off-the-shelf variable output AC/DC switching power supply?

[[email protected]]

i am trying to control the voltage to a DC brushed motor. it draws
about 1.5A at full load. its working voltage is 12V. i know that there
are many ways to do this, but I would like to do it with an off-the-
shelf, variable output (controllable with a external pot) switching AC/
DC power supply. by variable i mean 0 to 12V.

anyone know who might sell one of these? ive been looking about but
they all seem to be fixed output or only adjustable a few percent with
an on-board pot.

OR:

if there is an IC from linear etc... with a datasheet giving part
numbers for an equivalent circuit, that would work

 

[whit3rd]

i am trying to control the voltage to a DC brushed motor. it draws
about 1.5A at full load. its working voltage is 12V. i know that there
are many ways to do this, but I would like to do it with an off-the-
shelf, variable output (controllable with a external pot)

Lots of switchmode control chips can generate a
variable-duty-cycle square wave (a one-op-amp
triangle wave generator, a comparator, and a
potentiometer is all it takes). Add a switch transistor
and diode to the (presumably inductive) motor and
it's done.

Mostly, if you need 1.5A, you don't want to use an all-in-one
circuit. If you really are starting with AC, though, a
variable autotransformer and rectifier might be simpler.. don't
go over the 12V limit, or disregard the shock hazard if you
go that route.

 

[Rich Grise]

i am trying to control the voltage to a DC brushed motor. it draws
about 1.5A at full load. its working voltage is 12V. i know that there
are many ways to do this, but I would like to do it with an off-the-
shelf, variable output (controllable with a external pot) switching AC/
DC power supply. by variable i mean 0 to 12V.

anyone know who might sell one of these? ive been looking about but
they all seem to be fixed output or only adjustable a few percent with
an on-board pot.

OR:

if there is an IC from linear etc... with a datasheet giving part
numbers for an equivalent circuit, that would work

DRiving a brushed DC motor with a variable DC voltage is a very
bad method of speed control, for several reasons. For one thing,
you waste a lot of power (unless you have a magical adjustable
switcher), and for another, their torque drops dramatically.

What you do is control the _current_, which torque is directly
proportional to. My favorite way to do this is to PWM a current
source, for high-side drive, or a sink, for low-side drive.

This can be as simple as an NPN tranny with a calibrated
resistor at the emitter, and a specific base voltage to
give a constant current out put, whose voltage can be
anything up to the breakdown of the tranny.

Good Luck!
Rich

Oh, yes. Of course it makes a difference if it's series-wound
or shunt. ;-)

 

[[email protected]]

DRiving a brushed DC motor with a variable DC voltage is a very
bad method of speed control, for several reasons. For one thing,
you waste a lot of power (unless you have a magical adjustable
switcher), and for another, their torque drops dramatically.

What you do is control the _current_, which torque is directly
proportional to. My favorite way to do this is to PWM a current
source, for high-side drive, or a sink, for low-side drive.

This can be as simple as an NPN tranny with a calibrated
resistor at the emitter, and a specific base voltage to
give a constant current out put, whose voltage can be
anything up to the breakdown of the tranny.

Good Luck!
Rich

Oh, yes. Of course it makes a difference if it's series-wound
or shunt. ;-)

oooh! neat-o idea i like the sound of it. couple questions...

so the current for the motor passes through the calibrated resistor?
you cant mean that right? unless its really low value?

so the voltage at the base stays fixed, but its pwm modulated? so i
turn off-and-on a certain set voltage (which sets my current pulse) at
the base?