How to limit inrush current into DC motor simple way? - Page 1
Forum section: electronics / projects

author: Chriss
author_rank: Frequent Contributor
timestamp: February 18, 2020, 07:48:36 pm
content: Hi!I have to build a prototype of some DC motor controller which will be made with PWM...But at the very first moment I would like to make some mechanical testing where the desired DC motor will be driving a shaftbut the PSU which should power the DC motor is a switching mode PSU one.So, I think this kind of PSU's didn't like much the inrush current, especially not from a 12VDC motor which should consume around 6A continuously.I'm thinking somehow to protect the PSU from the inrush current until I finish the testing and for this I'm searching for somesimple solution to do this for the mentioned motor.Maybe to put a cap in parallel the motor terminal so when the motor is starting the cap would fast delivery the energy to the motor.But here I have a bit problem to take the correct value of cap.Can somebody help me to figure out what cap could I use for this ?Or is there a more simple solution to make this testing on the safe way?Thanks for any idea and help.My best regards.

author: Gyro
author_rank: Super Contributor
timestamp: February 18, 2020, 07:59:26 pm
content: Hi,A capacitor would do exactly the opposite, It would present a low impedance to the fast PWM edges, increasing the peak driver current! You can use capacitors for electricalprushbrush noise suppression, but that would be in the <100nF range.If you want to limit the peak current, then a low value series resistor would help, maybe around 0.1 -0.5R

author: Chriss
author_rank: Frequent Contributor
timestamp: February 18, 2020, 08:02:54 pm
content: Thanks for the faste response.Sorry, I didn't mentioned, for now in the testing time, I will not use the pwm...

author: Gyro
author_rank: Super Contributor
timestamp: February 18, 2020, 08:15:05 pm
content: Ah, ok. As mentioned, a small capacitor would help reduce high frequency brush noise getting back to the PSU. A resistor is still the simplest current limiter though, even though it will get very hot if you stall the motor!

author: Chriss
author_rank: Frequent Contributor
timestamp: February 18, 2020, 08:48:18 pm
content: The motor should never stall but I assume the resistor will go very hot even during the normal running time of the motor, cos the ~6A...What about to put an inductive coil in between the motor and the psu?That would also suppress the current I think.When I finish the test and make the controller then I will use the software soft start with pwm and I assume that will work, but for now to run the motor and the mechanic part for 1-2h it should serve but not kill the psu...

author: Ice-Tea
author_rank: Super Contributor
timestamp: February 18, 2020, 08:53:06 pm
content: NTC.

author: Chriss
author_rank: Frequent Contributor
timestamp: February 18, 2020, 08:55:47 pm
content: Can you pls help me to determine the correct ntc?Thank you.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 19, 2020, 11:38:45 am
content: Some redneck way for a cheap, high-power resistor, like running the current through a long iron wire, whatever you have around.Then implement the proper drive, this is, add current sense and current regulation loop to your PWM control.

author: exe
author_rank: Unknown
timestamp: February 19, 2020, 11:52:59 am
content: There are so-called "soft-start circuits". Most of them are just time relays bypassing a resistor with a mosfet.

author: Prehistoricman
author_rank: Regular Contributor
timestamp: February 19, 2020, 07:29:29 pm
content: Quote from: Chriss on February 18, 2020, 08:55:47 pmYou just need to consider their rated current, off resistance and on resistance. Run some numbers with power dissipation etc. and see if it would work.You could try to modify the PSU to ramp up the output voltage. It might even have an adjustment pot already.

author: Chriss
author_rank: Frequent Contributor
timestamp: February 19, 2020, 07:46:34 pm
content: Practically the mentioned PSU should be a PC PSU who can delivery 16A.This is just for the testing process, not for long term use.It should power the motor so we can see how the mechanical parts are react...The motor is a car windshield wiper motor.This motor should stay for long term use.

author: Benta
author_rank: Super Contributor
timestamp: February 19, 2020, 07:50:20 pm
content: Are you sure the PSU is not current limited? I'd be surprised if it isn't.Otherwise, use a motorcycle or car battery, they're cheap.

