LM317 to Regulate Wall Wart

My speakers (not these other ones) need 6V DC. The connector is 5.5mm × 2.5mm barrel jack, center-negative. The wall wart I used for these is 6V DC 500mA max center-negative too, but unregulated. The open-circuit voltage is 10.06V.

Thus far the speakers functioned just fine with this wall wart. However, I am not keen on waiting for the magic smoke to come out. So I made a regulator circuit to limit the input to the speakers at most 6V.

LM317 regulator circuit schematic showing the components and wirings.

The electronic schematic for the LM317 regulator circuit with 6V DC output.

I want to be able to handle the barrel jack input in whatever polarity. I mean the circuit should be able to handle center-negative input and center-positive input. I also want the same for the output. The easiest way to achieve this is to use DPDT switches on the input and output. However I didn’t have those so I implemented this using pin headers and jumpers.

The LM317 that I used here is a knock-off, even with just around 491mA current draw it dropped out of regulation. As comparison I tried the same circuit but with a known-original LM317 (salvaged from the board in a previous post). The original was able to regulate at around 600mA even without heatsink. But since I didn’t think the speakers won’t to draw that much current, I used the knock-off LM317 in the final circuit.

Initially the input reverse polarity diode (D1) was a BAT85S schottky diode. But according the datasheet the max forward continuous current is 200mA. I want more headroom so I replaced it with a 1N4001 diode. For R1 and R2 I had no resistors with exact values, so I put in several resistors in series to get closest values I can. I didn’t have the 25μF electrolytic capacitor for the C3, so I used the closest I had which was 22μF.

Another thing: the enclosure cracked when I drilled it 😔. It’s been awhile since the same thing happened before. I glued the pieces back using cyanoacrylate adhesive and resumed drilling it. Went fine. I put the circuit in the enclosure, fastened the lid, and put it to use.

That’s it folks.