Dropping 9V for a DC toy motor rated at 6V using Voltage Divider

Other Hardware General Electronics
1

I have a (sort of toy) motor controller which uses 9V power supply (and logic levels)

Ideally i should be giving you the schematic to be complete but it's not available.

Basically, it can handle up to four motors, but i only have two motors for it.

I now want to add two more motors but they are rated at 6V.

I thought i could use voltage dividers to drop the 9V to 6V, and this is how it would look like.

My question is regarding the scenario in reverse polarity, is it as straightforward as it seems or are there other considerations that one should take note of ?

Thanks for pointing out any potential newbie pitfalls.

FWIW:
The motor output terminals, M1 - M4 are also labelled O1 - O8.
So, M1 (O1 and O2) can also be used for non-polarity outputs, say a bulb, in which case the other end from O1/O2 would be the GND terminal on the controller.

Also, if any pedant says this is not Arduino-related, i later intend to make the afore-mentioned 'motor'-controller a slave to an Uno via bi-directional level-shifters.

The controller has 8 inputs.

2

A voltage divider should only be used to "power" things whose current demand is several orders of magnitude less than the no-load current through the divider. That usually rules out motors.

3

DaveEvans:
That usually rules out motors.

ohh, i see...

i have to start looking at relays or using transistors, then ?

what if i place power resistors in series with the motor - would that be enough to drop the voltage ?

EDIT:
let's say the motor takes 500mA, then a couple of 3 Ohm (5W) resistors (either side of the motor) would give it 6V, right ?

4

BabyGeezer:
let's say the motor takes 500mA

But you can't say that. It might have a nominal rating of 500mA, but the actual current will vary dramatically, depending on the load on the motor, and specially at startup. So your resistor will be the wrong value for much of the time.

I think you would be better off with a separate driver and power supply for your 6V motors.

5

PaulRB:
So your resistor will be the wrong value for much of the time.

i see - a motor is quite the dynamic device then...

PaulRB:
I think you would be better off with a separate driver and power supply for your 6V motors.

or it might end up being more efficient just getting two more 9V motors.

thanks for all the input guys - mixing and matching various spare parts and 'junk' is not so easy !!

6

The voltage rating of a brushed DC motor is usually the recommended maximum. You can run a 6V motor at 9V -- it will run faster but won't last as long.

If you use a motor driver to reduce the average motor current, that will lengthen motor life, so the two effects can counteract each other.

7

Thanks for the tips.

jremington:
The voltage rating of a brushed DC motor is usually the recommended maximum. You can run a 6V motor at 9V -- it will run faster but won't last as long.

yeah, that's a deal-breaker - these are (sort of) "Lego" motors that i've had for decades(!) and i'd like to handle them with care.

jremington:
If you use a motor driver to reduce the average motor current, that will lengthen motor life, so the two effects can counteract each other.

good to know, i will have to start measuring performances (of both motor and 'driver') and see what numbers come out as workable.