Cap safety gap (renamed) (fwd)
---------- Forwarded message ----------
Date: Tue, 16 Dec 1997 14:04:02 EST
From: Esondrmn <Esondrmn-at-aol-dot-com>
Subject: Cap safety gap (renamed)
In a message dated 97-12-16 03:09:12 EST, you write:
<< I would like to hear other folks opinion on this subject. I believe both
> Cox and Malcolm suggested the addition of the safety gap across the cap.
> Malcolm, you have the same C.P. caps that I have. What kind of safety gap
> you use, what transformer voltage, and how far do you open the gap?
> Ed Sonderman
The point Mark made about the high frequency ringing with the gap
across the cap is well made IMO. I never use a gap across the cap.
I *do* think it is a good idea as a *last* resort to stop the cap
dying from overvoltage until BPS etc. is optimized.
I've never run a real high power supply like a pig. The
only way a gap across the cap can fire is if the charging inductances
allow the cap to ring up to that voltage in between breaks. It can't
be energy coming back from the sec because you can't get more energy
into the cap by that route than you started with. A gap across the
transformer is a different matter entirely.
I've been mostly confined to neons and other transformers in the
2kW range at most. I think the Q of the charging circuit should be
lowered if the cap safety gap fires. It would also help if you can
continuously vary the break rate. You should be able to strike a
point for a given set of charging conditions where the safety gap
stops firing entirely. At the same time, coil output should also peak.
I would appreciate your thoughts on this. The last time I had this coil set
up (outside), I tried many combinations of primary set up. This includes
welder settings, resistance in series with welder, rotary speed and variac
settings. I added 600 ohm resistors in series with each H.V. feed line. No
bypass caps and no chokes in the H.V. feeds. With resistive ballast only, I
get a popping sound coming from the rotary gap that doesn't sound good and the
gap firing is erratic. When I add the welder in series with the ballast
resistance, the popping in the rotary goes away and the system runs much
smoother. The discharge sparks are much hotter and louder than with resistive
ballast only - at the same primary voltage and current. Is this an indication
of a resonant charging condition that is allowing the voltage across the cap
to ring up too high??
Should I increase the H.V. series resistors from 600 ohms in each leg to maybe
3,000 or 4,000 ohms? At 500 ma (5 kva at 14.4 kv), I will run into a
dissipation problem in a hurry along with concern about voltage breakdown
across the resistor. At 3,000 ohms, the resistor would be dissipating 750
watts with 1,500 volts across it.
The best performance in the above testing produced 57" sparks with the welder
set at maximum, series resistance at 4.8 ohms, variac wide open and rotary
speed at approx. 400 bps. Lowering the series resistance or increasing rotary
speed would cause the cap safety gap to fire. Primary input energy at this
point was only about 3.5 kva. I want to get the system dialed in to where I
can input all the power I have available - probably about 8kva. I have
produced 80" sparks in the past with this system.
Your idea of lowering the primary Q goes back to the original design where I
had all the primary components interconnected with two parallel sections of
RG213. Most of these recent problems have occured after I replaced all that
wire with 3/8" copper tubing - raising the primry Q.
Comments? Thanks, Ed Sonderman