Re: [EE] Inrush limiter

On Fri, January 24, 2025 7:49 pm, Spehro Pefhany wrote:
> At 08:03 PM 2025-01-24, you wrote:
>>I'm looking at making an inrush current limiter based on a series
>>resistors and a relay across the resistor that shorts the relay when the
>>inrush current has passed. The relay coil is across the load. During
>>inrush, the resistor drops the load voltage below the relay pull in
>>voltage. After the inrush, the resistor drops less voltage allowing the
>>relay to pull in.
>>
>>So, I'm trying to determine the resistor characteristics. Resistance,
>>power rating, and surge power rating.
>>
>>Does anyone know of any application notes about this?
>>
>>Thanks!
>
> Hi, Harold:-
>
> I've done this in a product. I just used the known worst-case charge curve
> of the capacitors (simple arithmetic) and a wirewound  resistor
> with guaranteed surge curves (with my usual conservative safety margins).
>
> I used a fusible 'cement' resistor so if  the relay failed to close
> it would be safe. I might have put a discrete fuse in series too,
> can't remember (client took it to production).
>
> Since my firmware was in control of the device being powered, it could be
> maintained in a very low power state until just after the relay contacts
> were safely closed.
>
> I also used a "bulletproof" reset circuit so that any possible glitches
> in the power would not result in inadequate charge delay, no matter what.
>
> Best regards,
> Spehro Pefhany
>

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



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