[Amps] Solid state relays..again
manfred at ludens.cl
Thu Feb 13 20:08:09 EST 2014
> The little black varistor thingies work quite nicely for eliminating
> that "THUMMMPP!" in small to medium-size equipment.
I don't think they would, really. They don't have enough resistance to
do that. Transformer saturation and the consequent magnetic flux through
the cabinet panels would still happen.
> I don't really know what they are
They are NTC resistors. As they get hot, their resiatance drops. A
typical one used in a switching supply for a few hundred watts at 230
VAC, might have 5 ohm when cold, dropping to less than one ohm when hot.
They are intended basically to avoid blowing the rectifiers with the big
inrush current of direct line-connected (transformerless)
rectifier/filter circuits. The resistance in these circuits is so low,
that when connected to a very low impedance AC supply, the inrush
current could exceed the absolute maximum transient rating of cheap
diodes. Adding such an inrush limiter is the cheapest fix for that, also
avoids welding any switch contacts, and works well except for one little
problem: If a brief power cut happens, it might last long enough for the
capacitors to discharge, but not for the inrush limiter to cool down. In
that case it won't limit the current to a safe value when the service
returns, and that creates a risk of blowing the rectifier, and explains
equipment failures happening during microcuts or due to worn power plugs
or any loose connections.
> but somebody gave me a whole bunch
> of them a while back. I have two or three different sizes.
The different sizes have different power dissipation ratings. Within
each size, several resistance values are available.
They are quite useful, but have their limits too.
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