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### Topic: Trying to understand pull-up resistors (Read 7632 times)previous topic - next topic

#### Grumpy_Mike

#30
##### Nov 14, 2012, 09:28 am
Quote
I believe that my USB 3.0 port provides less voltage, but more ma than the wall wart. I'm using this as a clue, but maybe I'm using it incorrectly:

No.
The current rating of a wall wart is only what it can supply not what it will supply. That is governed only by ohms law. If it is the same voltage it is the same current.
What could change is the grounding, connected to a computer you will have better grounding which will make your marginal signal better.

The capacitance from the long wire will slow down the rise time of your signal, a lower resistor will speed this up.

#### vdavidoff

#31
##### Nov 15, 2012, 01:49 amLast Edit: Nov 15, 2012, 04:02 am by vdavidoff Reason: 1
I still need to read the resources oric_dan(333) provided, and I'll be the first to admit that I don't understand what's going on here, but using a usb to power adapter (a spare Apple one I had, the little square with 2 prongs on one end, and a usb port on the other (so maybe it is a grounding thing? (the wall adapter is 2 prong, too))) instead of the wall adapter is working. I will keep trying to understand this but for now I'm just glad this is working. FWIW, I tried using both a 1k ohm resistor and a 180 ohm resistor along with the wall adapter, and neither helped.

Andy

#### oric_dan

#32
##### Nov 15, 2012, 06:13 pm
The info I gave last time was just general capacitor charging theory.

I've not used the DS18B20, but from looking at the datasheet, I'd think you don't need to worry about
what is powering it, as long as the voltage is in the range 3.0 - 5.5V. Current draw is only 9mA so about
anything will power it.

It shows sink current as 4mA, so you can't make the pullup value too small. 5V/4ma = 1.25K minimum.

The timing reqs for the device do look to be somewhat critical, and I can't say anything about that. If
it works with a short cable, but not with a long cable, and you're using a 1.2K pullup, I'd say you're
probably gonna need some different circuit.

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