Re: [EE] Inrush limiter

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