I wanted a NiMh charger so I bought one of these: https://www.ebay.co.uk/itm/1-2-24V-...e=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649
It arrived today and I realised that I hadn't read very carefully. This board controls a constant current (CC) and it has links to select a variety of currents but it does not control the voltage. You would have to charge for a controlled time which is definitely not what I want. But it was cheap.

Now, my question is how does it control the current?
It uses an LM2596 and the only other active component is a transistor. The only CC circuits I could find using this chip have the usual voltage control circuit and a sense resistor and op-amp.

 

I suspect that it uses the adjustable output version (Which seems to use a 1.23 volt reference voltage. If you look at the schematic (On page 9 of the TI data sheet.) for the adjustable output versions then if you make the value of R1 such that 1.23 volts is developed across it at the desired output current and put the load in place of R2 instead of in parallel with C out then it will adjust the current through the load so there is 1.23 volts across R1. I have done this with a LM2577S (Step up regulator.) for a constant current LED driver. This is the link to this idea http://lesjhobbies.weebly.com/led-driver-01.html.

Les.

 

Yes, I see. A very cunning plan. Thanks.

 

I wanted a NiMh charger so I bought one of these: https://www.ebay.co.uk/itm/1-2-24V-2-4-3-6-12V-Ni-Cd-Ni-MH-NiCd-Batteries-Charger-Modules-Charging-Boards/164299552107?ssPageName=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649
It arrived today and I realised that I hadn't read very carefully. This board controls a constant current (CC) and it has links to select a variety of currents but it does not control the voltage. You would have to charge for a controlled time which is definitely not what I want. But it was cheap.

Now, my question is how does it control the current?
It uses an LM2596 and the only other active component is a transistor. The only CC circuits I could find using this chip have the usual voltage control circuit and a sense resistor and op-amp.

A synchronous buck converter produces constant current using a feedback process where the feedback is fed to the PID controller which again control the duty cycle of buck converter hence controlling the output.