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Re: Old Capacitors


From: 	Thomas McGahee[SMTP:tom_mcgahee-at-sigmais-dot-com]
Sent: 	Tuesday, December 02, 1997 11:54 AM
To: 	Tesla List
Cc: 	100624.504-at-compuserve-dot-com
Subject: 	Re: Old Capacitors



> 
> From: 	Alan Sharp[SMTP:100624.504-at-compuserve-dot-com]
> Sent: 	Monday, December 01, 1997 4:46 AM
> To: 	INTERNET:tesla-at-pupman-dot-com
> Subject: 	Old Capacitors
> 
> Greetings all,
> 
> I've just aquired six 0.1uf 10kv DC capacitors.
> 
> They are 7" long, 1" diameter brown cylinders with stud terminals
> marked Hivoltage Capacitors Limited
> Londonderry N.Ireland
> +8% ripple 50Hz
> TPM 100-114
> 
> I was going to put 3 of these in series,
> then 2 series in //  for my 7.5KV AC 550mA supply - to give 0.066uf 30kV DC
> 
> but I'm wondering if they will take the current at 200kHz -
> would I be better to use 4 or 5 in series and get the other 0.04uF from
> my other capacitors? Or reduce the voltage to 5kV AC.
> 
> It also occurs to me that moving to a lower frequency - bigger coil
> will ease the load on the capacitors.
> 
> Any thoughts.
> 
> have fun,
> 
> Alan Sharp (UK)
> 
>
Alan,
For Tesla coil use, derate the DC value by 4 to get the SAFE RMS
value (i.e. equivalent Neon rating). We have to derate because of the
RMS factor, and also because of transients. I recommend putting
a safety gap directly across the cap system. That way if you move
the caps to a different system, the gap goes with it.

The dissipation factor of DC caps can be quite high, as many are
made with mylar. Thus RF heating becomes a problem.

Run these for short periods and immediately check them for temperature
rise. Remember that temp rise will be greatest at the center of the
cap and may take time to appear at the case. So run with reduced power
to begin with, and slowly increase the time of the runs and monitor
the temperature. If you can run them at reduced power for at least
a full minute without observing much temp rise, then you can try
increasing the power (open main spark gap) and observe the temp rise.
Do this in controlled stages, so that a high temp rise does not
catch you off guard or cause an explosion.

Guard against explosive effects by wrapping the caps in old cloth
or old clothes, and placing the wrapped caps in a wooden box or
other suitable container so that any explosion can be mitigated and
contained safely.

Hope this helps.
Fr. Tom McGahee