Capacitor to deal with inrush current in starting a small DC motor?

Kids have a Hot Wheels track. It eats batteries in no time... so a plan of a DC power supply came to my mind.

I got a switching 6 V 3 A DC converter. It works great, once it starts...

But it has a problem with the initial start. To spin the 2 motors current required is more than 3 A. This triggers "short circuit safety switch" (or whatever it is called) in the PS and kills power which comes back in about 0.5 s later to do it over again. It will pulse for about 0.2 s and dies again...

I can start it with help of batteries and then motors spin great with far more power than batteries - I guess less voltage drop.

Under no load it pulls about 0.5 A at 6.2 V.


How to limit the inrush current?

I thought of a capacitor to aid for the short time of inrush current, but I am unsure what size that should be. 4700 uF 25 V would be enough? Would it work at all?

I do not know what is the peak current. My multimeter is not quick enough to get the max reading.

Please advise,

by PLP
December 22, 2015

The simplest thing to do would be to try a big cap across the PSU output.

16V to 25V or higher electrolytic cap rating would be fine. Aluminium rather than Tantalum because Tants can be a bit fussy about big fast currents through them. Aluminiums are a bit more robust for this sort of application.

Higher operating voltages for a given capacitance is generally a good idea anyway but for electrolytics you need something around 2.5 to 3 times the nominaly voltage rating.

You'll get to the point where the cap is so big that it causes the short circuit cutout to trip anyway even without a load but if you're lucky you may find a cap value that's big enough to start the motors without tripping the cutout when they do and without just tripping when you switch on the PSU off load.

By Hot Wheels I understand something different from slot car racing so I'm not clear if these motors are in the cars and if they are then how the cars go from stationary to motors spinning and then physically moving.

That's important because if the cap idea doesn't work then it determines how some sort of alternative scheme might be made to work.

Ah! An idea for a slow charge/fast discharge cap circuit. Slow charge through the resistor so it won't trip on PSU turn on and fast discharge through the diode to start the motors:

by signality
December 23, 2015

Signality, You are one a heck of a guy. Thanks for a fast response. Resistor and switching signal diode make great sense.

As for what Hot Wheels is. It is a "ejecting cars" type or race track. You do not quite race two cars against each other, like in slot track. In my times it was Darda Motor tracks... slots as well, but never had one though.

There is a motor propelling two rubber covered wheels that spin at great angular speed and protrude inside the track from both sides. When the car comes the spinning wheels grab the car and shoot it forward. I'm not sure though if this particular track has two separate motors (it has two "ejecting platforms").

Now, going back to the setup. Is there a way to estimate what size of capacitor should be used to get this thing working? I understand it may need a fraction of second to charge the capacitor, but instead of blindly looking for a large capacitor, can I say that 1000 uF would work?

Also, can I leave the PS ON 24/7? Will it cause any harm to the capacitor?


by PLP
December 23, 2015

Here's a very much simplified version of the current limiter (drops about 200mV compared to 25mV of more complex but as yet untested design):

by signality
December 25, 2015

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