I need to build a simple power supply:- Mains input, 3 - 12 vdc (adjustable) output, max load 5A, no smoothing or high stability necessary as it's only to drive the motor of a small centrifugal pump. I've been out of electronics design for so long now that I don't know how such things are best done these days. Can someone point me in the right direction ?
Jim Hawkins
A simple solution is a rectified AC into a transistor with a pot altering the voltage to its base.
Other solutions are:
1) variac -> transfomer (for isolation)->bridge rectifer. 2) transformer -> bridge rectifier -> switchmode -> rectifier. 3) computer supply, 3 position switch between +3.3, +5, and + 12V. 4) bridge rectifier -> switcher with transformer. 5) 12V 5 A adapter with HUGE variable resistor. 6) 12V 5 A adapter with PIC pwm generator and power MOSFET. 7) Use parts from US spy satellite if you find them. 8) 12V battery, switch between cells. 9) Solar calls, with curtain to control output voltage. 10) Thermonuclear generator RTG with big potmeter in series. OK the next for somebody else..The Neural Net
Look into Pulse Width Modulation. It doesn't suffer from *low starting torque* problems.
Depending on the type of motor, PWM might be the way to go, albeit that's more involved than just a variac and a transformer. Most motors just give up on torque when you limit the voltage.
Good Luck! Rich
How about: Pop into a computer surplus shop. Get a rated used computer power supply. Guessing $20.00... Or try E*bay. That way you start off with 12V with lots of current capability.
I imagine then next step is to get or make a PWM controller to vary the motor speed. (Motor control is not my area of electronics.)
This is just a low power example I found in a 3 second Google search.
D from BC British Columbia Canada.
Computer supply -> pwm555 -> Motor
Sounds like a good solution. Skip JP2, Output on JP1 p2-p1, Maybe something beefier than BD140 is needed. Btw, how about a N-MOSFET or IGBT instead of a transistor..?
The PWM suggestion is good. Put a fast diode across the motor, that's important when switching an inductive load.
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