author: Chriss
author_rank: Frequent Contributor
timestamp: February 19, 2020, 07:58:00 pm
content: I'm sure this psu is not current limited, I tried the motor once or twice and it worked but I won't do the whole testingcos I think I could kill the psu.It is a good idea to use a battery but at the moment we don't have a car battery, and we should use then a charger toocos the battery would probably not last the testing time which could be 1-2h.Maybe we need repeat the testing etc.But we have two or more psu's laying around and it is more convenient.Maybe the best way would be to make a simple soft-start tool wit an ne555 so I can adjust the pwm signal to smoothly start the motor.Maybe that would be the fastest way to do and I would have that tool for future project too.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 20, 2020, 09:14:42 am
content: Instead of 555, give it a shunt resistor and a comparator, and you have a proper current limit with approximately the same complexity.

author: Chriss
author_rank: Frequent Contributor
timestamp: February 20, 2020, 09:21:52 am
content: Thanks for the idea.Can you pls share a schematic about your idea?Just to be sure how you mean to built that.Thank you very much.

author: Chriss
author_rank: Frequent Contributor
timestamp: February 21, 2020, 08:56:02 pm
content: I played with this kind of circuit formy soft-start stuff.The C2 should be replaced with the mentioned dc motor.I put it just for inrush current testing to this circuit.How good would be this circuit with this parts parameters to soft-start a 12v car viper dc motor?Thanks for any advice and ideas.My best regards.

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 21, 2020, 10:04:16 pm
content: The motor would be better modeled with a coil. This way the simulation will show what happens when the motor (an inductive load) is suddenly shut off: the voltage will suddenly rise across the motor--be sure to protect against this.In the case of your schematic, the motor would need to go on the high-side, between the positive voltage and the N-channel MOSFET. You will also want to put a resistor between the MOSFET's source and gate to 1) keep stray charge from building up on the gate, and, 2) to discharge C1 when the motor is turned off.Your circuit (with appropriate changes) might work but watch out for charge injection. When a voltage suddenly appears at the MOSFET's drain it can cause the voltage at the gate to shoot up which will allow more current to flow then you intend to. So it can basically foil your attempt to control the surge of current.The suggestion someone made earlier about using an NTC thermistor is probably the simplest solution. Although, if you can make it work with a MOSFET then you can solve the problem that the NTC will have--which is that you will need to wait for the NTC to cool down before you can start the motor again.

author: Chriss
author_rank: Frequent Contributor
timestamp: February 22, 2020, 10:28:50 am
content: Thanks for the suggestion. I can manage the stuff with the fet, no problem.Where I stuck a bit is :How to set up a testing circuit so I can check how good is my circuit?I mean, hiw can I monitor the inrush current without a coresponding amp meter ?I have a scope and could do the math.I thinking to put a shunt in series with themotor and measure the voltage drop through the shunt and calc the I based of Ohms law.Would that be a correct procedure?I can't really manage thesimulation in LTS. maybe somebody will be nice to help me to create a not so complex dc motor part so I can make some simulation in LTS?I woukd really appreciate that.I have no imendance meter so I can not check the impedance of my motor but I could set up a circuit with an R in series with my motor and feed some signal into and measure it with scooe and make the calc but all that would be a bit owerkilled now just to make the mechanical test...Thank you to all of good people who are helping.My best regards.

author: Zero999
author_rank: Super Contributor
timestamp: February 22, 2020, 02:37:25 pm
content: Quote from: Chriss on February 21, 2020, 08:56:02 pmThe only issue with that is the MOSFET is configured as a source follower, which means it will never turn fully on and drop a fairly high voltage, resulting in high power dissipation and the motor never running at full speed. You should put the motor in series with the drain, rather than the source.One way to reduce the size of the capacitor is to connect it between the gate and drain, making use the Miller effect to multiply the capacitance.

author: TinzerMotor-Riva
author_rank: Newbie
timestamp: May 06, 2025, 07:10:25 am
content: Hello, have you overcome this problem? To limit the inrush current of the motor? I have the same problem right now, if you solve it, could you help me too? My email is riva@ruishuoautoparts.com.Best regards.

author: Ian.M
author_rank: Super Contributor
timestamp: May 06, 2025, 02:18:19 pm
content: Quote from: TinzerMotor-Riva on May 06, 2025, 07:10:25 amChriss hasn't even logged in here for seven months, so don't hold your breath waiting for an answer. Also we don't do answers by email here, so I suggest editing your post to remove it, as all you will get is spam!I also suggest starting a new topic for your question to maximise your chance of useful replies, rather than tagging on to an old topic. I doubt you are prototyping a motor controller with the exact same motor and PSU as Chriss, so tell us about your actual problem!