Simple variable constant current source

[Pjdd]

I made this very basic variable constant-current source a long time ago for one specific purpose. It was meant to help me sort out an old collection of assorted zener diodes, most of which had had their numbers worn off. In some cases, I also wanted to evaluate their dynamic resistance. It has a range of about 0.5mA to 15mA.

It's one of my numerous minor projects and the performance is good enough for the stated purpose. There are alternative circuits with better performance, but this one is as simple as it gets and uses cheap commonly available parts. I expected to discard it after using it for for the said purpose but now, some 30 years later, I'm still using it for purposes other than the original one.

In case anyone is interested in making one for his/her own use, I can upload the PCB layout and add other details.

 

[Pjdd]

One application where I put this constant current supply (let's shorten that to CCS) to good use is to revive old rechargeable batteries. For example, when NiCad batteries were still widely used to power cordless phones and other gadgets, sometimes the battery's internal resistance became so high that it could no longer be charged with a normal charger and had to be discarded.

This CCS can supply more than 40V at 15mA. The high voltage can force current into the battery while limiting the current to a safe value. The forced charge reforms the battery's internal chemistry and gradually reduces the internal resistance. The battery's terminal voltage actually decreases steadily until it reaches the normal voltage. It can then continue to be charged with the CCS or with its own charger.

I did this a number of times. The revived batteries may not have quite the same mAh capacity as new ones but they are quite usable. Some of my clients continued to use them for a long time.