I'm trying to implement a bi-color LED connected to a Microchip MCP73831 battery charge management controller, so I can switch between red and green LED output. Currently my circuit is mixing them during charging and gives me an orange color, but it's hardly distinguishable from a green LED when it's fully charged.

I want to add an NPN transistor to the negative side of the green LED, so the transistor will switch it on when the STAT pin is high and it will turn off when the STAT pin is low.

Problem is that I couldn't find the STAT pin current when it's in the high state. I looked around in Google and almost everyone uses a limiting resistor on the base of NPN transistor, generally 1k ohm value.

Can anybody tell me what resistor value I should use for R3? Or do I need it at all?

Datasheet for MCP73831

Datasheet for the Nexperia PMBT2222 NPN transistor which I'm thinking to use. The peak base current is noted as 200mA, if that makes any sense.

schematic diagram

  • 1
    \$\begingroup\$ Did you read the datasheet? What you want is right there on page four. \$\endgroup\$ – Hearth Mar 16 at 19:00
  • \$\begingroup\$ at the end of high output voltage it says 4 ma i saw that but in this case can i get away without using that R3? \$\endgroup\$ – Kamen Boyadzhiev Mar 16 at 19:04
  • \$\begingroup\$ No, you cannot. If you look at the current rating on the pin, it's rated to source no more than 35mA, and you have to limit the current to that (or less) or else risk breaking it. \$\endgroup\$ – Hearth Mar 16 at 19:05
  • \$\begingroup\$ (5V - Vf of the LED) = R * 0,01A \$\endgroup\$ – Wesley Lee Mar 16 at 19:55
  • \$\begingroup\$ ok made some digging and its not very straight forward. according to some other forum sources my calculation looks like. load 20mA(green led)/100(transistor hFE) =0.2mA required base-collector current for saturation. base voltage-base emitter drop : 5v-1V=4V ohms law 4V / 0.0002A = 20k ohms so to make sure saturation current is reached we half that value 20k/2= 10k ohms is what i need. does that look correct? \$\endgroup\$ – Kamen Boyadzhiev Mar 16 at 20:10

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.