In the past I’ve built several of these small power supplies to use around the house, and they come in handy. But those others used a 12 VAC transformer instead of an AC adapter, so I had to add a fuse and other stuff to prevent shock. Using the AC adapter is much more convenient because all of that is included in the wall wart. All I have to deal with is low voltage DC. Those power supplies used old designs that didn’t have good regulation. I wanted to replace them with a linear regulator chip which gives excellent performance.
Years ago I had built a 9 VDC shunt regulated power supply, low current, only 40 mA max. A shunt regulator wastes the power that isn’t being used by the load, in this case it was built for powering a pocket transistor radio. A Zener diode is a good example of a shunt regulator. If none of the Zener’s current is used, then the current is wasted heating the Zener diode. So I dissembled the shunt regulator and used some of its parts to build a new regulator using a LM317. I also scrapped another power supply for the circuit board and some parts. Both of these used an old transformer type wall wart with outputs about 13 volts no load. So I used the adapter rated 9VDC at 600 mA.
I’ve been building these small RF circuits and I’ve been using a bunch of bench supplies to power them. The circuits need very little power, but the voltage needs to be stable, so that’s why I decided to build a few of these with adjustable regulator chips. A few other parts and I’ll have a 1.25 to 9 VDC well regulated supply, that can put out about 200 mA to a load, up to 8 volts. The wall wart output drops as the current goes above 150 mA and the regulated output drops below 9 volts. If I had a better AC adapter that wouldn’t happen. But I can live with the lower current.
This new power supply uses a LM317 with a 270 ohm resistor from Output to Adjust pins, and a 2.5 k wirewound pot from the Adjust pin to negative. This would allow me to adjust the voltage up to 12V, but the adapter can’t put out that much under load, so I put a resistor in parallel with the pot to limit the voltage to 9 volts maximum. But even at 9 volts, if I put a load of more than 150 mA on it, the voltage drops below 9 volts.
I added some capacitors and diodes to the basic design to filter ripple, and help protect the circuit from overvoltage or reverse voltage. This is pretty much standard practice with the adjustable VRs.
I used several EMI / RFI suppressor sleeves on the input and output wires to prevent the RF from the circuits from radiating from the supply. I added a 2200 uF, 16VDC capacitor on the circuit board to help the capacitor in the wall wart.
The circuit fit on a perfboard the size of a large postage stamp. I got the heatsink from an old PC power supply, but I should mount the heatsink on the aluminum lid of a project box for better heat sinking.
