Power from audio jacks

[nobody]

Is it possible to draw power from 1/4" audio jacks? I'm thinking of building something and I need to be very economical about space and size.

The reason I'm even entertaining this thought is because this page says:

"The universal transmitter is powered by a built-in rechargeable battery; the 3G/4G iPod model is powered by the iPod’s remote/headphone jack, which means it reduces your iPod’s battery somewhat."

Would I have to wire things in a certain way to make this happen, or do audio jacks happen to pass along enough current to power a small device anyway?

[loydb]

The examples I've seen are all wired specifically for power. Be sure you label that jack really, really well.

[blue hell]

The iThingies use a four pole connector; ground and stereo (headphones) out and then the 4th one for the mic - and that on has power.

Normal audio inputs never have a provision for power, some mic inputs do tho - as in walkman like recorders and stuff a low voltage for electret mic pre-amps, or the 48 V phantom power in mixers as used for condensor mics.

[nobody]

Hm, I guess I should rule that out as a power source, then. Thanks, guys.

[egasimus]

Standard audio jacks can be used for power, with a proper cable, of course. In theory, you can also transmit both audio and power over a single wire in the same way phantom power is implemented in mic preamps - have a decoupling capacitor at either end, take power from before it and signal after it. You won't be able to carry much current this way, though - 10-20 mA is the absolute maximum AFAIK. And I don't really know how you'll remove the signal from the power - I guess you're going to need some sort of high pass filter for that.

[magman]

Another point to note is that standard jacks get shorted as you insert the plug, so you have to have a PSU that can stand shorts for a small amount of time.

You can mitigate this to some extent if you use TRS (stereo) jacks and use the tip for the +ve voltage and screen for ground, but there is also the worst case scenario of a partially installed plug.

If you want to use bipolar supplies, look for another type of connector.

Regards

Magman

[elektro80]

Why not use one of the small DIN connectors that are intended for this kind of use?
These are pretty small and not expensive.

[nobody]

What I was wondering is if any kind of power already exists in standard 1/4" jacks, which I now see is not the case.

But I have another idea for power. I want to build small devices that plug into, for example, the jacks on the back of my synths. These devices need power, so what if I create a wall outlet power adapter that plugs into the first such device, which plugs into the second device, which plugs into the third device, etc. I assume there would be a limit to the number of devices chained in this way, and that limit would depend on the power adapter. Would this work? Could I perhaps build a power adapter that, when turned on, first sends a small signal through the chain to determine how many devices are currently connected and uses a step transformer or something to regulate the voltage appropriately?

(Kind of talking out of my ass here, my university electronics courses were quite a while ago.)

[jksuperstar]

Just choose a power supply that can supply more current than all the devices together need (say there are 5 devices that need 150mA each, then 5 * 150mA = 750mA or larger). No need to try & be "smart" about supplying just enough power.

If you used differential signals between those devices (on a mono signal) you can use the same setup as Phantom Power to send power along those lines.

[nobody]

You're right, that should work.