Temperature Shifts

Where we last left off, I had been able to use a relay-actuated electric heater and a temperature sensor to act as a simple thermostat to regulate the temperature for use when allowing root beer to ferment. Yesterday I upgraded the system to use a transistor to control the relay rather than driving it directly from one of the Arduino Uno microcontroller pins.  It looks like this may have solved some problems with losing serial communication to the Arduino IDE (at least the problem hasn't shown up again) but the trouble-shooting process revealed another: a temperature shift whenever the relay opens or closes.

I actually observed this problem the first day I was doing testing on the system:

If you look at the "Heating Pad on High" plot (the green one) you see that as soon as the chamber temperature hits the lower limit (and the relay closes to turn the heating pad on), the temperature shifts down almost a full degree.  Oddly this behavior doesn't show up for the "Heating Pad on Low" curve which is at least a little bit odd. 

My immediate guess, which turned out to be correct, is a sagging Vcc.  With all voltage supplies, as the amount of current being drawn increases, the voltage at the output will decrease.  Supplies that have very little voltage drop are considered "well-regulated"and there are various metrics that can be used to quantify this degree of regulation.

So I pulled out my multi-meter and measured the 5V supply:

Relay closed: Vcc = 4.93V

Relay open: Vcc = 5.03V

That 0.1V shift messes with the on-board analog-to-digital converter (and the math I do in software to convert the measured value to temperature).  The converter and math think they are working on a 5V scale but some of the time its actually a 4.9V scale.  Not a huge difference except that my temperature sensor measures from -40'C to 125'C and the temperature differences I'm trying to measure are less than 1'C.  In this application, very small changes in supply voltages can cause trouble with the temperature measurement.

Since I discovered the problem, I have though of three different solutions in succession:

1. Op-Amp Range Adjustment - Use a rail-to-rail op-amp (like the MCP6002 which I have used before and have on-hand) to magnify the output of the temperature sensor so that at say, 100'F the sensor/op-amp combination effectively put out 5V.  This would effectively increase the resolution of the sensor around the temperatures I care about at the cost of not being able to read temperatures over the whole sensor range.  Any temperature above 100'F would all produce the same 5V value.
2. Constant Load - If the changes in load are causing the voltage supply to drift, why not do something to keep the load constant?  Using a PNP transistor and a fixed load (something like this), I could provide a constant load on the voltage supply whether the relay was engaged or not. When the relay was open, a dummy load would be switched in through the PNP transistor (using the same control signal for the relay coil) and when the relay closed, the dummy load would get switched out and the relay coil would load down the power supply.
3. Change the power supply - It took me a while to figure out why, in the above graph, the "Heating Pad on High" showed this problem but "Heating Pad on Low" didn't.  The amount of power the heating pad draws does not affect Vcc on the Arduino; it is on a completely separate circuit. I eventually realized I had effectively changed the power supply to the Arduino between these two experiments.  When the heating pad was on low, I had been supplying power through the DC power jack and when I had changed the heating pad setting, I had unplugged this power supply for some reason.  Without the external power, the Arduino was being powered by the USB bus it was attached to and it doesn't surprise me that the voltage regulation was not near as good.

Obviously, solution 3 is the easiest to implement, though it didn't come to me until the next day.  I had already planned to power the Arduino from an external supply when the system was actually in use as it would be operating stand-alone, apart from a computer.  With the external supply in place I get the following Vcc measurements:

Relay closed: Vcc = 4.99V

Relay open: Vcc = 5.00V

This is probably within the range of error for my meter and should be good enough for my purpose. Here's what the thermostat performance looks like under USB power compared to external DC power:

The shift under USB power is very obvious and it is clear that there is also a slight problem still when being powered by the DC jack (the little squiggle at the lower control limit).  By the way, this data is when the heating pad is on high; way over-shooting the target temperature range.  But more about that next time.