I'm wanting to make or buy a kit to make a speed control for a 12v brushed
DC motor (BDC). I want to adjust and set speed by simple turn of a knob.
When under heavy load, I see that the motor draws 2.5 A at a voltage of
I'd like to controller to be able to drive the motor smoothly at small
speeds and it would help if it could maintain rpm under varying loads.
Can anyone recommend a suitable application note or kit or circuit? Thanks.
That is not a bad option very simple! Farnell has the chip at £2.13
I was going to suggest the Velleman kit K8004
Upto 35v and 6.5A. I have used one and it is a useful kit.
CPC has it at £13.61
and here it is £7.95 !!!!
As you get all components PCB and a set of destructions I would
personally consider this over the Micrel chip.
Best of luck
I must admit that I did not read your OP correctly and missed the bit
about "maintain the rpm under load".
You need to determine what sort of percentage variant you will accept,
and what cost you are happy to accept. I really feel you need to
specify some numbers before you ask again.
When you do come up with a spec please post here as well.
I'm just messing about actually with two old motors, which could be used to
drive various small things I can think of. I'm not sure seeking speed
regulation is that worth it, well, it depends on whether it's expensive or
really difficult to achieve. I think it might be made somewhat more
problematical because I don't have a Permanent Magnet motor.
My input supply range is up to 20V DC at up to 5A. The motor is 12V, series
wound, and has 11 commutator segments. It's a Smiths motor that was
once on an blower for an old car. You know, the old squirell-cage type
I've just found this to mull over:
And I'm searching application notes and other useful web pages.
I think definitely "speed controller" is a key Google search term.
I'm in the middle of a similar project, using 12v 'seat motors' out of
a vehicle with adjustable driver and passenger seats. The thought
being that if the motor can drag a 20 stone person about it should
have no difficulty in doing what I want.
As to the controls, luckily I have a friend who is an electronics whiz
and he has suggested the control layout.
Basically, as I understand it, to have a smooth control of a DC motor
from low to high revs a Pulse Wave Modulator is needed. See the link
to eBay below which shows the control I am using.
Most transformer/rectifiers don't give a 'smooth' output, the voltage
can fluctuate quite dramatically and will often produce a voltage a
lot higher than stated on the label. For the best results you need a
'smoothed' power supply and although it is possible to build
capacitors into the circuit it would be easier to use a car battery
and keep it charged with one of the intelligent battery chargers.
There is also the argument that if you are driving something which
resists being moved by the motor then you don't need a smooth supply
as this 'resistance' will do the job for you. But don't overload the
If you do go down the PWM route make sure you include in the circuit
some fast acting fuses to protect the PWM.
I think I should correct some misunderstandings, whether real or
imagined I'm not sure.
A PWM circuit just switches on and off very fast, and adjusts the length
of the ON and OFF pulses according to some input, often a static voltage
from a potentiometer.
This is useful as a sort of "partly-on" electronic switch, where using
eg a variable resistor would waste power (dissipated in the resistor),
as modern power FETs can switch on and off very quickly.
Normal simple voltage controlled PWM's will vary the ON-ness ratio
according to a static voltage, derived from the position of a
potentiometer - but this will *not* keep a motor at a specified speed
when under varied load, more circuitry is required to increase the ON
pulse width when the motor begins to slow down under load.
A speed controller needs more, it needs to have some method of measuring
the speed  - then compare that measured speed to the desired speed -
then create a difference input to the PWM.
If it does that fast enough, which if should be able to do, then the
speed of the motor shouldn't vary much whether it's unloaded or loaded
at full torque.
My point is, a speed controller will have this circuitry included - a
simple PWN won't. Others have said this, but I'm unsure who understands
 this is easy when a permanent magnet motor is used, as the
transitions between segments on the rotor, with changes in the flow of
the current, are easy to detect electronically
-- Peter Fairbrother
It is simple to measure the back EMF generated by the motor when the "switch"
is off. this gives a linear indication of speed and has been used for the last
three decades or so by good controllers.
Conversely, a decent shunt wound, brushless or permanent magnet motor has
fairly good speed regulation anyway. Series wound motors should be rewound
with shunt field coils if any sort of speed stability is desired.
As this is and engineering rather than electronics group, how about
building a centrifugal governor as found on old steam engines? Let the
moving arms adjust a potentiometer and you don't need any electronics at