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Having tackled my first project using a mains transformer (in the UK - 230V), I haven't put any inrush protection into the circuit. The circuit is a power supply, following these instructions: The Spyder - an Eight-Output Pedalboard Power Supply.

Schematic of The spyder - an eight-legged, hum free pedalboard power supply

The schematic shows a resistor/capacitor in parallel to the transformer primary, which I neglected to put in. My guess and hope is this is what is needed to limit the inrush current to the transformer. I have come humbly to you because I can find no other examples of this being used as inrush protection, and because currently (no pun intended) if I switch on the supply my TV will turn off then on briefly, which I can't see is doing it much good.

Is this resistor/capacitor combo intended to stop inrush current, and is it adequate?

If it is adequate, what component values would you recommend? And if not, what component(s) do I need and how should it be wired?

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    \$\begingroup\$ I don't think that's a capacitor. Haven't seen the symbol in a while, but I'm pretty sure that's a little neon bulb - and has nothing to do with inrush protection. \$\endgroup\$
    – brhans
    Commented Oct 24, 2014 at 0:11

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No, that will not affect inrush current and in fact it's just a neon lamp + resistor to indicate power on.

For inrush current limiting you can use an NTC thermistor or one like this in series with the primary. It's imperfect though- the resistance drops when it gets hot so if the user flips the switch back and forth with it hot, you can get the fuse blowing if it's not an appropriate slow-blow fuse type.

enter image description here

Another approach is to put a power resistor in series with the primary and short it after a second or two with a relay contact. You must use a fusible resistor so if (when) the relay fails, the resistor will burn out without causing undue excitement, sirens, etc.

Edit: BTW, it's also possible that the problem is not the inrush current (especially if this is not a toroidal transformer) but sparking when the power is removed or the switch bounces. In that case, an MOV across the input 230V at the transformer (after the switch) and/or a surge protector on the devices that are affected may help.

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  • \$\begingroup\$ If I were building something like that, it wouldn't occur to me to need inrush protection. Do you really think it's necessary? \$\endgroup\$
    – gbarry
    Commented Oct 24, 2014 at 0:16
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    \$\begingroup\$ @gbarry Yes, for some transformers. Audio guys like toroidal mains transformers because they have well contained magnetic fields (low stray flux), but they can have a really nasty turn-on surge that depends on the core state at the most recent turn-off. Perhaps 100A for a few hundred VA transformer, which can blow circuit breakers etc. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Oct 24, 2014 at 0:26
  • \$\begingroup\$ gbarry No, I didn't think it was necessary either, but when it was switched on the TV on the other side of the room switched off then on, and I was a little concerned that my PC was on the same outlet as the TV. I guess it's much cheaper to buy a resistor than replace the PSU on the computer. @SpehroPefhany NTC Thermistor it is then, it should just be switched on for an evening gigging or whatever else, plus I wont have space in the case for the more elegant relay solution. Thank you all for a staggeringly prompt and comprehensive answer! \$\endgroup\$
    – Charlie
    Commented Oct 24, 2014 at 1:47
  • \$\begingroup\$ So to conclude, what value thermistor am I looking for? My knowledge of electronics says 100 ohm thermistor would let 2.4 A through, which sounds ideal? \$\endgroup\$
    – Charlie
    Commented Oct 24, 2014 at 2:12
  • \$\begingroup\$ I don't know that it's ideal- without knowing the operating current it's hard to tell, but a WAG would have the input current normally at ~100mA or less so 100 ohms at 25C would not drop too much voltage especially when it warms up. If you have an old PC power supply you could scavenge a thermistor from there. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Oct 24, 2014 at 2:27
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Most AC transformers operate with their core slightly saturated and this means if any more current is taken the core saturates even more and things start to get tricky. The problem really occurs when the AC is applied to the transformer at the zero cross of the voltage waveform: -

enter image description here

After a short time the primary magnetization current and voltage settle down to their normal 90 deg phase shifted relationship and this would be seen if the AC voltage were applied at it's peak: -

enter image description here

Even a fairly perfect inductor (with no saturation) shows the problem when AC voltage is applied during the zero cross: -

enter image description here

The first cycle of current rises higher than under normal operation and will gradually assume an average zero value over several cycles. This has to happen due to the formula for an inductor's current and voltage being differential in nature. Clearly with a bigger peak current, a real transformer core is going to saturate more and, this in turn, draws more current causing more saturation.

So, one option is to build a circuit that applies AC at the point it reaches maximum. I know this sounds non-intuitive but it's true!

Picture taken from here.

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