Re: [EE] Inrush limiter

At 11:53 PM 2025-01-24, you wrote:


>THANKS! In this application, they do not know much about the load. This is
>a power control/distribution system that is typically used to control high
>power audio amplifiers. I suspect the steady state current is known, but
>the inrush would depend on what load was plugged in. I've seen switching
>power supplies that say they have a 30A inrush at 120V and 60A at 240V.
>But for how long? And if we limit the inrush current with the resistor,
>the inrush will be lower current but last longer.
>
>I really like the simplicity of the series resistors with a relay that
>shorts it out when the load voltage gets high enough to pull in the
>shorting relay. Real simple, but what resistor?
>
>Thanks!
>
>Harold

It's going to be a lot harder to pick a suitable resistor (and relay)
without going way overboard on size if you have little knowledge of the
range of possible loads.

For example, say I had a fairly hefty 470uF capacitor to be charged
to 170VDC from the 120VAC mains. That would be for something in the
few hundred W range. I might pick a time constant of
0.1 or 0.2 seconds so a resistor of about 200 or 400 ohms, which would
limit the current to less than 1A or 0.5A. We know that the energy the
resistor dissipates  is equal to the energy stored in the capacitor (CV^2/2
or about 7J) so not very much - 7 watt-seconds. A switching supply
in that range would have a similar surge if the output is not loaded.

However if you have to  allow for  someone plugging in a compressor
motor and we have a 1 second delay then it could see more like 100J-200J
(and it would do no good at all).  So, roughly speaking, more like
a 50W or 100W resistor than a 5W resistor.

You'd likely also want to use a TV-5 or TV-8 rated relay so there is
less risk of the contacts welding shut, since you can't guarantee
the peak contact current will be low when the relay closes. Hopefully
there's enough wiring resistance to meet the TV rating maximums,

Best regards,
Spehro Pefhany






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