Improved Simple Adjustable DC Power Supply

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Introduction: Improved Simple Adjustable DC Power Supply

Work in progress: I'll add more text explaining how this thing actually works and a schematic image in the next few days.

There are already a few instructables about using linear voltage regulator chips to power low-voltage experiments and projects. This is my variation on those, using a low dropout regulator to allow lower input voltages and a coarse/fine adjustment system.

Step 1: Parts and Tools

You will need the following components:

LDO voltage regulator such as MC33269
TO-220 heat sink and mounting screw
Capacitors to filter the regulator's input and output circuits
240 ohm resistor
Two potentiometers: 1Kohm or more for coarse adjustment, 100 ohm for fine
I used trimmers for both pots, since they're compact and hard to accidentally adjust
Power connectors: a PCB-mount coaxial jack for input and screw terminals for output are usually the easiest to work with
3-4 square inches of perfboard
Some hookup or bus wire

You should have these tools available:

Soldering iron
Small screwdriver
Pliers and wire cutters
A sharp knife that can carve your perfboard such as a regular #11 X-ACTO blade. (or a 1/8" drill bit)

Step 2: Circuit Schematic

Check the datasheet for your regulator for details about what capacitor values should be used and how to wire an adjustable regulator circuit. Instead of a single variable resistor to adjust the voltage, I added a second one for fine adjustment. With the two resistors in series, the voltage depends on the sum of the two, so adjusting the fine control (which has lower resistance) will change the output voltage slowly.

Step 3: Mounting the Power Jack and Regulator

The most common style of DC power jacks uses two large pins that are too wide to fit in standard perfboard holes. To be able to mount the jack, you need to cut wider holes in the board. There is also a third terminal on the jack used for a switch. Since this circuit doesn't use that terminal, and since it gets in the way, you can bend it out of the way or cut it off.

New method: I've also discovered that it's easier to just drill three holes with a 1/8" bit, widening the existing perforations.

Medium- and high-power linear regulators use TO-220 packages that can be mounted to a heatsink to safely dissipate more heat. Heatsinks made for TO-220 devices are usually a block of aluminum with a 4-40 threaded hole tapped near the top.

Step 4: Installing Other Components

Mount the remaining components on the board and solder them together according to the schematic.

Step 5: How to Use It

Connect a DC power supply (AC adapter, USB cable, battery pack, generator, etc) to the input connector. Connect a voltmeter to the output and adjust the potentiometers to select the voltage you want. Finally, connect your load circuit to the output to give it power.

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    33 Discussions

    what do you call that pad that you sandwiched between the voltage regulator and heatsink?

    Is it true that the MC33269 cand only handle only 800mA max? I am looking for a replacement for a LM350T that doesn't seem to exist on the local market here and I'm a bit stuck. I need a 1-25v regulator that can handle as many amps as possible(i was thinking at least 2 amps). Do you have any suggestions?

    5 replies

    hey, you wouldn't have happened to have any luck with that? i need 24v @ 2 Amps.

    2A is a little high for a linear regulator. You might be able to pull that much current from a standard LM317 (they're guaranteed to at least 1.5A), but you'll need serious heat sinking. A switching converter will work much better.

    Or he could try to use a LM138/338 . The datasheet also contains the schematics for using it, you just have to change the values of the two resistors acordingly.

    a switching converter? sorry, im kind of new to voltage regulation

    http://en.wikipedia.org/wiki/Switched-mode_power_supply

    Harder to design (I haven't had a chance to build one that works very well yet) but more flexible and more efficient.

    I used an LM7805 to yield 5 Volt output. But the power supply didn't work. Is it possible it's due to excessive heat that I used when i solder the LM7805?
    If yes, how should I plug it to the circuit?

    do you happen to know how to build a power supply like this with a higher output? im trying to power several circuits, each with different power requirements ranging from 9 volts to 15 volts. if need be i would make more the one regulator but the idea is to use one battery pack to get the job done

    2 replies

    As long as you have a battery with a higher voltage than you need you can connect as many different regulators as you want to create different voltage rails, just use multiple copies of this circuit with different regulators or feedback resistors in parallel with the same Vin and ground lines. If you need regulated voltages across a wide range it might make sense to run the lower ones from a tap in the middle of your battery pack to reduce power losses and heat. But then you also have to deal with some of your battery cells discharging faster than the others. If you need more current that your regulator can supply, sometimes you can wire two or more in parallel, or add an external transistor (regulator datasheets usually include a "current boost" application circuit).

    thanks, I didnt know you could use them in parallel, i will play around with that and see how it works out. thanks again

    how much current can this cope with when connected to 18v?

    how much do all of the supplies cost. and if possible can you make one for me and i will buy it from you

    With the parts I used, no. The MC33269T only handles up to 20V input, and the input cap is probably only 50V. Other regulators have different maximum input voltages, but I've never seen one as high as 120V (not that they don't exist).