I have two motors from the back wheels of an old Rollin' Rambler kids powered car (https://www.amazon.com/12V-Rollin-Rambler-Ride-On/dp/B00S713PW4). Unfortunately, I didn't get the wiring harness and controls out before it was thrown away. I'm thinking of using one of the motors to make a vibrating parts cleaner, but want a way to power it without having to use the battery. The car had a slow and fast speed, so I'm assuming it supplied maybe 6 or 8 volts to the motors for slow speed and 12 volts for fast speed. However, I don't really know how many amps the motor pulls. I can't find an exact one online using the numbers on the motor housing, but I think this one is similar: https://www.amazon.ca/SODIAL-15000-Electric-Motor-Spare/dp/B078CR3P86
That description has 35-45 W. If I understand it correctly, that equates to 2.9-3.75 A.

I also have an old laptop charger that outputs 19.5 V and is rated for 3.33 A. I'm wondering if there's any reasonable way I could use this charger and a resistor or something to power this motor at 12 V (or even less volts if that will reduce the current to an acceptable level for the charger).

From what I can figure, to use this charger to power the motor at 12 V and 3.75 A (I know it might even pull more amps on startup), I would need a 2 Ohm resistor:

    (19.5-12)/3.75 = 2

But then I think that would require the resistor to be rated for almost 30 W:

    (19.5-12)*3.75 = 28.125
And that just doesn't seem right/logical to me, but I've only really done anything with lower voltage and small watt resistors.

So, is there a reasonable way to power the motor using the charger, and are my calculations correct?

Here are pictures of the actual motor and charger. Motor enter image description here

  • \$\begingroup\$ Slow/fast may have been series/parallel connection. \$\endgroup\$ Sep 9 '20 at 11:18
  • \$\begingroup\$ That does not sound like a good idea, even if you were not running the motor 50% above the intended voltage as you will be. Perhaps if the load were very light, and not subject to jamming... but really you should find something more suitable, probably for both the power supply and motor roles. \$\endgroup\$ Sep 9 '20 at 11:34

Yes you can. It will however rotate at a higher speed and will get a lot hotter. Also if the now hotter motor is stopped abrupt, it might serve thermal damage (so its a good idea to use a speed regulator and to let it cooldown at a lower rotating speed). Also the lifetime of the commutator will drop.

But these cheep motors like the one on the picture are robust. So it will work for some while I guess...

And if you only turn it on for a short time, lets say a minute, it shouldn't get this hot and work well. Using Motors with higher power than rated - in the thermal limits - is normal e.g. for trains when they start rolling.

  • \$\begingroup\$ Keep in mind that the current estimate in the question is not credible beyond a light load and certainly does not represent a heavil loaded or stalled condition, where a motor will simply draw current until something sags, protects, or breaks. \$\endgroup\$ Sep 9 '20 at 11:04
  • \$\begingroup\$ Yes but the power supply should be able to power that motor in normal condition. And even if the motor is blocked, the supply should be short circuit save and at least be safe. \$\endgroup\$
    – schnedan
    Sep 9 '20 at 11:09
  • \$\begingroup\$ How do you know what "normal condition" in this custom application yet to even be built will be? Do you know what will be connected to the motor, and how it mechanically behaves? And do you really know the supply's overload behavior? The asker might be lucky and get away with it, but this is not an example of sound design. \$\endgroup\$ Sep 9 '20 at 11:24
  • \$\begingroup\$ Of course its not an example of sound design. But you can't tell every interested person he should attend some years of education and training. If it doesn't work he will learn from it. \$\endgroup\$
    – schnedan
    Sep 9 '20 at 11:29
  • \$\begingroup\$ It's not about obscure education, it's about developing a good sense of what is not a good idea, and having a healthy awareness of how motors behave. \$\endgroup\$ Sep 9 '20 at 11:33

Using the Laptop charger will reduce the performance of your motor, in turn kill your coils. I can say that safely because your coils will pull more current because of the voltage and power rating.

As an alternative I would suggest you purchase a 12V adaptor. These are easily available in any Electronics shops nearby. It has a max output current of 1A (depends on the company you buy it from). It'll suffice for your applications.

Hope this helped.

  • 1
    \$\begingroup\$ Why are you recommending a 1A power supply for a >3A motor? Also, please explain why a laptop charger would reduce the performance of the motor and "kill its coils"? What do you mean by "your coils will pull more current because of the voltage and power rating"? \$\endgroup\$
    – StarCat
    Sep 9 '20 at 10:08

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