[Amps] Solid state relays..again

Manfred Mornhinweg 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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