In rush Current limiting for AC circuit

Hi everyone,

I’m looking at how to limit the inrush current of AC/DC converter and am a little unsure of the details. I’ve done a bit of research and it seems that having a thermistor in parallel with a TRIAC is a good way to go as it means that the thermistor limits current initially but, once power has been established I can switch a TRIAC to bypass the thermistor. This means that the thermistor can cool down while the circuit is running and I don’t have to wait for it to cool down, if the power is turned off, before I can turn my project on again.

I’ve had a go at designing a circuit but, I am unsure if it’ll work, how to size components properly and if there is a better way I should do it. I have attached it (please excuse the horrible drawing).

Parts list:
-ac inlet with fuse and filter

The circuit needs to work at 230 and 115 VAC.

Would this circuit work? How do I go about sizing the components? Is there a better way to do it? (I think I could probably get away with only using the limiting circuit on the hot wire of the AC input)

I’m also a little unsure on how to size the fuse on the AC inlet as well (Ihave yet to do some serious research into the issue). If you have any input on that too, it would be much appreciated.

Thanks for all the help

What's the VA rating of your transformer? How much capacitance do you have in your rectifier circuit?

It seems most people don't bother with a soft-start unless they're using 500VA or more. You do need a slow-blow fuse, however. Otherwise, a fast-acting fuse needs to be several times your maximum current rating, which isn't much defense if the circuit misbehaves.

The data sheet doesn’t specifically mention capacitance or VA rating.

Its rated for outputting 480W continuosly. Its a switch mode power supply with active power factor correction. At 230VAC, the Power factor is greater than 0.95 typically and at 115VAC its greater than 0.99 typically. Typical input current is 6.5A/115VAC 3.5A/230VAC

I’m not certain how to calculate the VA rating here but, i guess it is
Apparent Power = Real Power/ power factor = Vin_rms * Iin_rms / PF = 230 * 3.5 / 0.95 = 847 VA at 230V
= 115 * 6.5 / 0.99 = 755 VA at 115 V

With regards to calculating fuse requirements, I have found a guide here
It talks about calculating the I2t value of the fuse. There are some different V vs I waves forms but it says the most common is the normal decay curve. It gives the formula
I2t = 0.5 * Ipeak2 * timeConstant
If the inrush current lasts for 0.5 to 1 cycle length so at 50Hz, thats 1/50 of a second. Would this be 5 time constants or only one? ie, would I use
I2t = 0.5 *402 * 1/50 or I2t = 0.5 *402 * 1/50 * 1/5

Ohh... I completely missed where you were using a prebuilt switching PSU. I assumed you had built one using a transformer, rectifier, and cap filter. That's a different animal entirely.

This site ( ) has some very good design ideas for soft-start circuits. His designs are quite a bit more complicated than what you're proposing. I don't have the chops to offer anything beyond that.

I do still wonder, though.. Why bother? The spec sheet says the PSU already has active inrush limiting. I imagine the quoted figures are for a [u]very[/u] short period of time (as a guess, 1-5ms). Any residential wiring should handle that with no trouble. You'll regularly see much worse from common appliances. The only case I can see to limit it further is if you're powering this from a generator or power inverter -- in which case, you're barking up the wrong tree by using this PSU anyway.