Powering DS18b20 Temp sensors off the VIN leg while using USB to power Photon


#1

Hi, I’ve read about using the VIN pin as a power source here, but it also cautions not drawing too much current from this leg while plugged in to a USB wall wort to power the Photon. I have tried to research but cant find how much each of these one wire temp sensors (DS18b20) draw, I am looking to hook up a few (4 for sure, maybe going as high as 12 if I can?).

Can anyone comment on whether it would be wise or not to use the VIN leg (which for me is reading about 4.7V when plugged in using the USB port, definitely within the range on the datasheet of the DS18b20), to power anywhere from 4-12 of these devices? As I understand the biggest limiting factor is the amps provided by the USB wall wort.

Also, for completeness, it mentions on the thread I linked above that you need to be careful which pins you use for the “data” wire as not all are tolerant of voltages >3.3V.

Thanks very much for any insight you can share!


#2

Well, reading the docs never hurts:
https://docs.particle.io/datasheets/photon-shields/#temperature-sensor-1-
https://docs.particle.io/datasheets/photon-datasheet/#power
The datasheet isn’t a bad place to look either: https://datasheets.maximintegrated.com/en/ds/DS18B20.pdf (page 19).
Or pretty much any google search for “DS18B20 current consumption” would’ve gotten you there :wink:
tl;rd: 4mA

From the pin description in the docs, we can gather the following:

VIN: This pin can be used as an input or output. As an input, supply 3.6 to 5.5VDC to power the Photon. When the Photon is powered via the USB port, this pin will output a voltage of approximately 4.8VDC due to a reverse polarity protection series schottky diode between VUSB and VIN. When used as an output, the max load on VIN is 1A.

tl;dr: 1A

12*4=48mA. 48<1000, so yeah, you should be good.

Info on the pins can be found here: https://docs.particle.io/datasheets/photon-datasheet/#peripherals-and-gpio

Happy Hack… ehm… Holidays :slight_smile:

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#3

Thanks very much, i was going through the Maxim doc and found that page as you replied!

Happy Holidays!