A 2.1mm DC adapter pack with battery bypass and any-polarity to any-polarity connectors.
I recently got a flamenco metronome that has a 2.1mm power connector but in centre-negative configuration. Unfortunately I don't have any centre-negative power packs on hand. I could of course just rewire one, but decided it might be the time to make up a generic any-to-any connector.
Features I'll add:
- centre-positive and centre-negative output plugs
- 9V battery bypass
- polarity rectifier, so that the input supports both centre-positive and centre-negative connections
- a power switch
- an LED power indicator
To protect against reverse polarity input power, I could just add a single rectifier diode in series. That would prevent the connection of power with the wrong polarity.
A little more fancy is a rectifier bridge that allows connection in either polarity. There is a price to pay however: loss of two diode drops from the power supply.
Since I'm not dealing with huge voltages here (planning for 5-9V), and DC operation so switching speed is irrelevant, there's a broad choice of rectifiers possible e.g.
- 1N4001-7 standard go-to rectifiers with a pretty high average Vf of 0.8V, but extremely low reverse current of 30µA
- 1N5819 offers extremely low Vf (0.34 V at 100mA), however the maximum reverse current at rated voltage is pretty high at 1mA
I decided to use the 1N5819 in the bridge rectifier to get the low forward voltage benefit. The reverse current is a little high, but at low voltages and currents perhaps negligible enough and in practice this should be swallowed by the bridge rectifier (just showing up as a reduced efficiency)
Some measurements to back that up:
| Diode | Input Voltage | Output Voltage, No Load | 1kΩ Load | 10kΩ Load | 100kΩ Load |
|---|---|---|---|---|---|
| 1N4007 | 8.72V | 8.13V | 6.88V | 7.54V | 7.78V |
| 1N5819 | 8.72V | 8.62V | 7.97V | 8.29V | 8.43V |
PS: the "evils" of centre-negative jacks has now been explored in EEVblog #1015 - some great background and demonstration of how these work.
