Hello to all !
At first, please leave a reply, I'am relatively new to bigger curcuit design so please forgive mistakes.
Most of the components i've selected in the upcoming curcuit are based on the stock that is available to me.
My goal of the project was to create a constant current source with four ranges: 20mA, 100mA, 1A and 10A.(by switching the shunt resistor) With a dual voltage measurment or differential measurment for compensation of wire losses at higher current ranges.
Because I've got some XTR110AG (normally used for voltage to current conversion of analog signals used in industry sensors and i/o expanders of plcs) lying around, I started designing a curcuit.
Because I am quite interested into statistics, I've tried to analyze my measurments.
In the attachement:
input_curcuit.jpg, you can see my voltage divider for up to 50V input voltage and the range switching(done by relay) to 0 to 5V by connecting the input directly to the buffer amplifier.
In the first prototype I've used Lm324 as OPs, but their offset voltage was a little scary high. So I changed to OP07.
To protect the ADC (VOLTAGE_IN flag) i've limited the current with the 4k7 resistor by clamping the voltage via schottky diodes to ground and the reference voltage. - Is this the right procedure ?
I've choosen the LT1011 comparator for overrange indication against the STM32L432KC(NUCLEO board).
The bigger part and questions:
Under
current_regulation.jpg i show the connection of the XTR110AG. The internal shunt resistor of the XTR is note precisely known, so I had the need to measure the shunt. (In my case 46 Ohm)
But I can't choose my shunt resistors for the different ranges (10, 1, 0.1 Ohm) according. So my idea was to change the span adjust potentiometers for a digital potentiometer and calibrate the different ranges by different digital potentiometer values.
I am using two irf9540 in parallel for easier heat ditribution.
The switching of the different shunt resistors is done by the schematic shown
shunt_switching.jpg.
For feedback of the real current i wanted to use a INA138 highside shunt amplifier. (You can see in
high_side_shunt_measurment.jpg).
Reference voltage source is a REF02CP.
dac.jpg is for completeness the MCP4921. The weird jumpers across the resistors and the in- and output of the op is for using different reference voltage sources (e.g. a 2.5V one, and the lowest input voltage range of the XTR110AG is 0..5V).
Should I filter the analog signal going into the ADC ?
I thought about using one of the buffer ops as an active filter.
I am happy to hear your thoughts.
PS: Buffer caps or on a different picture, I didn't included.
And excuse my crappy english !
Greeetings
Sebi