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Project Notes

#330 Power/Any2AnyPowerPack

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

Input Power Rectifier

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.








Credits and References

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This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.