current draw USB socket in car (12v)

builder22 · 7 Sep 2017 at 10:21

with a socket like this

http://img.dxcdn.com/productimages/sku_258722_4.jpg

I know current=power/volts, but how does the voltage reducer on these sockets affect the current draw from the battery to the socket?

say its a 2amp device being used, so that must be 2amps between the 5v side of the socket and the cable to the device. But what about on the 12v side of the socket to the battery? Still 2amps or does the voltage reducer increase the load?

leezer3 · 7 Sep 2017 at 11:00

Correct, there will be some additional power draw.

How much depends on the circuit design of the adaptor.
https://electronics.stackexchange.com/questions/127525/reducing-voltage-with-resistors

As a rule of thumb, cheap Chinese crud will be little more than a couple of resistors and a smoothing cap & might use something like 300mA extra, with decent stuff using ~50mA

Edited to revise lunatic figures.....

builder22 · 7 Sep 2017 at 11:24

hi...cheers

so,depending on the sockets circuit design... it would be roughly between 50 and 300w/12 + the amps drawn from the usb device?

that makes a big difference to the size of cable I thought I needed!

leezer3 · 7 Sep 2017 at 12:19

Arrgh.....

Try mA as the figure for the extra current draw, not watts

(Blame posting from the phone and not proofreading very closely)

You want reasonably chunky wire (Volt drop on 12v can get quite high), but nothing too silly.

builder22 · 7 Sep 2017 at 12:39

ahhh gotcha, that seems more reasonable! (anything beyond a single resistor I don't know how to work out figures...need to do some reading)

I have flat twin 1mm(squared), approx 5metres cable run.....voltage drop calculator on 12v planet says 3.6% (0.43v) drop for 2.5amps....sound ok?

touch · 7 Sep 2017 at 13:32

builder22 said:
say its a 2amp device being used, so that must be 2amps between the 5v side of the socket and the cable to the device. But what about on the 12v side of the socket to the battery? Still 2amps or does the voltage reducer increase the load?

(Ignoring any increased load from the device used to reduce voltage)
Drawing 2amps on the 5v side will give 10watts.
To get the same power on the 12v side, you only need 0.83amps.