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17Ah Charger with USB??


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If you already have the charger and want a little challenge, then why not add the USB port yourself? Saves the waste and money of buying a new one. Here's one way of going about it:

1. Find pin diagram

http://en.wikipedia....rsal_Serial_Bus

So pin 1 is +5V and pin 4 is ground. You won't need the data pins.

2. Find a cheap USB extender cable. That will give you the socket and it will nicely pre-wired. Chop cable. Use current tester to ID pins 1 and 4.

3. Get yourself a voltage regulator (they cost pennies) an LM7905 should work. It takes a DC voltage in (up to about 30V) and produces 5V and max 1A out. You'll just need a couple of capacitors. The data sheet is here: http://www.fairchild...s/LM/LM7905.pdf the simple circuit is on page 12. Top circuit. Maplins probably sells everything you need. The circuit is so small that you can wire it all together without a circuit board. Just use tape or heat-shrink tubing to insulate the bare wires.

You won't blow up your stuff. :) Just make sure the voltage coming out of the plug is 5V and that pins are right. If those two things hold then you're good. It's probably good form to add a fuse 1A fast-blow fuse upstream of the regulator.

N.B. I'm not electronics expert, but I've often saved on buying extra DC power supplies in this way and it seems to work. If I'm powering say, a 12V item and a 24V item I buy the 24V supply and use a regulator for the 12V item. Saves space, wall sockets, and cash. Note that the regulators can get hot if you're pulling currents near their rated max. You can use the screw-hole to attach a heat sink of some sort.

Edited by umadog
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I really appreciate your going to the trouble, but I wouldn't know where to start with this. It's like me and maths, technical stuff just doesn't compute. I need a new charger anyway as this one's old and not holding its charge like it used to. I hope your explanation will help somebody though!

many thank!

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The 79xx series are Negative Voltage Regulators?

It's the 78xx ones are the Positive Voltage Regulators..

From a fully charged pack you would be dropping 8.8v falling to about 7.5v as the battery discharges for a 1A load this means you have 8.8w to dissipate so the device would need heatsinking a 7c/w heatsing would sit around 50-60c above ambient...

Peter...

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Ah, thanks for that, Peter. I didn't notice that it was a negative voltage regulator. Edit: Now that I understand the heatsink bit it's very informative. Thanks!

Edited by umadog
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