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Grogster

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Joined: 31/12/2012
Location: New Zealand
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Posted: 03:21pm 01 Mar 2015

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Hi folks.

Manual, page 17 talks about using 18B20 temperature sensors.
You can use external supply or parasitic power.

Is it perfectly acceptable to use parasitic power, or is the PREFERRED method to use the pullup resistor and an external supply?

IE: Do you only use parasitic power if you are desperate - you should use external whenever you can?

Smoke makes things work. When the smoke gets out, it stops!

robert.rozee
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Posted: 05:59pm 01 Mar 2015

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http://datasheets.maximintegrated.com/en/ds/DS18B20.pdf :
"The use of parasite power is not recommended for temperatures above +100�C since the DS18B20 may not be able to sustain communications due to the higher leakage currents that can exist at these temperatures.
For applications in which such temperatures are likely, it is strongly recommended that the DS18B20 be powered by an external power supply."

so in applications where the measured temperature is likely to be high, parasitic power should be avoided. i would suggest this includes solar power applications, or when operated in australia

there are also several limitations on what features of the DS18B20 can be used when parasitically powered (one not available being end-of-conversion notification), and i assume that geoff has written the mmbasic routines to work around these limitations. it is possible to detect programmatically if the device is parasitically powered, so some of these features could, in theory, be made available in future mmbasic firmwares.

i'm afraid that a detailed reading of the datasheet is required to get the whole picture.

cheers,
rob :-)

ajkw
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Joined: 29/06/2011
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Posted: 09:26pm 01 Mar 2015

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I use external power in my application. Works fine as expected. Parasitic by name parasitic by nature, I think as you say,

Quote Do you only use parasitic power if you are desperate - you should use external whenever you can?

My 2 cents.

redrok

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Joined: 15/09/2014
Location: United States
Posts: 209

Posted: 08:51am 04 Mar 2015

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Hi Robert'

robert.rozee said http://datasheets.maximintegrated.com/en/ds/DS18B20.pdf
so in applications where the measured temperature is likely to be high, parasitic power should be avoided. i would suggest this includes solar power applications, or when operated in australia

I agree.
In reading the section called "POWERING THE DS18B20":
There is a method where the device can be delivered more power through the 1 wire buss without using resorting to the the use of a extra Vdd power wire.
In figure 4 they show the use of an external FET.
However, the Micro has a strong pullup when the pin is set to the digital output mode. The timing of this sounds to be a bit critical but could easily supply the 1.5mA required during the conversions. Then switch back to normal open drain mode for the data transfers.
I don't know if Geoff is doing this in his code. It would be cool if he is.
Does anyone know?
Another method is to use a storage capacitor connected to the Vdd pin. A diode, probably a Schottky diode, charges this from the DQ line. Since the DS18B20 conversion takes 200mS and consumes a max of 1.5mA a 1000uF cap would probably be sufficient.
redrok

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