50 µA Current Source

50 µA Constant Current Source

A small test source that puts out a steady 50 microamps, built to check the microamp range of a newly acquired multimeter.

A small gray 3D-printed box with a black lid, a label reading 50 microamps Out and a red and black pair of leads coming out of the side, on a cutting mat in front of a breadboard — The finished source in its 3D-printed case

I recently acquired a used B&K Precision 5470 multimeter. I wanted to check that its microamp range could be trusted, and a constant current source is a good way to do that: it gives a known current regardless of what it's connected to. So I built a 50 µA constant current source on a breadboard, from a design by Alan Wolke, W2AEW, who shows it in his video #324, “How to ‘calibrate’ the Simpson 260 VOM – BONUS: 50 microamp current source”. The current source part starts at about 14 minutes in.

I compared its output on the B&K against my calibrated Simpson 270 analog meter. At 50 µA the B&K was spot on.

50 µAconstant output

9 VDC supply

PETG3D-printed case

The build

Breadboard. I built the circuit on a breadboard first and powered it from my B&K 1601 power supply, using a Fluke meter to confirm that the input was 9 volts.
Checking the current. With the output measured on the B&K 5470 and on the Simpson 270, both showed 50 µA. A small trimmer sets the exact current.
Perfboard. Once it worked, I rebuilt it on a small piece of perfboard.
The case. I printed a box in PETG and put the perfboard inside, with terminals for the 9 volt supply and a pair of leads with alligator clips for the 50 µA output.
Using it. Apply 9 volts DC to the terminals, and the alligator clips give a 50 µA test current, ready to check any meter.

A small white breadboard with a few resistors, a blue trimmer, three transistors and jumper wires, with red and black leads and alligator clips around it — The circuit on a breadboard

A workbench with a B&K power supply under a yellow Fluke multimeter reading 9.16 volts, a Simpson analog meter beside it, and the breadboard at the front — On the bench: the power supply, the Fluke confirming 9.16 V, and the Simpson 270

A digital bench meter reading about 50 microamps on the left, the power supply with a Fluke in the middle, and the breadboard on the right — A bench meter reading about 50 µA, with the power supply and the Fluke

A B&K Precision 5470 bench meter on the right reading 50.02 microamps, the power supply on the left and the small case with leads clipped between them — The finished case connected to the B&K Precision 5470 (right), reading 50.02 µA

The perfboard version of the circuit, with transistors and a blue trimmer, next to a gray plastic box — The circuit rebuilt on perfboard

The back of the perfboard with soldered wiring, next to a hand-drawn diagram — The back of the perfboard

The open gray box with the perfboard inside, red and black terminals on the side, and red and black leads with alligator clips coming out — Inside the case: the perfboard, the 9 V terminals and the output leads

Parts

Transistors, a reference diode, resistors and a small trimmer, as in W2AEW’s design (see his video for the schematic and values)
A small piece of perfboard
A 3D-printed PETG case, from the Parametric Project Box on Thingiverse
Terminals for the 9 V input, and leads with alligator clips for the output
A printed label on the case

Links

W2AEW video #324 The case on Thingiverse

Credits

The circuit is a design by Alan Wolke, W2AEW. The case is the Parametric Project Box by Anonaab on Thingiverse, shared under a Creative Commons Attribution license.