Variable Power Supply using LM317 and 7805 ICs

My power supply front and inside board (hand drawn PCB)

Power Supply Schematic
(Let it be known that a cooling fan and some ventilation might be needed if you drive the voltage Adjustable Power supply schematic V2regulators past about 250mA. They get hot and to protect themselves they begin to limit the current output. A fan, some holes, and bigger heatsinks than the ones that I used can help this.)

The fan’s power supply is just like the LM317 on the original schematic. The JPG to the left is the new schematic.

One of my classmates, Jared, had the idea to make a power supply the other day. He found a site with a pretty good design, but we came up with something much better while we made his. It requires some extra parts and has a little more “engineering” behind it, but it is worth it. The total cost should be no more than $50.00, but it pays for itself in 9v batteries since I can’t seem to keep them around. The 5v output eliminates the need for a 9v battery and a regulator when I tinker with microcontrollers, too. It’s a fun project, but useful as well.

This power supply has two ground (negative) outputs and two positive outputs (5 volts and variable). We used a mixture of parts in the design ranging from to The main thing is that the transformer was the least expensive from Radioshack. The voltage is variable from 1.3 volts to about 23 volts, and is capable of delivering 1.4 amps from the LM317 and .9 amps from the 7805, not to exceed 1.5 amps total to preserve the transformer’s life.

The best thing to do is to hand draw the PCB. It isn’t a lot of connections and can be drawn in 10 minutes. That is much faster than designing it in EAGLE, finding a copy machine, ironing the toner, and filling any blank spots where the toner did not transfer.

Parts list: $50 MAX!
1 – 273-1512 – 25.2v 2.0A transformer – $10.49EA –
1 – 276-1146 – 50V 4A bridge rectifier  – $1.99EA –
2 – 276-1778 – lm317 1.5A adj volt reg – $2.29EA –
1 – 276-1770 – 5v 1A voltage regulator – $1.59EA – or
KA7805ETU-ND – $.50 –
1 – 271-1714 – 5k Ohm linear pot – $2.99EA –
1 – 274-407 – hexagonal knob for pot – $1.99EA –
1 – 270-1807 – 7x5x3″ box – $5.99EA –
3 – P824-ND – 1uF electrolytic cap – $.14EA –
3 – 493-1323-ND – 2200uF 35V electrolytic cap – $.79EA –
2 – J106-ND - red banana jack – $1.33EA –
2 – J107-ND – black banana jack – $1.33EA –
1 – P13405-ND – 24V cooling fan – $7.59EA –

1 - 220 ohm resistor
1 – 270 ohm resistor
1 – 4.7k ohm resistor
1 – 1.2k ohm resistor
1 – 1k ohm resistor
1 – on/off switch capable of 120volts/2Amps
1 – 1A slow blow fuse and holder
1 – power cord
Wire, solder, copper pc board, tools
1 – voltmeter (0 – 25 or 30 volts range) [buy a cheap analog meter locally and pirate the meter or look on ebay] If you pirate from a cheap analog meter, keep in mind that some require an inline resistor. Most meters are 1 microamp meters. If this is the case, full deflection would be .000001Amps. If you wanted full deflection to be 30 volts then you would need a 30Meg ohm resistor in series with the meter (Resistor = Voltage/Current) [R=30V/.000001A). Be sure you know before you go! If you are unsure use a 30Meg resistor to test the meter and lower it as needed until it deflects. Trial and error on the safe side when you do not have a datasheet.

Some of these parts can be taken from your junk pile. If you do not have a junk pile then you need to start one. They are indispensable. If your wife complains about the junk pile, get some plastic tubs and hide it under the bed or buy a barn.

The photos below show the fan mounted into position. I was testing the fan speed versus voltage in these photos. I had not built the power supply for the fan yet. I simply used the original LM317 to test the fan. Then I build a new circuit board and spliced it into the original circuit. I tested this power supply at 1.5 amps (15volts with a 10 ohm load). It held 1.5 amps for several minutes. The air entering the box was at 25°C (77° F). The air exiting the box was at 34°C (93° F). The cooling fan was doing a great job of moving heat.

fan mounted into positionside view of fan

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About robbie

I am an electronics enthusiest and a ham radio operator (W1RCP). I like to play with electronics. It's fun and educational. I looked forward to working in the engineering field in the future. I have a BS in Electronics Engineering Technology from DeVry University. I also have an Associate's degree in Marketing Management from Moultrie Tech, and a diploma in Electronics from MTC.

2 thoughts on “Variable Power Supply using LM317 and 7805 ICs

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