I need to design a power supply for a dc motor, 12V 4A. So far I have a 12V transformer, a KBU8M bridge rectifier and two 4700uF capacitors (in paralel) connected. I will also ADD a diode to the terminals of the motor. Since I don't have the motor with me yet I would like to ask for suggestions on this power supply. It will only have an on/off switch and nothing else to control the motor.
Anyone think this will be good enough for it to run properly?

 

If it was me, I'd pull the PSU out of an old computer. You might find one for free at a recycling dump. Almost any PSU will have a 12V supply and many will have enough capacity for your motor. PSUs are nice because they have built-in heat and over-current protections, a switch, maybe a fuse, a cord to plug in, and so on.

 

Do you think it will be able to properly supply the 4A of the motor, including any spikes in the start?

 

Assuming your motor is a DC motor, all you should need is the Br rectifier and the on off switch .

 

The thing I'm mostly afraid of is over-voltage on the motor. I quickly measured my circuit (the rectifier and two 4700uF caps in paralel) and I had ~17V measured on the DC scale. I know this was measured without any load in the power supply, but still, isn't it too much?

 

Do you think it will be able to properly supply the 4A of the motor, including any spikes in the start?

Yes, as long as it's rated for 4A or more. It'll have a label on it with the specs.

Like your bridge and filter arrangement, it may show 16v or more when not loaded. But the regulation at 12v will be very good under load.

Your unregulated circuit will probably be fine too but definitely not as precisely regulated. Your motor wouldn't be damaged by a voltage spike within reason - it's excessive prolonged current that will burn out the windings. If you have any doubts, measure the voltage across the poles while running. Anything under 14v wouldn't worry me too much as long as it's rated for continuous duty.

You didn't really say whether your transformer can handle that load continuously either. It should be rated for 5A or more, and that's a pretty big transformer.

 

Yes, as long as it's rated for 4A or more. It'll have a label on it with the specs.

Like your bridge and filter arrangement, it may show 16v or more when not loaded. But the regulation at 12v will be very good under load.

Your unregulated circuit will probably be fine too but definitely not as precisely regulated. Your motor wouldn't be damaged by a voltage spike within reason - it's excessive prolonged current that will burn out the windings. If you have any doubts, measure the voltage across the poles while running. Anything under 14v wouldn't worry me too much as long as it's rated for continuous duty.

You didn't really say whether your transformer can handle that load continuously either. It should be rated for 5A or more, and that's a pretty big transformer.

Thanks for the explanation.
My worry regarding the power supply is that the open circuit voltage was way too high, and I'm not sure of how much that drop would be when the motor would be connected.
But regarding the spikes, it was not with the motor I was worried, but with the computer psu. I will add the free-wheeling diode, but still, a psu burning due to the motor starting wouldn't be very nice
And yes, the transformer is rated 5A.

 

Depending upon the type of DC motor you may be able to operate it directly from the rectifiers without any filter. That will provide a pulsating DC voltage with an RMS value of 12V.

 

Good point crutschow. Leaving out the capacitors will avoid the tendency of the suply to charge up to the peak voltage of the sine wave, thus arriving at exactly the right voltage.

DC motors aren't very picky about their power supply. Be sure to measure the current at full load to make sure the motor isn't going to overheat and you should be OK.