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possible to power a voltage divider for a analog in by a digital out?
Hi
Probably a stupid question:
Is it possible to use a digital out to give the +3.3V to a voltage divider which is read bij an analog pin?
(aprox. 1000 Ohm  1000 Ohm divider).
Is it stable after 0.1 msec?
Alain

Senior Member
I think that might work... you could just try it...
.... is it to turn off the current when you're not going to be reading it?

It's indeed to only turn on the current when measuring.
Because I plan it to be battery powered and only need a measurement every few seconds (or even less), I want to conserve unneeded energy.

Yes, it does work. This is how resistive fourwire touch panels are read as well: a pair of pins is used as outputs, and the coordinate along that axis is read from either of the two other pins; then the roles are swapped for the other axis.

Senior Member
Originally Posted by
AlainD
(aprox. 1000 Ohm  1000 Ohm divider)
If the first 1000 in the OP was meant to be 100 that's a lot of load (33 mA by my oft faulty calculations)  I've heard 25 is the limit.
So 500 Ohm or greater I think to be safe.

Originally Posted by
oddson
If the first 1000 in the OP was meant to be 100 that's a lot of load (33 mA by my oft faulty calculations)  I've
heard 25 is the limit.
So 500 Ohm or greater I think to be safe.
3.3V  1000ohm or a bit more fixed resistor  the analog in  and a PT1000 (a bit above 1000Ohm) to the ground
So about 20002300 ohm from 3.3V to ground.

You won't have much accuracy at all doing it that way.
I would recommend you use a MAX31865 instead, with its voltage or ground switched via a transistor or MOSFET on demand by Teensy.
You can find cheap (45€) breakout boards on eBay for example. They typically have a 430 Ohm reference resistor, but for PT1000, a precise 4000 Ohm resistor would be much better. (Even just replacing the resistor with one whose resistance you have measured accurately  it does not have to be 4000 ohms exactly, 3.92k or 4.02k will be just perfect; all that matters is that you know or measure its accurate resistance, so you can do the math on Teensy correctly, giving you accurate temperature measurements.)
Plus, you can use a threewire PT1000 sensor, and with just a little math on the Teensy (as the PTC resistance as a function of temperature is not exactly linear) you can get half a degree Celsius/Kelvin accuracy; with a fraction of a degree resolution. (Meaning you can detect relative changes, on the order of a fraction of a degree, but with proper math compensation, your measurement can be accurate to within a degree or less over the entire range.)
The MAX31865 does not consume much current at all, no more than 10mA or so during conversion  that 10mA being just the figure you need to use for its power budget, i.e. using a small transistor with a maximum current rating of say 50 mA is plenty. You do need to power the MAX31865 for something like 10  20 ms from power on before initiating a measurement, each measurement taking somewhere around 5070 ms, so budgeting about 100 ms per measurement should work fine.
I have a few MAX31865's as well as 3wire PT100 (but no PT1000) temperature sensors, so if there is something you wish me to check, let me know.
I do also dabble with EasyEDA (but I am only a hobbyist bumblespork!), so if you want to make your own board, I may be able to help. The MAX31865 circuit is simple, just three capacitors and a precise reference resistor  the value itself isn't that important, as long as you know it precisely! , so adding a powercontrolling transistor with a currentlimiting resistor, and maybe an inrush current limiting resistor for the MAX31865 power too, would be a simple thing. I like EasyEDA, because it is easy enough for a hobbyist like me, and getting the boards manufactured at JLCPCB is easy; here are some of my own "designs". Other members here have much more experience than I, though!

Hi
Thanks
I checked it with two fixed resistors and it works quite well.
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