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      A Few Simple Rules   05/22/2017

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B.J. Porter

Low current, low voltage DC to AC inversion.

8 posts in this topic

So I wanted to add a better set of speakers to the ship's PC for movie night. Simple enough, the PC runs off of native 24V, I bought a 24V - 19.2V DC-DC transformer for the monitor and snipped of the brick and wired it in. Surely if I buy some PC speakers with a brick I can do a similar trick, right? Because down here in the Caribbean choices are limited in terms of what you can get, and I couldn't get anything loud enough that didn't run off of AC power. My hope was to run the speakers directly from battery power without having to fire up the 2400W 110V inverter, that way I could wire them directly to the power panel and dispense with power cords and bricks all over the place.

 

So I picked out a pair of low profile speakers and checked the power block - 10V nominal voltage. Odd voltage, but I should be able to find something. Sure enough, I can find a variable DC-DC converter from China on E-Bay that will let me convert from 5-32v DC in, to anywhere from 5-32V DC out. $13 with free shipping, what can go wrong?

 

Until...I looked more closed at the stupid power block for the speakers. It is an AC-AC block, producing 10V AC power (500 milliamp), not DC. WTF? I put a multimeter on it to be sure it is in fact 10V A/C, not DC like any right minded engineer would use.

 

Attempts to find a small inverter circuit to produce 10V AC power at very low currents (half an amp!) have proven fruitless. I don't need lots of wattage here at all, but I need to figure out how to get this odd voltage from my ship's DC banks without running a 40 pound inverter that wastes more power on it's overhead than I need.

 

I'd love to see 12V or 24V DC in, 10V AC out. Small and cheap; I already have a big inverter with wall plugs, and the stupid little bricks for charging cell phones and whatnot - what I am looking for is a solution to permanently wire to the boat out of sight to power the speakers like the rest of the PC is powered.

 

 

Any ideas?

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I would take the speaker apart and trace the power circuit and see where it goes to DC, measure the voltage and then wire in a 12V DC to DC supply there. You may be able to get away with feeding in 10VDC, but it really depends on what's next in the circuit.

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Low voltage AC input devices are designed to work with wall wart stepdown transformers.

 

That little black box that plugs into the wall spits out 10-20 volts AC which then lets the device it plugs into circumvent all the UL high voltage regulatory stuff.

 

 

Bottom line: You may be able to feed low voltage DC into that Low voltage AC input.

 

Why? First thing that happens when it gets inside your device is it hits some diodes and gets converted to DC anyhow.

If you feed it DC it only uses half the recitifier system.

 

Need to know more about the actual device you got to feed with juice.

 

Got schematic someplace?

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What Jerry2me said:

 

If they're cheap - plug in the 10VDC and see what happens.

 

Chances are that it gets rectified first thing anyway.

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This may not be very helpful, but I agree with posts 2-4. If these suggestions don't work, and most likely they will, you can move on from there. Plug the cheap sucker in!

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What Jerry2me said:

 

If they're cheap - plug in the 10VDC and see what happens.

 

Chances are that it gets rectified first thing anyway.

yeah but it could be a half wave rectifier. if you plug the DC in and it does nothing, reverse the polarity of the DC input and see if lights up then.

 

half wave rectifiers save a little money - fewer diodes - at the cost of noisier power.

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If you have 10vac going in, this will become 12vdc inside the speaker. As suggested in the previous post, look inside the speaker, the 10vac is being converted to 12vdc by a diode rectifier bridge. There will be an electrolytic capacitor( round barrel looking thing with plastic wrapping) at the output of the diode bridge.

Measure the voltage across this capacitor. It should be about 12v. If so connect the 12v here ( observe polarity).

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