Temp Probes: 3.3v vs 5v?

[Brian Hilgert]

So I'm getting ready to update my custom heatermeter, and iwas looking at the schematics for both the linkmeter and the rpi heatermeter, and noticed that on the linkmeter version that the temp probes (among other things) run off the 5v rail, but in the rpi version the temp probes are running off the 3.3v rail. I tried to find anything that 'indicated' to the 328p which voltage was used, as well as in the source (hmcore & grillpid), but found nothing. I am somewhere between really rusty and wet behind the ears, so I'm guessing I probably missed something, or does it not matter which voltage the probes are run at?

 

[Steve_M]

The probes shouldn't receive any voltage. They are thermistors that simply have their resistance read.

Edit: Looks like there is 3.3v on them.

 

[Brian Hilgert]

The probes shouldn't receive any voltage. They are thermistors that simply have their resistance read.

Edit: Looks like there is 3.3v on them.

Right, and if you check either LinkMeterHM.sch or LinkMeterHM.brd, you'll see that it is attached to 5v. My question is when the codes does an analog read, how does it distinguish between 5v on the LinkMeterHM and 3.3v on the HeaterMeterPI? Or was that your point, that is doesn't need to because it is only measuring the resistance on the themistor(s)?

 

[Steve_M]

Voltage shouldn't matter because whatever voltage is being fed into them will be multiplied and divided by the same voltage number in the temperature calculation code that calculates the resistance.