Usually in DC input motor controllers a relay is placed in series with the power supply ( Vbat+) a resistor is connected in parallel with it. This resistor is said to be a precharge resistor for the bulk capacitance (ex: 4700uF at 48v ).

I want to eliminate the relay from this circuit , and i want to know what are my options for limiting inrush current for the bulk capacitors , which as i understand stabilized the dc link from switching and supplies peak current for motor.

My question is:

1- can i use an NTC alone and have the same performance if yes how is the selection is made ?

2- should i place a mosfet in parallel with this NTC that opens after the capacitors are charged ?


simulate this circuit – Schematic created using CircuitLab

3-is there any recommended alternative ?

  • \$\begingroup\$ That's a fairly common way to do it if you can spare/live with the losses from the capacitor ripple current going though your MOSFET constantly. Since it will only be used once, can you consider a fixed resistor instead? \$\endgroup\$
    – winny
    Commented Dec 31, 2016 at 18:18
  • \$\begingroup\$ In 2, the MOSFET should be closed (i.e. fully conducting) after the capacitor is charged. \$\endgroup\$ Commented Dec 31, 2016 at 18:39

1 Answer 1


NTC is usually used allone. It affects reliability because it gets hot, also it's not very consistent.

Do exactly what you drew, but with a resistor (and better from the hugh side).

Another option- power line filter. It may have significant inductance, so inrush current is slightly limited.

  • \$\begingroup\$ can you clarify what is a power line filter ?? is it a inductor in series with power supply , because we are talking about 50A or more from the battery so this would dictate a huge inductor ?? \$\endgroup\$
    – ElectronS
    Commented Dec 31, 2016 at 15:19
  • \$\begingroup\$ It would be huge... Usually it's one or two common mode chokes and a bunch of capacitors. Just google it. \$\endgroup\$
    – user76844
    Commented Dec 31, 2016 at 16:35
  • \$\begingroup\$ By the way, i often use several kiloohm resistor and a relay to bypass it after several seconds, when the capacitor is almost fully charged. \$\endgroup\$
    – user76844
    Commented Dec 31, 2016 at 16:36
  • \$\begingroup\$ i like the idea of common mode chokes (although a bit expensive 50A at 15$ on mouser ) mouser.com/search/refine.aspx?Ntk=P_MarCom&Ntt=163737108 , but do they really work ? there series resistance is very low , maybe the ac resistance is what matters since we are looking to decrease the rise time (inrush current) , can you elaborate your answer and include how to calculate the required inductor , since it is the best and only answer \$\endgroup\$
    – ElectronS
    Commented Jan 2, 2017 at 16:10
  • 1
    \$\begingroup\$ Ok, two things. First, common mode choke, or better a filter based on common mode choke, is widely used to not radiate stuff with power cable. Second, such filter is never primarily intended to fight inrush current. It can help, and if you need it anyway- maybe it will also solve your problem. Otherwise- resistor and relay. \$\endgroup\$
    – user76844
    Commented Jan 2, 2017 at 17:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.