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Re: FAQ - Ballasting a power transformer.

Posted: Mon May 26, 2014 9:19 am
by John Futter
Just to add

The reason a ballast is used is to add a real impedance to the circuit to limit dI/dT and dV/dT both of these terms are very destructive to all systems so limiting voltage and current transients allows control circuits to act.
As Richard and Chris have said a value somewhere between 5% and 50% of the power available is about right unless you know all the parameters of the power supply and load where you may confine these limits to a tighter range.
Taking into account response time of the supply an unballasted system will appear to have negative resistance which is the most damaging of all conditions for the supply ie this can be asking the supply to to cope with more than twice of what it is capable of.
ie the time lag of the supply tends to reinforce not negate what is happening.
The Ballast slows system response so that the supply can correct /and /or limit what is going on.
Bode plots and their theory give the insight into this--it all comes down to phase margin (a measure of lag in the power supply)


Happy Ballasting

Re: FAQ - Ballasting a power transformer.

Posted: Mon May 26, 2014 10:40 am
by Johannes westman
I think I need to read it again, sorry.

Re: FAQ - Ballasting a power transformer.

Posted: Mon May 26, 2014 1:19 pm
by Richard Hull
The key is "KNOW... and understand your supply" If you don't, you will be --ssing into the wind. No neon sign transformer based system ever needs any form of ballast. A 25 kilowatt utility pole based power supply would need a small engineering effort to properly ballast the supply, both for and aft of the transformer.

What the he-- is your supply ready to supply without bringing down the power grid or melting your fusor grid instantly? 99% of all amateur neutron club attempt fusor supplies are hard pressed to supply 30kv at 10ma. Everything in them is weak and whimpy. As noted above, for these supplies, a wire wound, 25-100 watt ceramic resistor of 30kohm to 70kv ohms will be sufficient in the hands of a good operator to prevent damage.

A dim witted, not very fast on his feet person, who never built a well instrumented demo fusor and played with it for hours is capable of destroying most any supply system. There is no ballast or supply that can survive stupidity on the part of the operator.

Intelligently applied, real watts, have to go into any successful fusor and the operator is as much a part fo the ballasting system as the physical ballasting system, itself!

Richard Hull

Re: FAQ - Ballasting a power transformer.

Posted: Mon May 26, 2014 3:13 pm
by Johannes westman
I think I know where I went wrong now. My assumption, based on insufficient knowledge about transformers was that transformers (unballasted) will simply continue to draw an ever increasing amount of current.
(My supply, btw - will be a 50kv 8ma x-ray transformer, I also have a neon transformer at 10.5 kv 30ma at hand)

Re: FAQ - Ballasting a power transformer.

Posted: Mon May 26, 2014 3:14 pm
by Johannes westman
I plan on running the 50kv at a lower Voltage, and a slightly higher current btw.