EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: made2hack on June 21, 2016, 06:50:08 pm
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Hello all,
Here's the circuit I'm using with an MCP73831 (http://www.tme.eu/gb/Document/c098b907e182272dea149b0595062e17/mcp73831_73832.pdf) to charge a single 18650 LiPo cell. I have the /1 at the end and the STAT pin is a Tri-State logic output.
(http://i.imgur.com/PPajorq.png)
Can I use this to drive a common Cathode Red/Green LED? How would this work? I want say red when it's charging and green when it stopped?
Right now the led just stays on when it is charging as above and lightly flickers when the battery is disconnected.
Thanks
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You can't drive a common cathode LED without additional components. (additional components = anything other than current limiting resistors).
You can drive two separate LEDs without additional components, though inefficiently.
For common cathode LED's you would need 2 transistors, a NPN and PNP.
http://www.electronicshub.org/wp-content/uploads/2015/01/7.-Transistors-Driving-LEDs.jpg (http://www.electronicshub.org/wp-content/uploads/2015/01/7.-Transistors-Driving-LEDs.jpg)
Like above, only for the NPN, put the load on the emitter. You might also need to put the current limiting resistor up on top of the collector, for both cases.
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I have used these before and for one product, I simply used a common anode tricolour LED (that is, two LED's with their anodes commoned, not bicolour). Anode was connected to the input power supply, the green cathode was permenantly grounded via suitable resistor. The red cathode was on the STAT pin. So with no power supply, LED off. Charging - yellow, done - Green. But it was also green when no battery was present.
You would implement a 'red - charging, green - done' with same LED, but with an extra NPN transistor on the green cathode. So when the STAT is low, transistor is off, but pulls the red cathode low. When STAT is high, transistor on, green LED on, red off.
There's many ways to do this with relatively simple components just depends on what kind of indication you need.
Another option would be a bicolour LED, with one side connected to STAT and the other the centre of a voltage divider between charger input and ground. The resistors would permanently draw current (say ~500 ohms, total 1k, across 5V = 5mA) but would mean the LED would be one colour when STAT is high, another when low, and 'off' for high-Z. For projects where I have used this I replaced the voltage divider with a ~2kHz CMOS schmitt trigger oscillator. Has the same effect but reduces quiescent current consumption - handy if you're driving lots of these LED's.