motor guesstimates


I’m trying to vaguely guess something, don’t know if this is the right place to do it as it were.

we’ve got a 12V power supply 500mA

we’ve got an old compact bosch cordless screwdriver motor and the motor goes round, but the little led on the power supply goes off. A supply blew at one one point, is this the cause? What wrongheaded thing am I doing?

Is there a safe guessstimate for these types of cordless motors? Is out volts too high and our amps too low?

Any suggestions



I would guess that the motor is trying to draw more power than the supply is rated for.

is there a way of telling how much power the motor requires?


yes you can measure the current requirements with an ammeter. This is usually a function on a mutlimeter.
For power, multiply DC voltage X current.

For example:
motor on 12v supply consumes 1 Ampere
power = 12V X 1A = 12W or 12V at 1A/H

With a large gel-cell battery that provides 12V @ 7AH, you could run the above example motor for about 7 hours.


PS: those compact drills draw a lot of current… are you using a motor driver? you will need something like a 2A or more supply for that type of drill.

thanks alot. Just bought a multimeter today. V.helpful. Will begin tinkering



One thing about motors is that there current requirements can rise dramatically as force is applied to resist the turning of the motor. Most motors have there current requirements measured when there are running free and when they are stalled. It would not be unusual for the stall current to be 10-20 times the current requirement of the motor freely running.

Actual current requirements of the motor when operating will be between these two extremes; however, the drive circuitry needs to be designed to handle the stall currents.

hello Wadrson

great to get a sense of path of least resistance so to speak…

to reduce complications we were going to use a relay to switch it on and off. The motor is moving about ten files files in a cabinet using scanner mechanics, not heavy, but does stall occassionally.

Is it a matter of finding a robust power supply, or do I need to add circuitry to protect the power supply or the motor, and if so what??? (capacitor to handle spikes?)

cheers for your help



@wandrson is right, you will often need extra juice for startup and when the load becomes more stressful on the motor. Try to get something that is 2-3 times your current requirement. You can fuse it with similarly valued Slo-blow fuses if you;re worried about short circuits.


Hello everybody i’m working with electrosun on this project:)

After measuring with a multi-meter, it looks like the motor draws the following from our 12v .5a supply:
1.6a (no load)
2.1a (under load)

sooo, using the example Daniel helpfully provided, the power on our motor is:
12v X 2a = 24w

In which case, should we be looking to find a 12v 4a-6a supply? (tricky to find at short notice?) Or, if i’m getting my head round all this, could we also use a 24v 1a supply?

Ta for everybody’s help, its really appreciated!

P.S. Hmm, this post (& time spent on simar issues) is making me think that it would be really useful to have an “Idiots guide to choosing power supplies” in a similar vein to the “Getting started with Arduino” booklet.

That is about the power supply size I would choose; however, if you can’t find one think about re-engineering another product. A motor doesn’t typically need the level of voltage regulation that electronics do (hardly any) so a transformer, rectifier and filtering caps are all that is really needed.

One possibility would be to hack a moderately sized car battery charger for the parts/supply.

Don’t you keep that old Pentium II 75Mhz machine around somewhere in your cellar or some other dark corner of your House ? :wink:

Seriouly the cheapest you could get is to rip the powersupply out of an old Computer. This will at least give you 6 A on the 12V side (and plenty of amps on the 5V rail too).


Great stuff, unfortunately there aint no old PC’s laying round here, so it looks like a bit of a skip-diving or freecycle ahead!

Hello everybody i’m working with electrosun on this project:)
In which case, should we be looking to find a 12v 4a-6a supply? (tricky to find at short notice?) Or, if i’m getting my head round all this, could we also use a 24v 1a supply?


You need a 12V/ 2A or more power supply. Unfortunately the real-life math does not work the way you would like it to with the 24 supply, so stick with the 12V supply,minimum current 2A, preferably lots more. Hardware or automotive supply stores will often have unregulated 12V power supplies that would fit the bill.

Thanks Daniel:)