[Amps] Solid state relays..again

[MU 4CX250B]

I agree completely with Manfred's assessment, but just want to add a
couple of comments. First, these NTC resistors are usually referred to
as "current inrush limiters," and cost about a dollar.  Second, while
high current ones usually have a relatively low cold resistance (e,g.
5 ohms), lower current ones (1-2amps) can have a 25-50 ohm cold
resistance. I find the CL-140 type very handy for limiting inrush
current in transmitting tube filaments. I also routinely put one in
the primary winding of boatanchor receiver transformers

Because the devices run hot, I avoid clipping their leads short, and
use Teflon sleeving for insulation. It's best to keep them away from
other components.

73,
Jim W8ZR

Sent from my iPhone

> On Feb 13, 2014, at 6:09 PM, Manfred Mornhinweg <manfred at ludens.cl> wrote:
>
> Brian,
>
>> 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.
>
> Manfred
>
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