Re: Converting diodes to resistors

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts by way of Terry Fritz <teslalist-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

On 22 Jun 2003, at 11:55, Tesla list wrote:

 > Original poster: "S & J Young by way of Terry Fritz 
<teslalist-at-qwest-dot-net>" <youngs-at-konnections-dot-net>
 >
 > Semiconductor failure experts,
 >
 > I fried (over easy) a string of  15 1N5408s in my DC resonant charger power
 > supply.  Usually I have the spark gap across the supply & resonant choke and
 > the cap in series with the primary.  But I noticed in Richie Burnett's site
 > he showed the cap across the charging power supply and the gap in series
 > with the primary.  So I tried it.  BIG MISTAKE!  The RF caused a smoky and
 > stinky voltage breakdown in one of the MOTs I used for the charging
 > inductor.  The RF also wiped out a diode string.

I would guess that the slow recovery time of the diodes caused the
failure through excessive peak dissipation. You may well have better
luck using the UF540x series in that configuration.

 > Question:  The diodes all still have the usual forward characteristics, but
 > the reverse resistance on all of them has gone from higher than 200 megohms
 > when they are good, to around 12K.  They vary from 5 K to 20 K.  So, just
 > curious, what did the RF do to the diode junctions to make them act like
 > resistors?

Don't know. Re-arranging the doping; partially fusing the junction;
heat possibly causing decomposition of the potting in the vicinity to
form a carbon track all spring to mind.

Malcolm