NiMH battery charger - Page 1
Forum section: electronics / projects

author: quim
author_rank: Newbie
timestamp: February 04, 2018, 10:00:57 am
content: Hi!I've been designing an AAA NiMH charger recently and I'm having some troubles troubleshooting it. The charger is based on the cheap 1117 smd linear regulators to mantain a constant current. I've added two in series to decrease the power each one has to get rid of (probably not as good idea as I thought). The thing is the current has a lot of noise; in the order of 200mA with a charging current of 700mA. I'd like to know if there's any known way to stabilize these types of loops or if it is a bad design overall. If it is indeed a bad design for low noise I would like to know how bad is it to charge NiMH with these conditions.PD: I've tried attaching caps on the outputs of both regulators and things get even worse.

author: IanB
author_rank: Super Contributor
timestamp: February 04, 2018, 10:39:56 am
content: Quote from: quim on February 04, 2018, 10:00:57 amHave you thought logically about what it means to put two constant current sources in series? By definition the current through two devices in series is identical, and yet each constant current driver is going to be trying independently to maintain its own constant current. It is inevitable that they will fight each other and the control loop will go unstable.If you want to reduce the power dissipated by the regulator, why not just put a resistor on the supply side to drop some of the voltage? If you want to get more sophisticated you could put a voltage regulator before the current regulator so the current regulator sees just enough supply voltage to stay in regulation.

author: BravoV
author_rank: Super Contributor
timestamp: February 04, 2018, 10:45:27 am
content: Its really bad-bad design, NiMh cell should not be charged that way, its a recipe for shortening the battery's life.Google for battery university, read the section how to charge a NiMH cell properly.

author: quim
author_rank: Newbie
timestamp: February 04, 2018, 10:56:27 am
content: QuoteSorry I must have explained myself badly, there's no such thing as two constant current sources in series in my design; the regulator on the left is always 1,2 volts above the second regulator (basically the idea you explained on your post).QuoteWhy is it a bad design? I'm monitoring the dV/dt through a uC and also the dT/dt. I don't know if you where thinking of this or the noise in the charging current (which I can not find any reference of in the website you quoted).PD: thanks both for your answers!

author: David Hess
author_rank: Super Contributor
timestamp: February 04, 2018, 12:25:50 pm
content: The AMS1117 is a low dropout regulator so it requires an output capacitor for stability as described in the datasheet. This might not apply when using it as a current source however the similar LM337 does.The operational amplifier may require some extra external frequency compensation to maintain stability while driving the AMS1117. I would start by adding a 10k resistor in series with the inverting input terminal and a 0.1uF capacitor between the output and inverting input.It would be fun to drive the first AMS1117 with another operational amplifier so they they split the dropped voltage evenly. As you have your circuit configured, the first AMS1117 is providing 1.25 volts to the second AMS1117 which may be below its dropout voltage.

author: quim
author_rank: Newbie
timestamp: February 04, 2018, 12:50:10 pm
content: Quote from: David Hess on February 04, 2018, 12:25:50 pmThanks a lot! I think you might be right and the problem might be there's not enough room for the second regulator to regualte properly (the datasheet states maximum dropout of 1.3V). I'll try to add some more room with a diode. I'll also try adding the resistor and capacitor to the opamp if that does not work.