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)

Comments

[throbscottle]

Nice! What application did you use to draw your schematic?

[FreeHighFives]

Does the current remain the same at the very end?

[Adri3l]

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?

[zack247]

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

[stuuf]

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.

[Adri3l]

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.

[zack247]

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

[stuuf]

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.

[qwertyboy]

will the LD1084V work for this?

[xiexie]

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?

[gedion4000]

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

[stuuf]

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).

[gedion4000]

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

[12V]

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

[silencekilla]

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

[Charles IV]

Would this be safe hooked up to 120v DC?

[bwpatton1]

Where would you get 120v DC?

[stuuf]

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).

[Charles IV]

Alright thanks

[ReCreate]

whats the maximum input/output for this?

[joinaqd]

i made one of those just from an old headphone volume control...it started melting because the resistance made heat!!!

[qwertyboy]

can i use a LM317?

[stuuf]

An LM317 will work, but the dropout voltage will be considerably higher, which limits the output voltage you can get from a low-voltage supply. With a 5V input, the highest output the LM317 will give you is about 3.3V compared to about 4V for LDO regulators (if you put more current on the load you'll see an even larger dropout)

[ironaxe69]

what size capacitors???

[stuuf]

I think I used 100μF for the output and 10μF for the input, the regulator datasheet has some information about what sizes to use

[ironaxe69]

A simple question. where do you buy your parts? I started looking, and am having a hard time finding the mc33269t voltage regulator.

[stuuf]

I got most of those parts from http://www.allelectronics.com/

There are a lot of other regulator parts that work the same way that you could use instead, such as the LM1117

[frollard]

Great instructible!! Do you find that once a load is connected the voltage coming out of the regulator changes? i.e. a lot of wall-wart transformers will put out a much higher voltage when there's no load attached.

[stuuf]

The reason why the voltage from a simple transformer will vary depending on the load is because they don't have any kind of regulation; as you draw more and more current, the voltage drops across the transformer windings, diodes, and wires will increase, reducing the voltage available at the end.

[frollard]

Awesome, thanks for the info.

[alex-sharetskiy]

well if you don't short it out, the voltage will pretty much stay the same

[Derin]

he is probably talking about the open circuit voltage also,if the circuit is forced to the top current(like short) it will drop