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Old 04-07-2008, 11:29 AM   #1
WRXVT
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Default calling all electrical know-it-alls... USB charging question

-I have a device (gps navi) that came with a car cigarette lighter charger. The charger reads 5V, 1A. The other end of the cord is a mini-USB which plugs in to the GPS to charge it. I assume from this that my device requires 1000mA or 1A of current.

-I bought a wall charger that receives a type A usb cord (plug the mini-usb into the gps and then into the wall charger). It reads 5V, 1A as well.

My questions are:

1. I read that USB cords carry a maximum of 500mA in current. Does that make the wall charger half as strong and therefore potentially not usable? If yes, doesn’t that apply to the car charger which is also mini-usb on one side?

2. Does it really matter how many amps of current I give the gps to charge it? I have a charger for my Motorola razr that reads 5V 500mA, and another one that is 5V 750mA… can I use those?

3. The manual says to use a mini USB charging cable- are all mini USB charging cables able to carry the same amount of current?

Thanks
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Old 04-07-2008, 12:13 PM   #2
KitC
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Quote:
Originally Posted by WRXVT View Post
1. I read that USB cords carry a maximum of 500mA in current. Does that make the wall charger half as strong and therefore potentially not usable? If yes, doesn’t that apply to the car charger which is also mini-usb on one side?

2. Does it really matter how many amps of current I give the gps to charge it? I have a charger for my Motorola razr that reads 5V 500mA, and another one that is 5V 750mA… can I use those?

3. The manual says to use a mini USB charging cable- are all mini USB charging cables able to carry the same amount of current?

Thanks
I can only answer questions 1 and 3 authoritatively, question 2 I'm less certain on.

1. The USB standard actually requires a minimum current load of 500 mA for powered root devices, with a maximum of 5.0 A. This is shared amongst all devices drawing power from the port. You are well under the limit for your 1A device.

2. I think so, but it should take longer to charge, given a lower current delivery. I wouldn't recommend it.

3. The USB spec is pretty clear on the physical gauge of the wires in all USB cabling. I would suspect that any mini-USB cable would be within some small tolerance of being identical to pretty much any other mini-USB cable.
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Old 04-07-2008, 12:19 PM   #3
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2. yes. The charger's rating is a maximum, your device may use less - it will only "take what it needs" so to speak.

For some unregulated power adaptors, it would be unwise to us a 1 ampere adaptor for a device that originally had a 300 ma, but I bet your 1 ampere adaptor is well regulated. The unregulated adaptors have open cirucit voltages a bit higher, then they sag down to rated voltage at rated current. Even then, you'd probably be ok - it would be a pathological case where you'd have a problem. My nephew's kiddie CD player was like that - I used a my well regulated 12v adaptor because the original was lost, the internal regulator got hot and shut down. Sorry, I ramble.
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Old 04-07-2008, 03:31 PM   #4
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thanks for the info... its seems that USB cables are pretty cool... that a plain jane mini usb cord can be plugged into a wall outlet and power my gps at 5v 1000mA and also serve as a usb 2.0 data transfer...
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Old 04-07-2008, 03:39 PM   #5
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So, the answer is actually going to be determined by what your charger will put out. I know that for customers of mine doing usb outputs, I always, always, always have to current limit to 500mA max.

So chances are pretty good that your charger will only put out 500mA. Don't assume either that the current will simply stick at 500mA. Some of my less pricey competitors make it easy on themselves and instead of really limiting to 500mA, they simply shut off when they exceed 500mA.

So the answer is.....depends on what exactly....your charger will do.

jack
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Old 04-07-2008, 03:47 PM   #6
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Quote:
Originally Posted by Jack ffr1846 View Post
So, the answer is actually going to be determined by what your charger will put out. I know that for customers of mine doing usb outputs, I always, always, always have to current limit to 500mA max.

So chances are pretty good that your charger will only put out 500mA. Don't assume either that the current will simply stick at 500mA. Some of my less pricey competitors make it easy on themselves and instead of really limiting to 500mA, they simply shut off when they exceed 500mA.

So the answer is.....depends on what exactly....your charger will do.

jack
My charger says output is 1A on the back- are you saying it won't put out 1A? and is it the usb cord that is limited to 500mA (trying to figure out why you always always limit current to 500mA max? I'm confused because post #5 conflicts with post #2
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Old 04-07-2008, 04:21 PM   #7
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Quote:
Originally Posted by KitC View Post
I can only answer questions 1 and 3 authoritatively, question 2 I'm less certain on.

1. The USB standard actually requires a minimum current load of 500 mA for powered root devices, with a maximum of 5.0 A. This is shared amongst all devices drawing power from the port. You are well under the limit for your 1A device.

2. I think so, but it should take longer to charge, given a lower current delivery. I wouldn't recommend it.

3. The USB spec is pretty clear on the physical gauge of the wires in all USB cabling. I would suspect that any mini-USB cable would be within some small tolerance of being identical to pretty much any other mini-USB cable.

dude, 5 [A] would fry the USB cable, and any person that touched it. 500 [mA] is the max (USB spec, section 7.2.1). The PORT supplies 500 [mA], the hub may draw more power to support the ports that it has, but if it only draws from another USB hub, it cannot draw morethan 500 [mA] (it should be powered externally to support more current). Can you quote what part of the USB spec you got that from?

The charging device dictates what it requires to charge up properly. voltage and current have an inverse relationship. If the device requires 1 [A] to charge at 5 [V], it will kill a source that is spec'ed to output 0.5 [A] at 5 [V]. The device manufacturer should know this. If they violate this requirement, it is out of spec.

http://www.usb.org/developers/docs/

Last edited by CarlEToast; 04-07-2008 at 04:26 PM.
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Old 04-07-2008, 04:32 PM   #8
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Originally Posted by CarlEToast View Post
dude, 5 [A] would fry the USB cable, and any person that touched it. 500 [mA] is the max (USB spec, section 7.2.1). The PORT supplies 500 [mA], the hub may draw more power to support the ports that it has, but if it only draws from another USB hub, it cannot draw morethan 500 [mA] (it should be powered externally to support more current). Can you quote what part of the USB spec you got that from?

The charging device dictates what it requires to charge up properly. voltage and current have an inverse relationship. If the device requires 1 [A] to charge at 5 [V], it will kill a source that is spec'ed to output 0.5 [A] at 5 [V]. The device manufacturer should know this. If they violate this requirement, it is out of spec.

http://www.usb.org/developers/docs/

ok so where does that leave me? I'm confused because my charger states 5 [V] at 1 [A] as the spec'ed output, but it uses a plane change mini usb cable which according to you is limited to 500mA. I have it plugged in to my 110V home wall outlet.
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Old 04-07-2008, 04:41 PM   #9
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Originally Posted by WRXVT View Post
ok so where does that leave me? I'm confused because my charger states 5 [V] at 1 [A] as the spec'ed output, but it uses a plane change mini usb cable which according to you is limited to 500mA. I have it plugged in to my 110V home wall outlet.
the 110 [V] is the external source for the extra current. Your charger is not specifically a USB hub, it's a charger. So it doesn't need to comply to the USB spec I guess.

I am just saying that you need to make sure and not assume that a USB port can always double as a charger.

You're probably safe with the current loading that the device is requiring.

Last edited by CarlEToast; 04-07-2008 at 04:54 PM.
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