Most common household appliances (TVs, LED drivers, etc.) have short inrush current (50-60 A). As I understand it is for charging input side capacitors. Can I use pre-charge system with resistor, to slowly charge the device? I need it because my AC supply cannot handle such peaks. Is 230 VAC pre-charge a common practice? I would just put ~10 Ω resistor for 100 ms and then enable main relay.

  • \$\begingroup\$ Can you section your appliances over several breakers and turn them on one by one? \$\endgroup\$
    – winny
    Aug 9 at 12:13
  • \$\begingroup\$ I'm not sure how you figured an inrush current of 50 to 60 amp for a TV or LED driver. If you had a huge motor, you might see inrush in that order of magnitude, but for a TV, I highly doubt it! \$\endgroup\$
    – Julien
    Aug 9 at 13:25
  • 1
    \$\begingroup\$ Why do you say your AC supply can't handle short high peaks of current? \$\endgroup\$
    – Finbarr
    Aug 9 at 13:33
  • 2
    \$\begingroup\$ @Finbarr It's a common problem on inverter-based sources that have relatively little transient energy storage (car converters, solar, battery, UPS, inverter generators). It's becoming a problem for utilities too, as solar, battery, wind, and DC transmission becomes prevalent. There it's called grid inertia, referring to the energy stored in rotating generators. \$\endgroup\$
    – user71659
    Aug 9 at 21:09
  • \$\begingroup\$ @Julien My laptop’s supply brick reliably sparks at the cord plug every time I plug it into mains. And it’s not a little spark. You hear it very clearly - a “pop”. It’s drawing way more than 15A then. A cold 1.5kW space heater doesn’t make such pops when plugged in. \$\endgroup\$ Aug 10 at 11:29

2 Answers 2


Commonly devices handle the inrush limiting by themselves if they need to. Such resistor limiters are commonly seen on audio amplifiers that take a large current surge to charge the supply bulk capacitors on the secondary side.

And usually power supplies already have NTC or small resistance to limit the surge current.

If a device does not expect external resistive limiting, it might not work properly, because it may try to take the current it needs to start up and it may need the current within certain time to make the supply voltage rise quickly enough. One such example might be power supplies with power factor correction (PFC) on their input.

The 10 ohm resistor might be too high as well. Usually it is lower. The resistor would need to pass a lot of current and it would momentarily need to dissipate a lot of power.

If your supply can tolerate a single device being turned on one at a time, it might better to just build a time delay which energizes one device at a time.


The idea isn’t bad.

At the most basic, a hand-controlled rheostat will do the job. Doesn’t need to be rated for full device power since it will dissipate heat only for a couple of seconds. The most critical rating is the continuous current. A bypass switch for when the rheostat is turned down to zero takes care of that.

That “switch” could be an end-switch-activated DPST relay that holds closed until the power is turned off. Another such relay, at the high-ohms end of range, can turn on power to the rheostat. That way it’d be safe in case of power loss and absent-mindedness. To turn it on, the rheostat wiper needs to hit the high-end switch. As soon as you then turn it down to zero, it gets bypassed. Power loss will cause the relays to drop out and the contraption is off until manually handled to restart it.

Main problem with pre-charging is that the device doesn’t “know” that it’s pre-charging. As soon as the DC link capacitor has charged high enough, the power supply will start up and will draw a fairly high current since the DC-link voltage is at the low end.

But today most devices start up in some kind of a low-power mode, so that shouldn’t be an issue most of the time.

You’ll have to experiment. Instead of a fixed resistor you may also use a synthetic resistance that ramps down to zero instead of just switching to zero. The synthetic resistance works by using a low power variable reference resistor, and “duplicating” it in series with the load.

The variable reference resistor can be a variable gain op-amp, a photoresistor with variable light source, a digitally adjusted potentiometer, a motorized potentiometer, etc.


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