[AMRadio] DRIVER XFMR UPDATE

[Donald Chester]

> > If it's impossible to find 125 mfd's (which I doubt...they can
> > be found

I see loads of recently manufactured electrolytics both used and unused, 
about 125 mfd @ 450 volts, at hamfests.  Many of these are packaged as 
"computer grade".  They usually measure above the rated capacitance.  I 
always test for leakage before using them.  The simplest test is to put them 
across a HV supply at near their maximum working voltage and charge them up. 
  Disconnect them and let them sit for a few minutes.  A good cap will 
retain the charge (I have found that out the hard way, by getting a jolt 
from charged electrolytics that have been lying around for a week or more).

The next test, if the cap appears to hold  charge, is to put it back across 
the high voltage, fully charge it, then read the leakage current by placing 
a milliammeter or microammeter in series with it.  Be careful; if the 
capacitor shorts you can ruin an expensive meter that way.  Make sure the 
cap is fully charged before placing the meter in series, as the charging 
current can ruin a sensitive meter movement.

Since the internal resistance of even a marginally good electrolytic is 
likely to be several megohms, it wouldn't hurt to place a current limiting 
resistor in series with the meter - just calculate, using ohm's law, the 
resistance needed to make the meter read approximately full scale at the 
power supply voltage.  This resistance will be much lower than any leakage 
resistance (unless you are using a fractional microammeter), and will have 
little effect on your leakage resistance reading.

Don K4KYV

_________________________________________________________________
The new MSN 8: advanced junk mail protection and 2 months FREE*  
http://join.msn.com/?page=features/junkmail