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Reply: Voltage Regulator Question



Jan 1, 1970
I need a voltage regulator to supply a small electric DC motor with a
variable voltage of 3 to 6 volts at about a max of 5 amps, the input
side is a 14 amp, 13.8 volt DC power supply.
A search of Google found a few things that might work but not exactly,
like the LM317 but it only is rated for 1.5amps max.

Any help or plans is greatly appreciated, I have never built a power
project so I am a definate newbie.


Five amps is a fair amount of current. Dropping 9 volts in the
regulator will mean generating 45 watts of heat to provide 15 watts of
power to the motor. No self respecting designer would do that.

You have two better options.

Chop the power (modulate on and off) rapidly with a variable pulse
width - "on" time varies. The motor probably won't care that the
power is switching on and off; its inertia is too great. Plenty of
simple 555 timer circuits to provide PWM and the output of the 555 is
great enough to drive a pass transistor (large transistor carrying the
current to the motor). These types of circuits are very efficient and
you would waste 3 watts or less (small heat sink)

If you are more flexible regarding the power supply, use an SCR
(Thyristor) speed control. see "basic version of the SCR throttle
circuit" I use that
circuit with some modifications to control four DC fans I use for
ventilation at home. With an SCR type controller you need AC into the
input and the SCR varies the firing angle (portion of the AC sine wave
that gets sent to the motor) The big advantage of this circuit is
that the motor sees nothing but smooth DC voltage. Waste power 3
watts in the control element and another 3 in the rectifier (something
that would happen inside your 13.8 supply anyway).

The problem to you: you already have a DC supply - but you could tap
into the stepped down voltage on the internal transformer (assuming it
has a large power transformer and isn't a switching supply) Or just
buy a power transformer and use that instead of the DC supply.

The chopper circuit can be further modified to supply smooth DC power
by adding an inductor to store energy while the transistor is off -
otherwise known as a switching power supply - but it isn't necessary
for an ordinary DC motor - but would be for a "brush less"
(electronic) DC motor. Waste power 3 watts or less. A little more

check out robotics web sites - those guys are always dealing with just
what you want to do - model railroad sites ditto. There are tons of
info on the web already.

All the good solutions require more work than just using a three
terminal regulator. (incidentally there are 3 term regs that can
carry five amps if you have a heat sink large enough to get rid of the
waste heat) See the LM338 regulator: 5 amps and 1.2 to 32 volt. has a simple PWM motor
speed control links to more