ESP8266 Community Forum

- Tue Mar 08, 2016 5:44 pm #42710 Lee,

The important thing is that the 0.1u goes between your ADC and ground, -not- between your point "B" and the ADC.

It is extremely unlikely that you've damaged the thermistor (if it is a thermistor) with such a low voltage/current supply. They're pretty tough devices.

-John-

- Tue Mar 08, 2016 6:36 pm #42715 Lee,

On the second part of your question ...the 200K for 5C indicates that the thermistor is an NTC part (the resistance will drop as the temperature increases), but thermistors are non-linear, so you can't plot a straight line relationship between temperature and resistance; it's a curve.

You have a couple of givens here. One is that your ADC can only handle 0~1v. The other is that your thermistor will be at room temperature at starting point, so the resistance will be in the 100-200k range (which you've already determined with your tests). The critical variable is what the resistance values are going to be in the 180~220C range (typically what you'd be cooking at). It may very well be (in fact, it's fairly likely) that you won't be able to cater for the high input voltage which your ADC will see at room temperature and still have enough resolution at cooking temperatures to give you accurate temperature readings. Unfortunately, the limited input range of the ESP ADC isn't very well suited to such a wide, dynamic input range. You'd probably need to build a buffer (such as an NPN emitter follower) to overcome this limitation.

Another thought which just occurred to me ...are you absolutely sure you have a thermistor? It's quite possible that your probe is actually a thermocouple. Does the packaging explicitly state that it's a thermistor?


-John-

- Wed Mar 09, 2016 3:23 am #42733 Hi John

Despite buying the capacitors on the way home, I was unable to run the test due to a broken probe! Hope to get one today.

I have 2 of these meat probe thermometers (both exhibit the same behaviour)
1. I bought in the UK from a thermometer company so should be fairly decent.
2. The second one I bought cheaply one of these - http://m.ebay.co.uk/itm/Digital-Probe-L ... nav=SEARCH

The one I am referencing above is the 2nd one. The first one I got working some time ago on the Arduino. I calculated the Steinhart coefficients for it and the temp calculated using the equation was +/- 0.2 deg C. This one also seemed to initially work on the Esp but then behaves the same as the other one.

To be honest I have no idea whether it is a thermistor or thermocouple. I had assumed it was given the item was so cheap. Any idea how I can tell?

Before my broken probe last night I tested the resistance of just the probe with the wire coiled up and straightened. I could be imagining it but it did seem more stable coiled. Also, another odd thing was that the reading was different depending on which way around I connected the meter to the probe. I don't have exact measurements as I noticed it checked my theory and the probe broke before I could do a controlled test but... It was something like 110k vs 45k (I can't say for sure this was k or not but the number was definitely around 45).

Thanks once again for your help. I definitely think you are hot on the trail of the root cause

Thanks

Lee.

- Thu Mar 10, 2016 3:01 pm #42845 Hi

Quick update....

I set up the circuit without the link to ADC and cap to make sure my voltages were correct before connecting to the chip. The initial voltage divider seems to work a treat reducing the voltage across the probe to around 1.9v. When I connect point A to the second divider I get almost no voltage. 0.048v at point A and 0.018v at point B. Clearly I have goofed something but I can't work out what! I have tried replacing the link between point A and the second divider with a 3.3v feed and the second divider also works as planned.

Any ideas? I have now checked this about 5 times.

Thanks

Lee.