Adjustable voltage circuit
Ok, so the components have arrived. No doubt I’ve forgotten something and will have to order some more.
I’m starting with building the variable voltage circuit.
Parts list:
- Micrel MIC29152WT low drop out adjustable voltage regulator
- TO-220 heat sink
- small piece of strip board (also called vero board)
- solid core wire
- solder
- 5kΩ potentiometer
- 560Ω resistor
- 10µF capacitor
- 22µF capacitor
Tools required:
- Soldering Iron
- multimeter
- 12v power supply
Schematic:
This schematic is taken straight from the voltage regulator data sheet. Vin is the 12v from the power supply, R1 is the potentiometer and R2 is the 560Ω resistor. The capacitor on the left is 10µF and the capacitor on the right is 22µF.
I calculated the resistor values using the equation in the schematic. These values should give a good linear range. If you have a different value potentiometer just try and keep the ratio roughly the same between it and the resistor.
If you don’t understand the electronic symbols then i strongly recommend you look them up. There are guides out there with pictures instead but you will soon get stuck when it comes to the more advanced circuits.
The pins on the voltage regulator have a 1.7mm spacing so to get them to fit in the strip board i simply bent them as shown here:
Since this is a very simple circuit, i didn’t bother drawing up a circuit layout. I just built it on strip board:
If you look closely in the circuit above, you can see i’ve inserted a small piece of plastic between the regulator and the resistor. This is because they are very close to each other and i was worried about them shorting out. I haven’t attached the heat sink yet, I didn’t need it for testing and i forgot to buy anything to fix it on with.
I tested this circuit by hooking it up to a bench supply at work and testing the output with a multimeter but if you don’t have access to a bench supply, you can power it straight from the atx power supply. Connect the +ve to a yellow 12v wire and the -ve to a black ground wire then short the green wire on the power supply to any black wire. If your supply doesn’t turn on, it may be that you need to draw more current from it. Try attaching a fan, power resistor or old cd drive and trying again. There are many instructions on doing this on the internet, just search for ‘jump start power supply’ on google or follow this link
TG's Electronic exploration 
Awesome instructions…. I find your blog very informative …
Just a question. Its quite difficult to find Micrel MIC29152WT hence I thought of replacing with a LM317 LDO. It has similar properties.
And do you see any problems replacing the 5k pot with a 10k pot ?
the LM317 should be fine, however there are a couple of things to note:
1 – it has a slightly higher dropout voltage so your maximum voltage won’t be as high (should be around 10.5v)
2 – Its output voltage equation/typical circuit is slightly different, this means you will need to swap P1 and R2 over in my schematic.
Replacing the 5k pot with a 10k should be fine, but i’d recommend also doubling the associated resistor (R2)
Great project..I really like the idea of using a simple ATX PSU to build a powerful Bench PSU.
I have just graduated in electronics engineering and my final year project was a AC to DC inverter. I will soon start up with this project of yours and try out myself…
Great series. As a total beginner I was able to build piece togther the PSU with no problems following your directions.
Can we ground the variable voltage circut to the -12V line out to create a 0- ~24v adjustment? I’ve read that amperage will be limited by using the -12V rail, but was hoping it would still be feasible for low wattage applications. I tried putting it togther (with an LM317 for the AVR) but only get 18V out instead of the 21-24V I was expecting.
You can use the -12v instead of ground to get an additional 12v out of the supply. The lm317 has a dropout voltage of between 1.5v and 2.5v depending on current draw so you shouldn’t expect to get more than 22v in your application anyway, but i would expect more than you’re getting. To figure out why you’re not getting the full range of output from the voltage regulator, try looking at the data sheet for the LM317. Somewhere in there will be a standard circuit and an equation to calculate the output voltage, check your resistor values allow the full range.
So If I read the schematic correctly the 22µF capacitor needs to be polarized and the 10µF needs to be nonpolarized?
They can both be polarised. It doesn’t matter for this circuit
If you raise the voltage of the supply, you should also change all the filter caps, and other voltage sensistive componets in each circuit. Could the variable voltage be done by adding a POT to the sense line on the 12 volt protection pins? I’m just looking at the PS224 chip data now, and was hoping to find someone who has already explored this possibility.
Just wondering the wattage values you used for the POT, capacitors, and resistors.
I believe the resistors are 0.25W, but not sure. The POT is probably higher but its not necessary, 0.25W will be plenty. Capacitors don’t have a wattage rating so that’s not relevant. They do have a voltage rating though, which I believe is 36V, but again this is overkill as they should never exceed around 13V.
Hi, in the datasheet the is recommended minimum current load 5mA. I have same valued R1 and R2 as you, but when I calculated the minimal volatage current, it cannot be 5mA, it is only around 2mA (only R2 value, cos R1 is zero when pot is down). How is it please ?
This has been an interesting project. Thanks for posting this. I have run into one problem that I can’t figure out. Maybe someone can help.
As from an earlier suggestion, I used the -12V to get 24V in total. Works fine with a 10K Ohm pot. I’m using an arduino to measure the voltage on all of the different output. the 3.3,5 and 12V output have voltage dividers but since the 24V line requires a -12V ground line to get a result within the 1-5V range for the arduino to read, I’m not sure how to proceed, If anyone has any ideas, I would really appreciate it.