I use a constant current control design to control the current through the halogen lamp, which typically runs at 800-900 mA
Resistor MPN: PCS2512DR1000ET
The circuit itself works as I expect it to. The issue I have is with the ambient temperature rising or dropping, which reduces the current that goes through the lamp.
I tested heating with the heat gun the electronics only, or the lamp only, and the result is the same. Increasing temperature on the electronics only increases current on the lamp, and increasing temperature on the lamp only reduces the brightness of the lamp (its performance eventually). The current increase/decrease is in the scale of mA (2-3mA) and 5mA max, but it is enough to cause some trouble in our measurements.
Since we make precision devices that are calibrated with the halogen lamp's brightness/intensity, we want the current to be as steady as possible during the variations of the ambient temperature.
We do try to keep the ambient temperature steady for the measurements, but some measurements might take more than 1 day and it's preferred if we did not leave the air condition on all the time. So I am trying to figure out a way to compensate the ambient temperature changes.
I have thought only one practical way of doing this. It is to add a NTC resistor in series with the halogen lamp:
I have not tested the circuit with the NTC resistor yet, but I will in the next weeks. I might as well try to use a 100mΩ NTC or PTC resistor instead of R1 and see how the circuit behaves, but overall the idea is to play with NTC resistors.
The physical layout:
The white line is the high current flow. The black/red wires on the left go on the resistor from the multimeter to measure the voltage drop for my tests. In the picture I do not have the heatsink soldered on the MOSFETs. (Those are two parallel MOSFETs)
Bottom layer is a copper pour GND layer
I know there is no a way to solve this issue 100%, so I try to come op with ideas to reduce the impact of ambient temperature in this system.
Other ways I have thought of reducing this problem:
Measure the ambient temperature and adjust the DAC voltage (aka change the current drawn based on ambient temperature). This is tricky since DACs have a quantum scaling, like the one I use is 1024 steps, and each step is 17 mA, which is a lot, considering 4-5 mA difference due to ambient already cause us trouble. But I could try making a system that has the ability to control the current on the 1-2 mA range, that would be useful.
Use Low ppm/oC components. That's the most obvious one. I actually set my soldering iron to 200 oC and was touching different components, turns out the DAC and the MOSFETs are the ones that when hotter contributed more to the current change. (Still, we are talking about 2-3 mA of change, but to out measurements its important)
Use heatsinks and forced air to cool the components. This helps in the summer, but in winter when we use the air condition vs during the night when the air condition is off, we see the differences.
I am also thinking of separating the control circuit (the op amp and the DAC) with the "high current" side (the Halogen lamp, the MOSFET and the resistor). Because the GND voltage near the high current electronics is messing with the DAC's accuracy and reference voltage of the Op amp. I guess when the temperature rises, the GND voltage around the high current components also rises due to the increase of the resistance of the copper. I want to separate them using a MOAT like so: https://electronics.stackexchange.com/a/430095/252145
I measured the current that is drawn from the power supply while I heat up the 100 mΩ resistor only, and the current drawn reduced. And this resistor has positive temperature coefficient. I was thinking, to replace that resistor with a one that has a larger temperature coefficient. So when all the components get their temperature increased and as a result, more current is drawn, the resistor with its higher temperature coefficient will balance it.
Any ideas (except of the ones I mentioned) to improve my system's performance over temperature change?
Is any of my ideas not leading me in the right direction? Especially the 5th idea and the NTC in series resistor idea.