# Driving a DC motor

Discussion in 'Misc Electronics' started by ptek, Jan 4, 2004.

1. ### ptekGuest

Hi,

I pretend to control a small dc motor (about 5 - 6V) using a
microcontroller (PIC) and I think I need some precautions since the
motor may be an inductive load...

What I want to do is this : the PIC connects to the base of a
transistor working in saturation zone, so that is behaves like a
switch turning on and off the motor's power supply (you know, the
motor is wired to the power source and connected to the transistor
colector, the usual stuff.)

By doing PWM on the signal outputed by the PIC to the transistor's
base, it's possible to control the motor speed (the motor is "quickly"
beeing turned on and off).

Since the motor is an inductive load (all dc motor are? correct me if
i'm worng) each time I power it and "unpower" it, a spike of current
is generated so the life of this circuit would be seriously shortened
....

What is the easiest yet functional way to protect it agains these
matter ... A small schematic or a detailed description of an example
circuit is really welcome.

Thanks a lot

Pedro Duarte

2. ### Jerry G.Guest

You can use a transistor to drive the motor. Use a series resistor on the
base of the transistor to limit the base current. You would have to work
out the value according to the amount of required base current required.
This will isolate the base a little, and thus give the transistor some
current limiting.

Put a small capacitor arrangement across the motor. Take care for the
polarity of the cap (as normal). You can typically use a 10 uF / 25 V in
parallel to a 0.1 uF / 100 V cap. This is done very often in many types of
devices using small DC motors. These are very common type voltage ratings
at the values indicated. The idea of the smaller cap is to suppress any
high frequency noise that the larger cap cannot deal with because of its
structure. The larger cap is the main suppressor for the lower frequency
noise or spiking factor.

Across the transistor you can put a silicon type rectifier diode. Take
regard for the polarity of the diode. You want to have the diode arranged
so that it only conducts across the transistor inversely. This will suppress
any reverse polarity noise from going across the transistor. Only when the
transistor is forward conducting the motor should run.

The PWM effect should still work. The cap arrangement across the motor is
not large enough to cause any delay effects, or cause excessive loading.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================

Hi,

I pretend to control a small dc motor (about 5 - 6V) using a
microcontroller (PIC) and I think I need some precautions since the
motor may be an inductive load...

What I want to do is this : the PIC connects to the base of a
transistor working in saturation zone, so that is behaves like a
switch turning on and off the motor's power supply (you know, the
motor is wired to the power source and connected to the transistor
colector, the usual stuff.)

By doing PWM on the signal outputed by the PIC to the transistor's
base, it's possible to control the motor speed (the motor is "quickly"
beeing turned on and off).

Since the motor is an inductive load (all dc motor are? correct me if
i'm worng) each time I power it and "unpower" it, a spike of current
is generated so the life of this circuit would be seriously shortened
....

What is the easiest yet functional way to protect it agains these
matter ... A small schematic or a detailed description of an example
circuit is really welcome.

Thanks a lot

Pedro Duarte

3. ### cpemmaGuest

You can put the same inverse diode across the motor instead, which will also
protect other electronics on the supply line, as shown
http://www.cpemma.co.uk/555pwm.html

4. ### Myron SamilaGuest

Actually, some brushless DC fans are impedance protected.

Also, in professional stage dimmer applications, we use opto-isolators and opto-couplers
to isolate the sensitive electronics from these types of problems you mention. ie: MOC
3021.