Review and test a 0 - 1mA constant current source

RE: Review and test a 0 - 1mA constant current source

DAB — Tue, 02 Dec 2025 21:08:41 GMT

Nice post Jan.

RE: Review and test a 0 - 1mA constant current source

Jan Cumps — Tue, 02 Dec 2025 19:28:51 GMT

I own an EEVBlog µCurrent. I had some incorrect low current measurements when I used it lately. Time to check if it's a test setup error, or if the µCurrent is off.
The current source of this blog is a decent source to compare the µCurrent measurements vs a reliable bench meter (Keithley DMM6500).

I put the µCurrent in series with the DMM. First nulled out the 2 devices. I dialed in a somewhat random current on the source. Then checked if the result is somewhat in the same range.

Turns out the µCurrent is still good:

I used the µCurrent's µA setting, where 1 µA generates a 1mV output.

RE: Review and test a 0 - 1mA constant current source

jc2048 — Sun, 10 Mar 2024 23:39:09 GMT

Good blog!

Few random thoughts and ideas

1. Output compliance with an old LM324 won't be 4.5V on a 5V supply (the top driver is a darlington pair, so there's at least 2 Vbe drops down from the rail for it to operate).

2. I don't think the LM324 version will be very accurate. May not matter - depends what you want to use it for.

3. The spread of offset voltage for the MCP6024 is -0.5mV to 0.5mV. It looks from the datasheet to be 'normal' (bell shaped curve), so, if you have a few of the devices, it might be worth testing them to see if you happen to have struck lucky and got a particularly good one in amongst them.

4. If you put some resistance in series with pin 5 of U1B, the MCP6024 version might survive the output being connected to 5V.

5. At high frequency, the PSRR of opamps isn't that wonderful, so you may want to examine the supply side of things in more detail. At its simplest, that might be a 10R resistor before the decoupling cap, or something like that for a bit of filtering.

6. How much variation are you seeing in the current as you sweep the output across the compliance range?

RE: Review and test a 0 - 1mA constant current source

Jan Cumps — Sun, 10 Mar 2024 09:41:49 GMT

LTSpice simulation

[View:/cfs-file/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-c8264f76-4871-4c68-9f84-334df49de529/3157.constant_5F00_current_5F00_source.zip:640:360]

The current lands slightly above the 1mA, because I set Rs as 33 ohm. It should be 100 ohm / 3 = 33.3 ohm. In that case the line lands on 1 mA for Vctrl = 1 V:

RE: Review and test a 0 - 1mA constant current source

DAB — Sat, 09 Mar 2024 19:34:09 GMT

Nice post, well written.

RE: Review and test a 0 - 1mA constant current source

shabaz — Sat, 09 Mar 2024 15:30:50 GMT

The PCB layout; it's just a small 1-inch square test board.

The KiCad files and Gerbers are in the following zip file:

[View:/cfs-file/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-c8264f76-4871-4c68-9f84-334df49de529/constant_5F00_current_5F00_rev1_5F00_files.zip:640:360]

RE: Review and test a 0 - 1mA constant current source

shabaz — Sat, 09 Mar 2024 14:24:38 GMT

Hi Jan,

Thanks for trying it out, and for all the analysis and measurements. Great work with LabView, it's made me realize I should upgrade my PSU one day, for one with programmability for such purposes. Hopefully the current source can come in handy in the future. In the past, I used this circuit:

..but it only supported fixed 50uA/100uA outputs. It was still useful for applying low currents to semiconductors etc, but not great for sweeping through a whole range of course!