I have a 500VA toroidal transformer I am going to use with my UK mains outlet to step mains voltage down to 25V. I am using a 13A fuse with an RCD (30ma).

I only just realised that the inrush from my transformer will certainly blow the fuse in the plug if not trip a breaker.

I have searched and read some solutions on here where a thermistor is used in series to limit this inrush current, however I tend to frequently switch the power off when I am touching the circuit. So this method would require me to wait for the thermistor to cool every time I alter the circuit.

I have read a post where a user switches the thermistor out after a few seconds but he relies on a secondary 12V supply for this. Can anyone suggest an alternative?

  • \$\begingroup\$ Here is some good info on inrush current limiting. \$\endgroup\$
    – Biduleohm
    Jan 3, 2015 at 15:18
  • \$\begingroup\$ Thanks for that, i didn't realise that the thermistor cools in a matter of minutes (<2) as stated in that link. Im going to go ahead and order one but ill continue looking for a soft starter. \$\endgroup\$ Jan 3, 2015 at 15:31
  • \$\begingroup\$ Soft start it is not the best solution. \$\endgroup\$
    – GR Tech
    Jan 3, 2015 at 20:25
  • \$\begingroup\$ for now ill use a thermistor and switch it out manually after a few seconds so that it will cool for when i re-power. \$\endgroup\$ Jan 4, 2015 at 21:46

1 Answer 1


One alternative is to add a series resistance, which you then bypass after a few cycles. This gives enough time for the field inside the transformer to build up, and to fill the input capacitors. You short this resistor after a certain amount of time and continue normal operation.

Like this:

enter image description here

Don't be alarmed by current spikes at the end, they are under 15A effective and should not trigger a fuse.

This circuit shows an application where you generate a rectified DC voltage. the current spikes are due to the capacitor being charged. The same voltage is then used to control a relay which triggers at 300V to bypass the input resistor.

Forget the thermistor. You will still have to wait for it to cool down if you cycle things fast.

Biduleohm suggested a nice link. There is also the circuit that uses the principle I suggested: Series resistors solution

You can find many good circuits there, the engineering is solid.

What is your application?

  • \$\begingroup\$ Thanks for your reply! That solution is much like a series resistance starter for a dc motor. Can i simply add a decent POT and turn it to zero in the first 5 seconds? \$\endgroup\$ Jan 3, 2015 at 15:21
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    \$\begingroup\$ The 'decent pot' that can handle the amperage you are talking about here would be interesting to see. \$\endgroup\$ Jan 3, 2015 at 15:31
  • \$\begingroup\$ haha your right it would. I could use it to heat my hot water tank i guess haha. My brain likes to post-process power dissipation, i am beginning to see a consistent pattern with all my ignitions haha. \$\endgroup\$ Jan 3, 2015 at 15:39
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    \$\begingroup\$ Would you touch a pot (actually rheostat) connected to line voltage? depends on the pot (rheostat) :) \$\endgroup\$
    – iggy
    Jan 3, 2015 at 15:43
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    \$\begingroup\$ @biduleohm and iggy -- The link you both suggested has a domain that is no longer active. Any idea where all that content went? I'm trying to prevent that inrush for 10A 120vac, from poping a breaker for ANY microwave oven. Here's a good discussion: en.wikipedia.org/wiki/Inrush_current_limiter \$\endgroup\$
    – DaaBoss
    Feb 3, 2019 at 17:44

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