Re: (not so) Easy Caps

Nick and others,

The built-in resistors are a safety feature to slowly discharge the cap
when the microwave oven is powered down.  This is supposed to partially
compensate for careless or foolhardy technicians who neglect to short the
cap before working on the guts of microwave ovens.

The value is typically 9 or 10 megohms, plenty high enough that you can
just ignore it for most purposes - no need to remove them.  MOT caps are
great for DC supplies where the frequency is low - 50 or 60 Hz.  At RF
frequencies, the dielectric (something like mylar) will dissapate far more
energy than the safety resistor.  If you run enough of them in series for a
low power TC, you might get away with their use in a primary resonant
circuit.  But beware they can overheat quickly and explode, so encase them
in a sturdy box. 

Also, some of these caps also have a HV diode built in as well as the
safety resistor.    The diode typically is between the case and one of the
cap posts.  This will be indicated on the cap label.

Best to stick with poly caps for power RF use, e.g. TC primaries.

> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Easy Caps
> Date: Sunday, January 24, 1999 3:23 PM
> Original Poster: NickandSim-at-aol-dot-com 
> To All,
>           i have found the best source of quick n' cheap caps.  MOT caps
> a resistor built in.  But the resistor can be easily removed (it is right
> the top, just saw off the flanged end of the cap and pull it out) 
leaving you
> with a 2kV 1.1uF cap and a workbench covered in oil.  It is almost
> to cut the top off without losing most of the oil so I need more to make
> work.  If anyone knows where to get this oil please tell me.
> Thanks in advance
> Nick Field