iphone and 1000mA car chargers

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Level 1

15 points

iphone and 1000mA car chargers

hi

this is more a question on how usb car chargers work.

does anyone know if car chargers that can delivery up to 1000mA (1A) of power via their usb port ALWAYS delivery 1000mA, or if they drop down their delivery depending upon what's plugged in?

ie. if a product only requires 500mA of power, will it only draw that much from the usb port or will the charger always delivery 1000mA and fry that device?

hope that makes sense.

regan

Message was edited by: reganyelcich

xserve 10.4.11, Mac OS X (10.4.11)

Posted on Aug 31, 2008 2:50 AM

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Answer 1

Aug 31, 2008 5:03 AM in response to reganyelcich

Any electric power source delivers at a fixed voltage. The device "draws" whatever current it needs. So no it won't fry a device that needs 500mA.

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Answer 2

Aug 31, 2008 5:56 AM in response to reganyelcich

nedhamilton is correct.

Let me add this though; If you connect a device that requires 1000ma (1 Amp) to a 500ma charger, you will more than likely burn out the power supply unless it has overcurrent protection (fuse, thermal breaker etc)

Peter

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Answer 3

reganyelcich Author

Level 1

15 points

Aug 31, 2008 3:51 PM in response to Peter Monahan

Thanks guys.

So if I understand it right it doesn't matter what a USB charger can "deliver" (500mA/1000mA/2000mA), the device that's plugged in will only ever "draw" what it needs to charge (regardless of whether the USB charger is plugged into a power socket or into a car lighter). But if you try to charge a device that will "draw" more than can be "delivered" then it may fry the charger.

Examples...

1. 1000mA charger
Plug in iphone 3g which requires 1000mA - won't fry device.
Plug in (camera, ipod...) which only requires 500mA - won't fry device.
Plug in TomTom GPS requires 2000mA - may fry charger.

2. 2000mA charger (with 2 usb ports)
iphone 1000mA alone - won't fry device.
Camera/Ipod 500mA alone - won't fry device.
Tomtom 2000mA alone - won't fry device.
TomTom 2000mA + iPhone 3G 1000mA - won't fry devices but may fry charger.

Am I right on all of that?

Thanks!

regan

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Answer 4

Aug 31, 2008 4:42 PM in response to reganyelcich

Plug in TomTom GPS requires 2000mA - may fry charger.

A standard USB port can supply only 500 mA, so the TomTom should not draw more than that unless it recognizes the port as a special high-current USB port.

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Answer 5

tonytone

Level 1

10 points

Sep 1, 2008 8:31 AM in response to Peter Monahan

Peter Monahan wrote:
nedhamilton is correct.

Let me add this though; If you connect a device that requires 1000ma (1 Amp) to a 500ma charger, you will more than likely burn out the power supply unless it has overcurrent protection (fuse, thermal breaker etc)

Peter

Unless I'm misunderstanding/missing something...I think you have it backwards? I mean, if the charger is feeding "only" 500mA to a device that requires 1000mA...how exactly will/does the charger wind up burning out the power supply?

Side note--what exactly is the iPhone 3G's current draw anyway...is it 500mA, 1000mA, somewhere between 500mA-1000mA, or more than 1000mA? Just curious, especially given that the Apple AC USB adapter puts out 1000mA (1A), while some (many?) iPhone 3G-compatible car chargers like the Griffin PowerJolt put out 500mA.

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Answer 6

rimshaker

Level 3

520 points

Sep 1, 2008 9:12 AM in response to reganyelcich

I rather not bore everyone by copying/pasting paragraphs from some old EE textbooks, so just stick with these facts for the 3G:

- The iphone works at 5V, either 500mA (by usb) or 1000mA (wall plug) is fine.

- Don't deviate from 5V, and don't get anything over 1000mA.

If everyone keeps asking "why," this post will start to become an online electronics class.

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Answer 7

reganyelcich Author

Level 1

15 points

Sep 2, 2008 3:03 PM in response to rimshaker

Thanks rimshaker - I posted my question because I wanted to know how the usb car chargers actually work - ie draw of power vs output etc. If it takes an "electronics class" style thread to explain it, then that's fine by me.

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