MicroCC is a constant current source, but focused on the low current microamp range. It uses the Touchstone Semiconductor TSM6025 voltage reference, a drop in replacement for the MAX3025 with better than 15PPM stability. The TSM6025 used both as the reference voltage and regulator for a TS1001 op-amp (previously covered here) powered by a single 3volt CR2032 coin cell.
A 20 or 25 turn 10k trimmer pot serves as the current source adjustment. 0.1 percent resistors at the load allows current measurements in the micro amp range to be taken with any volt meter capable of 1mV resolution.
The MicroCC uses the TS1001 in a modified voltage follower mode. The feedback is taken from the emitter side of a transistor instead of the output of the op-amp, which translates voltage regulation into current regulation. This is by no means a new type of circuit, however it demonstrates that a simple circuit is capable of operating in ranges usually reserved for more complex and more expensive circuits.
Initial testing confirms power consumption is less than 0.3mA, allowing a single CR2032 battery to last up to 600 hours. One of our multimeters produced a large amount of noise during tests on the uA setting, and we are looking into this.
Calibration is unnecessary as there is no part to calibrate, however we had best results when the power has been on for a few minutes to allow the circuit to stabilize before attaching a load.
There are plans to improve the range and accuracy, but the MicroCC preforms better than expected. Improvements could include adding a precision analog microcontroller, and the ability to to go all the way down to null current.
- R3-R5 1k 0.1% resistors – this is the load, the higher tolerance parts allows confident readings with a standard volt meter.
- TSM6025 – 2.5V voltage reference. could also use TS6001 for greater stability.
- TS1001 – super low power op amp.
- T1 FMMTA14TA – NPN Darlington transistor, the maximum collector cutoff current is 100nA, most will be lower than this.
- R7 89PR10KLF 20 turn trimmer, allows finer control of current vs. a single turn pot.
- BK-883-TR CR2032 coin cell holder.
- break away pin headers were used for power switch and connectors.
- C2-C5 1nF ceramic capacitors
- R1 1k to 10k resistor, for the base of the transistor
Unfortunately our instruments do not have sufficient resolution to fully characterize the operating conditions more accurately, but we hope the design encourages further experimentation with micro current sources.
The MicroCC is licensed CC-BY.
Thanks go to Touchstone Semiconductor for providing samples and demo boards.