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Topic: [SOLVED] DS18B20 Waterproof Digital Sensor Thermal Probe --- Resistor Spec (Read 118 times) previous topic - next topic

R79158

I am assuming that the probe does not come with the resistor and so I have to buy one. I do not know how to judge what resistor to buy for 4.7k pullup.

Is this ok?

Value 4.7k/4k70 ohm
Power 0.25w
Material: Carbon Film
Tolerance: 5%

Robin

Note:

Parasite power mode
When operating in parasite power mode, only two wires are required: one data wire, and ground. In this mode, the power line must be connected to ground, per the datasheet. At the master, a 4.7k pull-up resistor must be connected to the 1-wire bus. When the line is in a "high" state, the device pulls current to charge an internal capacitor.

Normal (external supply) mode
With an external supply, three wires are required: the bus wire, ground, and power. The 4.7k pull-up resistor is still required on the bus wire.

(taken from http://playground.arduino.cc/Learning/OneWire)



MarkT

Any 4k7 resistor will do, is not critical.  With short cables 10k will do, with really long
cable (more capacitance) you might want to try a smaller value than 4k7.
[ I won't respond to messages, use the forum please ]

R79158

Thank you for your reply. In my  quote there is a reference to adding the resistor at the master. Do you know what is meant by the master?

 

olf2012

The 18B20 is a OneWire "slave" device. It is controlled by a "master" which is your Arduino. Connect the resistor between Vcc and your designated OneWire pin.

R79158

Thanks both of you for your help which has solved my problem.

Paul__B

I suspect you will find it much more practical not to run it in "parasite power mode" but simply to connect the supply wire and ground to 5V and ground on your Arduino.

R79158

I suspect you will find it much more practical not to run it in "parasite power mode" but simply to connect the supply wire and ground to 5V and ground on your Arduino.

Yes, that is what I had thought I would try first. I ordered the resistor today because I will need to connect it to the supply wire, even though I will not being using the probe in parasite mode.

Paul__B

I will need to connect it to the supply wire, even though I will not being using the probe in parasite mode.
Correct. :smiley-grin:

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