Your help is required. For my thesis I do measurements on a reversible solid oxide cell (not really important). The issue that i have is that the measurement system is old and fully manual, so i want to update the system using adruino's. The cell acts like a power source producing a voltage that is dependent on the current. So if i can control the resistance in the circuit i can control the voltage it puts out. So here is the problem the voltage ranges from 0-1.2V and the current between 0 and 8 amps. In the picture I have put two graphs on how the voltage and current correlate and the resistance i need per current (between 7-0.05 ohm). And i have attached a proposed circuit, which is really basic.
Does anyone have an idea how i can create such a small but controllable resistance? If you have an idea i will be greatly appreciated.
Behaviour sounds a bit like solar cells (current source rather than voltage source); maybe you can apply MPPT techniques (PWM based) as is commonly used for solar cells to your fuel cells?
There are a couple of "challenges" - You'll have to include the resistance of the connecting wires and connections/contacts. If you use a switch or relay to switch resistance, the contact resistance is likely be significant and it may be inconsistent.
Measuring the resistance (so you can include this "extra resistance") requires a special 4-wire Ohmmeter that can accurately read low resistances.
A single [u]rheostat[/u] or pot probably won't give you range & resolution so you'll probably have to switch-in different resistances.
For the very-low resistances you can use [u]regular copper wire[/u]. A couple of feet of 24AWG wire is 0.05 Ohms.
For the higher resistances you can use Nichrome wire or you can buy low-resistance resistors.
...A million years ago when I was in college we had some big low-resistance power resistors that looked like electric heaters. From what I remember they weren't adjustable so we wired them in series or parallel to adjust the resistance. (You don't need high-power like that).
"...A million years ago when I was in college we had some big low-resistance power resistors that looked like electric heaters. From what I remember they weren't adjustable so we wired them in series or parallel to adjust the resistance. (You don't need high-power like that).".
Me too, but they had a metal bar across the resistor with a sliding contact for varying the resistance.
I used to have a similar device that came from a 1920's Russian marine radio transmitter. It was used to adjust the filament voltage for the tubes. Don't know where it went.
Paul
PS: I just looked at Ebay for "rheostat" and found MANY that would fit the OP's needs.
Maybe OP wants "Arduino controlled low resistance high current resistor" for automatic measurement. I think power MOSFET with some (closed loop?) control is the right answer.
Thank you all for your inputs! As far as I can see now, the rheostats and resistance wires are static resistances. I would need a controllable resistance that i can change over time with an arduino (and control loop).
So i could use the rheostats in parallel and use switches to change the number of parrallel resistances to dynamically increase or decrease the total resistance.
@Smajdalf, how would such a circuit look like? because, at the moment, I know to little about mosfets to know how to use them as resistances.
Most conductors I know of do have their resistance changed by mechanical stress. I can make a wire more resistant with a hammer but I can't undo that. Perhaps a strain guage would be more suitable.
Another resistance changer is temperature. BJT's have temperature curves, you'd just need to control temperature.
How many ways to control current flow? A small coil can EMF in two directions, assist or resist.
An appropriate N channel mosfet controlled in its linear region can serve as the adjustable resistor. The mosfet will need a heat sink and a 10 vdc DAC controllable by the Arduino to control the mosfet on resistance via the gate voltage.
The voltage and amperage can be measured with an INA219 module. Due to your higher amp range, the INA219 will need a lower value shunt resistor, this one should do the job:
You’ll need to calibrate the current against a standard for two reasons. 1) the ideal shunt resistance value is not available and 2) to eleimate any resistance within the INA219 since it does not support a Kelvin connection. You can then adjust the INA219 calibration data to achieve accurate direct output of volts, amps and power via the Arduino.
With R1 and V2 known you get the current. V1 is probably the value that interests you (or you may calculate load resistance from V1 and current). If you don't need quick measurements you may use filtered PWM to drive the MOSFET's gate, changing the voltage changes effective resistance of the MOSFET.
If you are interested in only a few values of resistance you may copy this circuit for each (fixed) resistor and turn on only one of them.
Smajdalf:
If you are interested in only a few values of resistance you may copy this circuit for each (fixed) resistor and turn on only one of them.
This is what I'd do, turn on/off current through R1 and use R1 for the load resistor. Then the current can be calculated from the voltage over the known R1 (V2).
Whatever you do to measure the cell, make sure that your instruments are capable of making such close measures as the by-hand methods doubtless chosen for their accuracy. Production grade techniques will be inferior.
