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I want to make a adapter from a vehicles 24v to a 5v usb phone charger. Can I simply connect the wires from a cigaret lighter jack to a usb and add a resister in between (using a voltage divider)? If so does it matter how strong the resistors are (assuming the ratio is correct?

Daniel
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  • there are usb chargers which could be mounted on cigarette lighter jack. Oh you need to build it DIY. Then why don't you study some reference designs. – Standard Sandun Sep 27 '13 at 11:33
  • why down-voted? – Standard Sandun Sep 27 '13 at 11:33
  • A DIY car to phone charger should work the same way that a tank to phone charger should work. If I would have asked help for a tank to phone charger I thought that less people would think that they could answer. – Daniel Sep 27 '13 at 12:05
  • @Daniel, 24V is common in trucks as well so 24V to USB charger would be a better title. I'll remove my downvote but hopefully you can see why it's better just to ask a question directly rather than "beat around the bush" and leave speculation. Edit - just realised my vote is locked in now, so you'll need to edit it before I can change my vote anyway so roll it into a more direct question. – PeterJ Sep 27 '13 at 12:17
  • thanks, but while I am at it I might as well practice with electronics and build it from scratch if it is not to hard. – Daniel Sep 27 '13 at 12:24
  • If it's a tank (military vehicle rather than large container of water) does the design have to have extra electromagnetic immunity features? What about the 24V supply being fused to prevent fire hazards in the "cabin"? What about problems with ensuring the design doesn't ignite ordnance/ammo stored in the tank? – Andy aka Sep 27 '13 at 12:25

2 Answers2

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No you cannot just connect the wires direct from the power jack through a resistor to the phone.

You can find a specific resistor that would drop 24V to 5V at a specific current. However the voltage in a vehicle is not constant and the current drawn by a phone during charging is not constant. This means that a single resistor value is not going to deliver a constant 5V to the phone load over the range of operating conditions.

What you will need is a DC to DC converter that is designed to accept the 24V in and produce the 5V out at the current levels needed for charging the phone. These are readily available at low cost for 12V applications in autos and small trucks but for 24V you may have to do some research to find one or build your own.

Michael Karas
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You can buy suitable modules.

Here's one example of many found by Googling for 24V DC DC automotive regulator 5V:

enter image description here

Whether the output of this is stable enough for your needs I don't know, but it's cheap enough to try.

If you intend drawing large currents, you might be wise to buy one of the ones built into a heatsink.

From your question I infer that you will find this easier and probably cheaper than creating your own solution with an equivalent performance.

RedGrittyBrick
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