side, I would put two of these 1.1K resistors in-series,
to lower the peak initial discharge current to 0.18A,
and the combo of the two resistors would be able to dissipate up to 6 W.

Cheers,
- Sylvain.

Steph,

For regular caps ~470 - 5K, 2W resistor is supposed to be good  high wattage resistor of about 100 ohms/V of the working voltage of the capacitor ( http://arcadecontrols.com/files/Miscellaneous/crtfaq.htm#crtcsa ).

I have a 10 Meg Ohm high voltage probe that can be used to discharge monitors through my meter, (which I can bring up on Dec. 6th if you can wait) but it doesn't say the Wattage (bigger is better obviously).  Slow discharge makes it less likely that you have to repeat discharge (I think) BUT you need to let the anode rest for 15 - 30 minutes after discharge and then discharge again as the charge builds back (though you likely already know this).

I'll swear I saw a project to build your own probe somewhere on the net but I've lost track of the link.

Quoted from sylvain

I assume you are talking about discharging the filter caps in the power-supply,
and not the picture tube's 2nd anode.

Correct.   (...Deflection Board actually ... )

Quoted from sylvain

Which cap value and voltage ?

4700uF  50V

Quoted from sylvain

One example: cap 100uF operating at 400 VDC.
400 V / 1100 ohms = 0.36A peak
(will come down quick, as the energy/voltage stored in the cap
dissipates as heat thru the resistor).

So I'm looking at

50 V / 1100 = 0.045A

Quoted from sylvain

To be on the safe side, I would put two of these 1.1K resistors in-series,
to lower the peak initial discharge current to 0.18A,

(50)(50)/1100 = 2.3 W

So I'm probably fine with only 1 resistor.

But thanks for the refresher couse on Ohm's Law,
...it's all coming back to me now..



Steph