Resistance to protect flybacks, etc.

[abstruse1]

I see recommendations to use a resistor with HV outputs of flybacks and other transformers.  Apparently. this can be done two ways, in series or in parallel with the HV output.  Is this right?

In parallel, it seems that this makes sure the output is never completely open.  But the resistor is always in place, thereby reducing a bit of the transformer output.  Too low resistance would kill the output, while too high might not be adequate.  What is a good range of resistance for this purpose?

I'm not sure what function the resistor affords in series. Perhaps just giving a microsecond or two before the full transformer output is called upon.  Or what?  In any case, what's a good resistance range for this function?

Thanx, guys.

[johnnyzoo]

A parallel resistor will provide a base load for the transformer. This may be beneficial or necessary with flyback/boost-type converters and certain resonant converters which may be damaged by overvoltage if operated without load.

A series resistor can be used to limit the output current. Whether this is necessary or useful depends entirely on the transformer/converter and the application.

Good resistance value also depends entirely on the transformer and the converter driving it. As for TV/CRT transformers, I can't immediately recommend any specific value, but a suitable parallel load resistor would probably be in the mega-ohm range or higher.


edit: I just noticed the subforum - are you asking about HV transformers in general, or cascade multipliers specifically? 

[abstruse1]

Thanx, Johnny. I was asking about xfmrs, but the reason I’m working with them is to drive CW multipliers. Your explanations are helpful.

[abstruse1]

With a typical TV flyback, approx what's the range of resistance for parallel use?

[johnf]

Parallel 10-1000 meg-ohms per kv depending on circuit topology
as for series this depends on power of the supply but I tend for a minimum of 1000% of the max current ie a supply of 100kV 100mA you would use a 100k ohms supply impedance = 12 meg ohm.
More if you want to protect your supply or less if you are adventurous