SILICON CHIP has produced a number of DC speed controllers over
the years, the most recent being our high-power 24V 40A design featured in the
March & April 2008 issues. Another very popular design is our 12V/24V 20A
design featured in the June 1997 issue and we have also featured a number of
reversible 12V designs.
For many applications though, most of these designs are
over-kill and a much simpler circuit will suffice. Which is why we are
presenting this basic design which uses a 7555 timer IC, a Mosfet and not much
else. Being a simple design, it does not monitor motor back-EMF to provide
improved speed regulation and nor does it have any fancy overload protection
apart from a fuse. However, it is a very efficient circuit and the kit cost is
There are many applications for this circuit which will all be
based on 12V motors, fans or lamps. You can use it in cars, boats, and
recreational vehicles, in model boats and model railways and so on. Want to
control a 12V fan in a car, caravan or computer? This circuit will do it for
Fig.1: the circuit uses a 7555 timer (IC1) to generate variable width pulses at about 210Hz. This drives Mosfet Q3 (via transistors Q1 & Q2) to control the speed of a motor or to dim an incandescent lamp.
While the circuit can dim 12V halogen lamps, we should point
out that dimming halogen lamps is very wasteful. In situations where you need
dimmable 12V lamps, you will be much better off substituting 12V LED lamps which
are now readily available in standard bayonet, miniature Edison screw (MES) and
MR16 halogen bases. Not only are these LED replacement lamps much more efficient
than halogen lamps, they do not get anywhere near as hot and will also last a
great deal longer.