# Electric circuit AC plug to DC Design Help

I'm new to designing electric circuits... but I have a simple project that I really need help on. All I need is a AC plug source to be connected to a 100 WATT bulb and a 3V motor(motor should only have 3V across it). Also, Ive read that I need a rectifier to convert AC to DC for the DC 3V motor? Can anybody point me in the right direction on how to do this... or how to learn how to do this?

Here's what my schematic so far looks like (it doesnt work)

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Accepting an answer so quickly is discouraging to others who otherwise also would have posted an answer. Now you'll possibly get less replies/ideas. (I don't think your currently accepted answer is a very good one.) –  Federico Russo May 16 '12 at 5:12
@FedericoRusso, if you think it a poor answer why not downvote? –  Kortuk May 16 '12 at 8:21
@Kortuk: yeah, right, to get a revenge downvote again? I still have one from last time! :-( –  Federico Russo May 16 '12 at 8:29
@FedericoRusso, You win, you successfully made me almost choke on my drink when I laughed. –  Kortuk May 16 '12 at 8:31
I will clear these comments out soon as off topic but I will say although that answer is not great it probably does not deserve a downvote. @stevenvh, I cannot even tell exactly whom voted for what on a single question, that is a dev only ability. I can check total counts to look for voting rings and such though. –  Kortuk May 16 '12 at 8:44
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No, that circuit won't work. You have a closed circuit for the lamp (the right loop), but the motor and resistor are just shorted; the loop they make doesn't have a voltage source in it.
The solution is a DC power supply, which unfortunately consists of quite some components.

First you'll have to lower the 120V/230V mains voltage to a more workable voltage to go to the 3V. That's what a transformer does. Next, you'll have to rectify the low voltage AC that the transformer outputs.

The bridge rectifier takes care of that. The smoothing capacitor is needed to remove the bumps from the rectified voltage. You then have a more or less smooth voltage, but it will vary with varying mains voltage and with a changing load of the motor. If you need a clean 3V DC you'll have to end with a voltage regulator.

If you don't need the regulator you'll need a 3.6V transformer, which will be hard to find. A 5V transformer will give you 5V DC across the capacitor. Replace the 7805 in the schematic by a 3V LDO voltage regulator to drive your motor.

edit
jippie makes an excellent suggestion.

Wall-warts like this one exist that output a regulated 5V DC. This includes everything in the schematic. If your motor accepts 5V you can directly connect it to it, if it has to be 3V you'll need the LDO voltage regulator.

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In these cases I always dive into my box of old telephone (and alike) mains adapters and find one that comes near to my requirements. Most are already DC. For example, I have one lying on my desk here that says: INPUT: 100-240V~ (which means it can be connected to mains) and OUTPUT 5V= 2000mA (which means it delivers 5V DC [DC is indicated by the '='] at 2000mA. Surely we can write a bit more on this topic when needed. –  jippie May 16 '12 at 7:19
@jippie - Didn't think of that. Thanks for the suggestion. Edited my answer. –  stevenvh May 16 '12 at 7:26
You can't beat the price of these things with a DIY power supply, not to mention the output specs. –  jippie May 16 '12 at 7:33
And with a circuit diagram as in the question, I would suggest working with mains is an unwise proposition until a little more experience is gained. –  Cybergibbons May 16 '12 at 8:05
@Cybergibbons - Yeah, especially since he would need a 40Hz transformer... –  stevenvh May 16 '12 at 8:51