Re: The MMC debate continues . . . . . .

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Terrell Fritz <terrellfone@xxxxxxxxxxxxxxxxxxxxxxx>

Hi,

At 04:13 PM 2/23/2007, you wrote:
> At the risk of sounding stupid....How do you determine the proper 
> size for bleeder resistors.I'm not using a MMC but I am using 
> several MOT caps in a voltage doubling set-up for a dual MOT power 
> supply.Are the resistors built into the caps okay?

The first thing to figure out is the time it will take to discharge 
the caps.  This is:

Time (seconds) = 5 x R x C

So 10M ohms and 150nF will discharge in 5 x 10,000,000 x 150e-9 = 7.5 seconds.

That time should be less than the time it takes you to turn off the 
power and possibly come into contact with the caps.

Then the power dissipated in the caps which is:

Power = Vrms^2 / R

For Vrms, I take worst case or 2000VDC for are typical MMC 
caps.  2000^2 /10e6 = 0.4W.  You always want this to be less than the 
resistor's power rating.

Hard to say if the resistors in the caps now are "ok"...  With the 
cap disconnected and completely discharged, you should be able to 
measure about 10M ohms across it with a good meter.  Personally, I 
would just use external resistors anyway that you can see and inspect 
far more easily.

The numbers do not have to be exact or anything.  So for a 1uF 2500V 
microwave cap a 2M ohm resistor will discharge it in 10 seconds.  The 
power dissipation is 3.2 watts.  So perhaps two DigiKey resistors in 
series.  Part Number 1.0MW-2-ND is 1M ohm at 2 watts so two in series 
gives 2M ohms for four watts.

I checked two microwave caps here and they were both 10Mohm  The 
bleed down time is 100 seconds in that case which seems "too long" to 
me, so "I" would rather use the two added resistors.  It sort of 
depends on how long it takes from power off to someone possibly 
touching it.  Of course, you should directly dead short the caps in a 
safe way to be extra sure if you are going to work on it and such.

Cheers,

        Terry