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The new Samsung Gear watch won't charge from my 1amp nor 2.1amp USB charger brick ports. I'm assuming this is because its own power charger is rated at 5v .14amp? So the watch dock has some kind of protection circuit.

I am VERY rusty on my electronics (been almost 30 years!), and Current was always my weakest area to begin with...

I'm planning to build a current limiting circuit: http://www.zen22142.zen.co.uk/Circuits/Power/add-on.html

My question is:

Based on the formula on the circuit design I=.7/R2, to limit to .14amp I would need an R2 value of 5ohm 1watt. Since I'm having a tough time finding that, I'm assuming that I can use two 10ohm 1watt resisters in parallel?

EDIT Not sure why this was tagged as a "duplicate" of a power supply question, when my question was specifically related to parallel resisters. I prefaced my question, (which was good because I learned a few things) but I call out my specific question in bold, and tagged an answer that was specific to that question.

ZacWolf
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    The real reason it won't charge is probably related to what the power bricks do with data lines. Some devices are very picky about this, and it sounds like this watch is one of them. Adding current limiting is not only unnecessary, it won't make the power bricks charge the device anyway. – Matt Young Jan 07 '14 at 15:51
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    Most consumer power supplies are voltage supplies, not current supplies. The basic principle is: give the device the correct voltage, which is backed by **at least** as much current delivery ability as that device requires, with some margin to spare (keeping in mind that some power supplies want a minimum load too: so there is such a thing as too "big" a supply). Resistors drop voltage. If your device needs 5V, and you try to limit the current with a series resistor, the device won't get 5V any more. – Kaz Jan 07 '14 at 15:56
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    As Matt says several branded phones or smart devises use some kind of "handshake" with the charging device to prevent use of non branded chargers. – alexan_e Jan 07 '14 at 16:28
  • Thanks for the comments! Kaz, that was my understanding as well, but when it didn't work I thought it my be some kind of over current protection. – ZacWolf Jan 07 '14 at 18:00
  • Further experimentation with other USB chargers of various max amps (.14-2.1) resulted in charging of the watch so I'd say that's pretty good proof of @MattYoung 's comment. That is frustrating since I have a very versatile AC/DC charger brick that I've used for a few years now. THANKS! – ZacWolf Jan 07 '14 at 19:15
  • Shorting the data wires together from the watch was all that was necessary to allow it to charge. Seems this is a fairly common Samsung design. – ZacWolf Jan 07 '14 at 21:57

1 Answers1

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[instead of a] 5 ohm [resistor ...] can I use two 10ohm 1watt resisters in parallel?

Yes.

For resistors in parallel the combined resistance is the inverse of the sum of their inverses.

$$R_{tot} = \frac{1}{\frac{1}{R_1} + \frac{1}{R_2}} $$

As others have noted, the answer to your question doesn't actually help with your charging issue.

Phil Frost
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RedGrittyBrick
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  • Since the questions was technically about the parallel resisters, thank you for your answer. :) – ZacWolf Jan 07 '14 at 21:54