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#41705 - 09/12/16 06:26 AM USB Output
pbGuy Offline


Registered: 08/04/09
Loc: Portland, Oregon
I have a desk lamp with USB port, having output spec at DC 5V / 1A.

I recently purchased a bluetooth headset, which has an electrical plug with a detachable USB cable (for the power connector to the headset). The electrical plug has printed output specs: 5V / 750mA.

Will it be Ok to detach the headset USB cable, from the electrical plug, and use the USB output port safely, on the desk lamp?

Apple's iPod USB adapter (with older-style folding prongs) is rated for output of 5V (no mention is made about amps output). [The Apple USB adapters with the fixed prongs, show output at 5V / 1A.] Can the headset USB cable also be safely used with this iPod USB older-style (folding prongs) adapter?
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#41716 - 09/12/16 05:02 PM Re: USB Output [Re: pbGuy]
joemikeb Offline
Moderator

Registered: 08/04/09
Loc: Fort Worth, Texas
According to the USB standard for charging power the maximum allowable amperage for a 5 volt connection is 3 Amps or 15 Watts. Although your headphones apparently only require 3½ Watts to charge (5V x .750A = 3.5W) assuming they were designed and built with the USB standard in mind there should be no problem using a charger that delivers 5 Watts (5V x 1A = 5W).

But read the instructions that came with the headset carefully. I have seen devices built with minimal protection circuitry that come with a warning not to leave them on charge too long.

By-the-way the Apple's iPad charger with the folding prongs is rated at 12 Watts or 2.4 Amps
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#41718 - 09/12/16 06:58 PM Re: USB Output [Re: joemikeb]
pbGuy Offline


Registered: 08/04/09
Loc: Portland, Oregon
Thanks for your reply as well as showing the power calculation.

By the way, the bluetooth headset, underlying my inquiry, is a Plantronics Voyager Legend which is for phone connection to my iPhone. Plantronics' products are good; so, I have no concerns about quality of product parts.

Originally Posted By: joemikeb
....Apple's iPad charger with the folding prongs is rated at 12 Watts or 2.4 Amps
...What's the rating on the iPod adapter with folding prongs?
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#41719 - 09/13/16 11:48 AM Re: USB Output [Re: pbGuy]
joemikeb Offline
Moderator

Registered: 08/04/09
Loc: Fort Worth, Texas
Originally Posted By: pbGuy
.What's the rating on the iPod adapter with folding prongs?

I don't know, I found the power ratings for those on the Apple Store site and there is no charger shipped with the current iPods and none is listed as an accessory for the iPod. But since the iPod touch is for all intents and purposes an iPhone less the phone components, I would assume it would normally be the same 5V 1A 5Watt charter as the iPhone has always used. Yes, I am well aware of the danger assuming.
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#41736 - 09/14/16 12:40 PM Re: USB Output [Re: joemikeb]
Virtual1 Offline


Registered: 08/04/09
Loc: Iowa
Originally Posted By: joemikeb
According to the USB standard for charging power the maximum allowable amperage for a 5 volt connection is 3 Amps or 15 Watts.

Please remember that's not how much a charger must make available to a peripheral, it's the maximum possible configuration a peripheral can expect, IF the charger is capable of providing it. "Just because that cup comes in small, medium, large, and extra large, doesn't mean ALL drinks served are available in extra large".

Drop resistors on the data lines are supposed to inform the peripheral the maximum amount of current they're permitted to draw. If they choose to draw more, (since current is supplied on demand, the peripheral doing the demanding) the only recourse the supplier has is to shut off the power, and sometimes pop up a warning. Cheaper chargers will "sag" the voltage to keep up with the current demand because they're limited to total power(watts) delivered (watts = volts x amps) but that violates USB specifications.

I have a USB charge bar in my truck that has a total capacity of 50 watts. It has two 2 amp (10w) ports, and five 1 amp (5w) ports. So total draw shouldn't exceed 45w. That does NOT mean you can draw 3 amps (15w) from any of the ports.
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