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DC Motor Soft Starter

Thread starter #1

I'm working on a starting schematic for a 12V DC brushless motor. The armature resistance is ~0.5ohms, and I would like to limit the starting current to 2A. The motor must start via a push-button switch, and I would prefer not to insert additional resistance into the armature circuit. In order to satisfy the push-button specification, I am not able to manually ramp up the voltage by turning a rheostat. I've thought about using an LM555 monostable timer, but am unsure about how to implement it. I think the best solution might be something that could slowly ramp up the voltage from about 1V.

Thanks in advance for any ideas!
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New Member
A 12V motor with a coil resistance of 0.5Ω is a very high load and won't do anything starting out at 1V.

I suggest to make a soft start applying 1/2 voltage (6V) initially and after a predetermined time switch to full supply voltage of 12V.

The delay can be held in the range of 2 to 3 seconds (max) as the motor winds up pretty quickly.

You might either use two 6V supply voltages and connect one initially, which doesn't require a voltage (and current) limiting resistor or use low value series resistor which will be shorted after the starting circuit times out.

Ramping the supply voltage from 6 to 12V means a lot of circuitry and also a lot of losses.

The simplest way is using a monostable (NE555) which controls a power MosFet transistor to add or remove the resistor.

The resistor won't have to have a power rating of 144W (1/2 of the motor power) since it will be engaged for a short time period only. Instead of using a high wattage resistor you might try using some strands of a cable reducing the initial power.

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Well-Known Member
if all you are concerned about is limiting the start up current just put it in place of the inductor of an LED current regulator circuit and short the "LED"

Thread starter #5
Thanks for the ideas!

I found a 12V/2.5A AC/DC (wall plug) power adapter from radioshack that is able to start the motor without exceeding the current rating, however the motor moves jerkily once it is started. I've tried putting capacitors in parallel with the motor to smooth out the motion, but it has had little to no effect. Is there anything else I can try, or is the power adapter simply not able to power an inductive load??

I appreciate any help!


New Member
You are offering about 1/10 of the nominal current to the motor.



Well-Known Member
sounds like a switcher going into current limit hiccup. What are you driving with the motor? the current demand is proportional to the load (torque)



New Member
Yeah, I've had some trouble with those Radio Shack wall-mount switching regulators going into fits near the current limit. Sounds like you need a bigger power supply.
Here's a simple soft start I designed awhile ago. It enables me to run a 6V motor off a 5V SMPs, I tweaked up to 6V, without tripping the current limit.


Brilliant design!
It shows that one does not require a complex microcontroller or fancy analog circuit to achieve its purpose.


Well-Known Member
linear... you could do a rudimentary PWM with one opamp as an oscilator and another comparing the oscilator capacitor to your RC to give you the slow startup with out the dissipation.

There are a lot of options to choose from depending on how picky you are and how much power the motor actually uses since that will determine how simple it can be.

It can handle any load with suitable power resistors.

60 years ago, before the advent of power semiconductors, if one had to start a 250HP, 600VDC motor, one would use huge resistors in series with the armature. As the motor picked up speed, one by one of the resistors would be shorted out with a contactor, until the full voltage was applied to the armature.

The key here is that the resistors would have to survive the inrush current for only a few seconds. The resistors were made with heavy nichrome wire suspended on ceramic insulators.

Today, soft starting is best achieved with a PWM circuit
Hi all,

I found this post which is kind of old but i kindly ask you if someone can help me with same circuit schematic but for 12V and 10A ?

Thank you in advance

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