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.
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.
- Attribution for the center-positive symbol: Wikimedia Commons.
- Attribution for the center-negative symbol: Wikimedia Commons.