read temperature from a thermoucouple through MAX6675 mounted on a protoboard,
control the current to a 30W-heating pad through a darlinton NPN (BDX53C).
All the circuit is powered by a 12V-5A AC/DC adapter.
All things seem working, but I have realized that the temperature reading is shitfed downward by 1-2°C when the heating pad is given current. What could be the cause?
I suppose that the power surce supports the load, as the voltage with and without the load of the heating element are respectively 12.43V and 12.07V; in fact also the regulated voltage out of the arduino board (that is also the power source for the MAX6675) is 4.91V (heating off) and 4.89V (heating on). Could this small 0.4% variation in voltage account for that error in reading the temperature?
I have also checked for cold-junction heating effects, but this seems not to be the problem.
but I have realized that the temperature reading is shitfed downward by 1-2°C when the heating pad is given current. What could be the cause?
Ground lift.
How is it wired?
You should use star wiring for the ground, that is where all the ground wires from each element of your circuit meets at one point.
Grumpy_Mike:
Ground lift.
How is it wired?
You should use star wiring for the ground, that is where all the ground wires from each element of your circuit meets at one point.
Thank you Mike,
I have tried to learn about "ground lift" and "star wiring" through the Web; as I can understand, the point is checking that there are non ground loops, but I can't see any stuff of this kind in the circuit.
I have attahced a scheme for any further suggestion.
This is an occasion where a photograph of your circuit would be good as it is not the I terconnections as such but how they are arranged. If it is like your schematic then this is fine.
It is not ground loops, it is the increase in voltage on the ground lead due to heavy current flowing through the finite resistance of the wire. This makes the ground wire not quite ground but a small voltage greater than ground. This in turn upsets any sensor on the wrong end of the wire because this extra voltage is added to the voltage from the sensor.
Grumpy_Mike:
This is an occasion where a photograph of your circuit would be good as it is not the I terconnections as such but how they are arranged. If it is like your schematic then this is fine.
It is not ground loops, it is the increase in voltage on the ground lead due to heavy current flowing through the finite resistance of the wire. This makes the ground wire not quite ground but a small voltage greater than ground. This in turn upsets any sensor on the wrong end of the wire because this extra voltage is added to the voltage from the sensor.
OK, now I understand the "ground lift". Anyway, I can't find the faulty piece of wire; in the photo the ground wires are shown; they all converge to the power socket.
Suggestions?
KeithRB:
The devil is in the details. Is your heater close to the thermocouple? Do they share a ground wire?
Thank you KeithRB. The heater has to be nearby the heater because the device has also a thermostatic aim. Their ground wires converge to the power socket, as shown in the picture above.
Well you were not looking for that.
However the wiring looks fine. What looks not fine is the lack of decoupling capacitors on your voltage regulator. Normally you have something like a 10uF in parallel with a 0.1 uF ceramic on both the input and output of the regulator, close to the regulator.
Since you get digital data from the 6675, the problem is that the heater is interfering with the thermocouple. You might need to shield the thermocouple wire or electrically isolate it from the unit with some epoxy or something.
Solved. By trial and error, I have found that placing a condenser between the leads of the thermocouple solved the problem. I suppose the effect is due to some frequency enclosed in the power source, that is a swithcing adapter.