LM317 to Regulate Wall Wart

Joshua R. T. Purba
Electronics

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.

The labels are: Panasonic PQLV19BX AC Adaptor, Input: 220-240V ~ 50/60Hz 40mA, Output: 6V ⎓ 500mA. Under that there's some regulatory/compliance symbols. Under that: Caution, indoor use only. A center-negative symbol. Lastly: Made in China.

The wall wart I used for my speakers.

Using a multimeter we can see the 10.06V DC open-circuit output of the unregulated 6V DC wall wart.

Open circuit output of the wall wart.

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.

Test setup: knock-off LM317 with 9V DC input, open-circuit output is 6.064V DC. It drew 5mA from input.

Test on breadboard: knock-off LM317 open circuit output.

Test setup: knock-off LM317 with 9V DC input, load is 100Ω, circuit output is 6.09V DC. It drew 64mA from input.

Test on breadboard: knock-off LM317 output on 100Ω load.

Test setup: knock-off LM317 with 9V DC input, load is 10Ω, circuit output is 5.481V DC. It drew 491mA from input. As you can see, the knock-off LM317 dropped out of regulation. I also redid this test with heatsink on the knock-off LM317, same result: dropped out of regulation.

Test on breadboard: knock-off LM317 output on 10Ω load.

Test setup: original National Semiconductor LM317 with 9V DC input, load is 10Ω, circuit output is 6.08V DC. It drew 619mA from input. I ended up using the knock-off instead of the original National Semiconductor.

Test on breadboard: original National Semicondictor LM317 output on 10Ω load.

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.

The soldered circuit on the perfboard. I snapped off the corners so the perfboard can fit inside the enclosure later.

Top view of the soldered perfboard.

The soldered circuit on the perfboard viewed from the bottom.

Underside of the soldered perfboard.

The knock-off LM317 on heatsink tested on the perfboard. Power from the wall wart. The load is 100Ω. The unregulated wall wart voltage dropped from 10.06V to 8.48V. The regulator output was 6.023V.

Test on perfboard with 100Ω load.

Test setup: power from the wall wart. The load is 10Ω. The unregulated wall wart voltage dropped from 10.06V to 6.115V. The regulator output was 4.002V. I had tested that my speakers worked even with 4V, so 🤷.

Test on perfboard with 10Ω load.

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.

The enclosure broke when I drilled it. Just my luck 🍀.

Broken enclosure.

I glued the pieces back together and resumed drilling.

Fixed enclosure.

After the holes were done, I fitted the perfboard in the enclosure. This photo made it clear why it was necessary to snap off the corners of the perfboard.

Fitting the perfboard in the enclosure.

Double checking that the output plug voltage and polarity was correct (6.039V open-circuit, center-negative).

Verifying the output plug polarity.

Testing the circuit as intermediary between the unregulated wall wart and the speakers. The power indicator LEDs of the speakers lit up (dim in this photo because I used flash). The stereo plug is not connected in this photo. Afterward I connected it to my computer and it functions OK 👌

The circuit was powering the speakers.

That’s it folks.

Notes: