# Inrush Current Limiter

#### FBorges22

Joined Sep 11, 2008
109
Hi there,

I am trying to project an inrush current limiter for a full-bridge diode rectifier. I designed a circuit that uses an phototriac to activate another TRIAC to bypass an inrush current limiting resistor of 1K. Also, the phototriac is activated by an 5V step signal supplied by an microcontroller.

What do you guys think of this design attached in inrushlimiter.png? It will work ok?

Best regards,
F.Borges

#### DickCappels

Joined Aug 21, 2008
10,104
With a 1.5 second time constant (1k and 1500 uf) you would probably have a pretty large turn-on delay, depending on how much current your triac can safely handle (maybe add a couple of ohms in series to limit the peak current rather than depending on the ESR of the capacitors.

Those two 100k resistors are there to balance the voltage across the capacitors. For that to work the point where the two resistors connect should also connect to the "center taps" on the capacitors.

The more conventional approach is to use a negative temperature coefficient resistor in series with the AC. It has a high resistance when power is applied but quickly heats up and drops in resistance.

Something like this:
http://www.es.co.th/detail.asp?Prod=017501849 <== Notice the datasheet on that page

You might also find the discussion at the link below to be helpful.

#### dannyf

Joined Sep 13, 2015
2,197
It works in principle. I would have used a solid state relay but the basic principle Is the same.

The 1k resistor may be a little too much. But I guess you can wait a little longer with that kind of value.

#### dannyf

Joined Sep 13, 2015
2,197
I can understand using a MCU Herr if it is already bused somewhere else.

Otherwise, activating the start up resistor based on rectified DC output is probably more robost.

#### FBorges22

Joined Sep 11, 2008
109
Hi there,

Those two 100k resistors are there to balance the voltage across the capacitors. For that to work the point where the two resistors connect should also connect to the "center taps" on the capacitors.
In order to balance the voltage across the capacitors should I connect the resistors this way? I also lowered the inrush limiting resistor to 10ohms@5W

#### DickCappels

Joined Aug 21, 2008
10,104
Yes, that is how I would connect them. If that is 240 VAC your 10 ohm resistor will limit the peak inrush current to 31 amps worst case (220 x 1.414 /10).

#### FBorges22

Joined Sep 11, 2008
109
Okays... thanks a lot for the assistance I will proceed to the PCB design.

#### ronv

Joined Nov 12, 2008
3,770
Hi there,

I am trying to project an inrush current limiter for a full-bridge diode rectifier. I designed a circuit that uses an phototriac to activate another TRIAC to bypass an inrush current limiting resistor of 1K. Also, the phototriac is activated by an 5V step signal supplied by an microcontroller.

What do you guys think of this design attached in inrushlimiter.png? It will work ok?

Best regards,
F.Borges

View attachment 116915
It's not a lot of capacitance to charge. Are you sure you need it?

#### FBorges22

Joined Sep 11, 2008
109
Yes the inrush current limiter is necessary because I will use this AC-DC module in a Half-Bridge DC-DC converter and the converter can only start when the capacitors are fully charged. Recently I blow my MOSFETs because of this problem.