I'm having problems hot swapping a powersource to my board due to the input capacitance, I have a big inrush current (around 80A peak) and hence I see arching (sparking) on the connector when I power my board. I believe this could lead to damage down the line so I want to avoid it.

I tried NTC Thermistors to limit the inrush, they worked, I managed to get down to around 30A peak but still there is sparking.


This device limits the inrush current in a unique way, by creating a slow transition on the output voltage. But alas this is a very specialized device for military use, and hence must be very expensive, and also looks quite bulky.

Can you recommend a similar device but for commercial products? Or any other way to duplicate the behavior of this device?

Input source: External Power Supply Brick. 24V DC Steady State Current: 3A Max Peak Current under normal load (momentary): 8A Max Inrush Current: 80A


1 Answer 1


You should be able to control inrush current with eg a series MOSFET with a network on the gate which controls the rate of rise of gate voltage. Their DVCL (Delta Voltage Current Limiter) controls the maximum rate of input voltage slew (dV) BUT by simply applying a slowly rising voltage to a MOSFET gate you will produce a "resistor" of decreasing value as the gate voltage rises. The characteristic will be non linear and voltage rise will be nowhere near linear as they show BUT will be able to be made as slow as desired.

The diagram below shows their claimed DVCL results - see their video for scales.
Yellow = Vin to DVCL and Green = Vout to circuit proper.
Blue = current.

enter image description here

If you place say a P Channel MOSFET in the +ve lead (source to Vin, drain to load, with a resistor from dare to ground (-ve) and a capacitor from the gate to ground then:
As gate voltage rises exponentially, as it rises above Vgs_th the MOSFET will start to conduct and the MOSFET will be fully on typically when Vgs is in the range 1V to about 5V above Vgsth. A little playing or some simple caluclation will allow the correct RC time constant to be calculated.

Excellent Motorola application note AN1542 - active Inrush Current Limiting using MOSFETS expands on this basic concept and provides much detail and background. Their not apprecioably more complex fig 5A provides an example real world solution.

enter image description here

The MOSFET will obviously need adequate voltage and current ratings and a suitable Vgsth. Rdson needs to be suitably low for your application.

You note that Iin = 3A steady and 8A peak and Vin = 24V.
Effective load resistance at 8A = V/I = 24/8 = 3 ohms.
Loss in MOSFET or in load = I^2 x R, so a MOSFET with Rdson of 0.3 Ohm will dissipate about 10% of the power used. But at 30 milliOhm Rdson dissipation will be 1% of power used and lower Rdson is better.

Circuit power at 8A, 24V =~ 200 Watt so peak power dissipation at Rdson = 30 milliOhm ~+ 2 Watts. See MOSFET datasheet for acceptable pulse-time ratings to decide if a heatsink is needed. A sensible TO220 device should be safe with startup times of around 1 second or faster and no heatsink. TO89 / SOT223 probably still OK especially if a little PCB copper is available for heatsinking. The excessively enthused could probably design a low Rdson SOT23 MOSFET into this circuit with due care.


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