DC Voltage Reduction

I have recently run into an issue that involves electronics that, as a mechanical engineer leaves me clueless.

I have been investigating how to build an auxiliary battery power source for my handheld GPS and the mechanism for doing this using the cable power source connections on a spare UBS computer cable is very straight forward.

I have determined that the UBS power supply voltage is 5v DC but here is where the issue arises that I cannot solve. I have an adapter power cable for connecting the an auxiliary car power socket to the GPS so I wanted to determine the supply voltage from that 12v DC source to the GPS; and when I did so by using my digital voltmeter at the GPS end of the cable it was also 5v DC. So, while I understand the old school method of using a resistor in series in a flowing current circuit to drop the voltage to a series connection in the circuit what I don't know what type of electronics can reduce an input 12v DC voltage to an outlet voltage of 5v in a circuit with essentially zero current as is the case with a volt meter as the only outlet connection in the circuit.

The importance of understanding this, other than the fact that I don't like not understanding things, is that I would prefer to use standard rechargeable batteries in my auxiliary power source and it appears to do this correctly I must condition the output voltage to the GPS to 5v.

Any input on this issue will be appreciated.

I can't help with the understanding part but have you thought about buying a cheap in-car charger (thinking £1 shop or dollar store cheap) and taking it apart? I get the feeling that they probably use a heat shielded regulator chip to keep it compact. I hate giving this answer but it might be worth asking Professor Google (or preferred search engine Professor).

Hope you find some part of this helpful though.

Cake of Doom:
Actually your input was helpful in that it reminded me to do what I often fault others for not doing. A quick web search provided a design and construction instructions although I am not sure it is what is commomly used for a simple low current GPS connector.

Also, I am going to test my exisitng automobile cable with some lower input voltages to see if a modified version of it might work for the pupose as well. At the same time, unfortunately it may all be a futile chase since, from the information I have found, such IC chip devices are rather inefficient using more input current that output, which is fine if you are using a 12v automobile connection source but may well defeat my purpose when using a small compact rechargeable battery set.

Thanks for the input.

Happy to be of service.

I can see how that kind of inefficiency could be a set back, although, similar things exist for mobile phones which are +/- in the same voltage ball park as your GPS. [url=http://www.ebay.co.uk/itm/Battery-Extender-Charger-for-Apple-iPhone-5S-5-iPad-4-Mini-iPod-Touch-Nano-7G-/321300371827?_trksid=p2054897.l4276]Link to typical iPhone version[/url]. Could be good for inspiration.

we can use this product to reduce the DC voltage [h=1]Jtron12V to 5V DC-DC Reduction Voltage Power Supply Converter Module w/ DC Jack - Black[/h]