Simple programmable load with gain

Today I needed to test a new PCB design for a load cell module. In order to evaluate the load response of the excitation circuit, I have to find a way to control the load current while observing the excitation voltage. The load curent needed is less than 20mA.

I quick search online and I found a simple circuit from Simple programmable load

constant current schematic

The circuit above was designed for much higher current than I need. I also need to read the current. I decided to stay with a 50mA range which coincide nicely for the 5V supply that I’m using with a gain of 0.1V/mA, 5V output = 50mA.

Schematic_Simple-programmable-load-with-gain_Sheet-1_20180406140518

I’m using MCP6002 since it’s cheap and I have plenty of them. It’s not a very precise opamp but precision is not a requirement for me. What’s more important is the rail-to-rail input/output. The high input offset does cause some issue when the control input is near zero or 5V. R2 will help set the gain and the ratio for the current reading. R3 is used when I was testing to make sure everything works as expected.

I prototyped this on a 2 sided perfboard and then hooked up to Analog Discover 2 to make use of it’s waveform generator and oscilloscope to verify my excitation circuit.

Simple programmable load with gain - prototype

I kept the circuit public here: [EasyEDA]Simple programmable load with gain