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# How do I regulate voltage in my power supply? Answered

I'm a beginner so pleaase deal with me. I'am building a AC-DC power supply. The transformer I'm using has an output of 12.6V and 3A. I used a full-wave rectifier to turn AC into DC. Now I need a way to regulate the power supply's output voltage. I did some research but I'm still not sure what I should do. I could use a potentiometer, or substitude one of the resistors in a voltage resistor for a potentimeter ... whats the difference? If I do use a potentiometer  how do i choose which one I need? Do I use an adjustable volage regulator?The load Ill mostly use it for requires 2.3A and a voltage of 3-12.6V. But I want the freedom of using it for other stuff. Also, in what way can I regulate the current? Sorry for all the questions, Im just confused after everything Ive read online.

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## 4 Replies

Just get one of these simple solutions:

http://www.amazon.com/DROK-Converter-Constant-Curr...

There are quite a number of different variants of this, they can supply up to 3A usually (make sure to glue a small heatsink onto the LM2596.)

Linear voltage regulators like the 7805 and adjustable lm317 are nice for small loads, but when there is a big difference between the voltage on the input and output (like 12.6V input, and 3V output) that times the current (2.3A) is power dissipated: In that case, its about 22W!!!! You need a huge heatsink for that! That is how much heat a soldering iron produces. If you need to buy a heatsink, then you need to know some thermal equations to calculate the heatsink needed. The equations are really easy and straightforward lucky.

You can even go totally bare-bones circuitry and use a simple transistor like a MJE3055, and have the collector tied to the positive 12.6V tap, then the emitter does to the load, and the base is connected to a voltage reference capable of delivering 100mA. Thats a little much current for a potentiometer, so that should be attached to another transistor stage, like a preamplifier. That emitter-follower can then drive the big 3055 emitter follower, and you basically end up with a darlington pair. That will have decent load regulation (output voltage doesn't change much when small or big loads are connected) but the output voltage is dependant on the input voltage, so it has bad line regulation. *(Basically none) A Zener diode can be used as a reference voltage instead. Here is that circuit I am referring to (note: that one uses just one transistor. Replace it with a darlington pair or a Sziklai pair):

iceng (author)2015-02-02

Do Not use a potentiometer .... there is heartbreak potentially if you do !

Follow the suggestions of max, Steve and mpilch to use a power semiconductor to do the output you need.

steveastrouk (author)2015-02-02

You need a regulator, but you always have to lose some voltage across the regulator. The 317 always loses a couple of volts.

mpilchfamily (author)2015-02-02

You need to use a voltage regulator like an LM317. A resistor is used to set the voltage output. Refer to the date sheet for details. But you can use a pot to adjust the output voltage. You'll be limited to the current rating of the LM317 unless you run a couple in parallel. You'll want to look up so known good designs here and follow them. As for current. You don't regulate the current but you can limit it. The projects you use will draw the current they need. If they exceed the current rating of the regulator then you can damage the regulator.

Building a Power supply and getting it right can be tricky and most electrical engineers will avoid PSU design if they can. Depending on what you want to power you may need to ensure you have a good clean voltage. So you'll need additional circuitry to clean up the noise from the AC line. Which can be difficult to design without the proper equipment. If you plan to use this to power projects you may want to buy a suitable bench top power supply or build off an existing power supply like a PC PSU.