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How to reduce the 1A current to 0.4A? Answered

I am using an AMS1117 to regulate the voltage at 5V and 1A, and this tension must recharge a battery that accepts a maximum 4.2v and 0.4A

I reduced to 4.2V using voltage divider, but I do not know what to do with the current.

Suggestions? Thank you!




Best Answer 2 years ago

What you need is a Constant Current power supply, that will enter Constant Voltage mode once the battery terminal voltage approaches 4.20V. Below is a pretty good video that discusses constant current sources. They generally work by regulating the voltage across a small value resistor, hence controlling the current through it as well as the load. Dave from the EEVblog also has a great video talking about safe charging techniques for lithium ion batteries.

The easiest way to do that might be to build a constant current driver like the one in the video and set it up to output 400mA, and use a LED and some series diodes to create a 4.2V shunt regulator, that will also light up the LED when the charger has entered CV mode.

One thing you want to watch out for though, is being careful not to exceed a voltage of 4.2V. That voltage is very critical, exceeding it by even a small amount has a large impact of the life and health of the lithium ion cell.

Excellent videos helped me a lot. I managed to complete my project. Thank you man!

Afrotechmods and EEVblog are some of the better YouTube channels especially for electronics. I also have a YouTube channel, powermax (same as my instructable identity) and I post electronic tutorials as well, and plan to add more as I get the time to do so.

I'll take a look, my English is not the best, but I can understand many things. thank you


2 years ago

A current limit will do it and probably eliminate a voltage drop because the regulator forward drop will be below 5 volts.

Remember at first a discharged battery will be low voltage which will climb as you charge it with current flow.

See (click) the simple circuit diagram.


Perhaps you want a voltage watcher to turn OFF the battery charge when the voltage reaches charged battery...

5v 317constCurr400ma.JPG

What about 5v to 4.2v? Any suggestion?

great idea stop charging the battery when it is full.


Try this but may need the 12v source instead of 5 because two regulators will drop too much volt...


I was using 9v, but I must have a source of 12V as well. thank you

This excellent website has a working circuit, with the correct safety circuits, and good notes on its use. I recommend you read it.


I had already seen, this was a great help. Thank you anyway


2 years ago

Thanks for the replies.

Steveastrouk, the battery I am using is attached.

Downunder35m, I thought about it, but did not think it was not the best way, you think I should do it, or change my circuit? If you think I should change could direct me to a tutorial for charging Li-ion batteries?



Ohm law as a starting point.
Calculate the resistor value needed to limit the current to 0.4A.
If you want to be precise also calculate the voltage drop of this resistor and adjust the input voltage accordingly.
Last but not least use the voltage drop and amp use to calculate the Watt needed for the resistor.
When done ssimply add the resistor between power and battery in series.
But for obvious reasons it would be best to use a proper charging circuit ;)

Er. You should have used an adjustable AMS1117....

What kind of battery are you charging ?