# Supply a DC motor with a SMPS power supply

I want to supply power to a motor from a SMPS power supply. But the problem is SMPS supply have a feedback loop and a fail-back mechanism, so on the startup motor shows a near zero resistance [according to the electrical model of the DC motor ].

I don't have privileges to open up the SMPS unit and bypass that mechanism. So I need to find a walk around on this. Any idea guys?

I know motor soft starter unit will work, but I don't have that luxury to afford a one. Any other quick and dirty trick to make this work?

• Add a dummy load resistor parallel to the motor? Oct 23, 2015 at 3:13
• motor is a 12V motor in vehicle. So how about a one ohm and 5W resistor ? Oct 23, 2015 at 3:14

You can try something like this

simulate this circuit – Schematic created using CircuitLab

Set the limit resistor high enough to keep the SMPS from current limiting, and the RC for enough delay to let the motor get up some speed. The relatively slow rise of the gate voltage will allow further motor acceleration during turn-on, but make sure the FET is beefy enough to a) do a good job of shorting the limit resistor, and b) soak up the power dissipated during turn-on.

• any idea how could I decide the value of RLIMIT? Oct 23, 2015 at 3:25
• Sure - divide the SMPS voltage by the current limit and add a bit. After all, when you start the motor it's acting like a short circuit, right? Oct 23, 2015 at 3:51
• Dirt simple soft start. I love it. Definitely filing this one away. Oct 23, 2015 at 4:44
• so this is a common circuit configuration, "dirt simple soft starter" Oct 23, 2015 at 6:00
• I recommend a reverse biassed diode across R1. If you lose power momentarily It'll discharge C1 into the power rail, resetting the soft start.
– user16324
Oct 23, 2015 at 9:09

For anyone wanting to play with the values here is a working schematic (using @WhatRoughBeast and @Brian-Drummond responses) that you can run the simulation (DC Time domain) with:

simulate this circuit – Schematic created using CircuitLab

You'll notice at 4v the power MOSFET is triggered