hello everyone,
i have been working on my own 4v 18650 li-ion battery charger based on a MAX1555.
the charging is working.. however i have a probem with the indication.
you see, the MAX1555 has a sink pin for when it is charging... for connecting a LED, however, i wanted to have 2 LED's to tell if it is charging or not.
now, i drew it out, even tested it in yenka (i donno if any of you know that... it's basic but it works).
however, connected it doesn't work, anything i did wrong? please help, it is driving me nuts.
at first the red LED staid on, and now only the green one... thats no good.
added info:
schematic:
https://gyazo.com/4c29436a43964905e518dc1380c75f5dSoldering:
https://gyazo.com/036ab77533a5e60f2cecc8009c8f0a9dcomponents:
https://gyazo.com/e24e8853d6974b87ecf934ed44fdc0edMAX1555 datasheet:
https://www.sparkfun.com/datasheets/Components/MAX1551-MAX1555-1.pdf
The schematic doesn't say which LED is red and which is green.
I suppose then when it is charging D2 should be on and D1 should be off.
And once complete D2 should be off and D1 should be on.
If I do a quick simulation it will show that there will always be a small current through D1 so it would be lit up a bit.
(
http://tinyurl.com/zyxwq86)
Why Q2?
It sinks current away from Q1 but the MAX1555's CHG output is already sinking current.
Q2 would just raise the base of Q1 by .6V
To turn off D1 you want to turn off Q1 and therefore pull it's base as low as possible.
Put Q1 between VCC and D1/R3.
What are you measuring in the circuit?
sooo, basicly what your saying... is i am using the NPN wrong?
The NPN will conduct when the base is .6V (one diode) higher than the emitter.
If you tie the emitter to ground then any Vb-gnd above 1 diode will make it conduct.
If you tie the base to a current sinking transistor that is also connected to ground then the Vb-gnd will never get lower then 1 diode so it will conduct a tiny bit.
If you place the NPN on top of a 2V diode then you need a Vb-gnd of 2.7V to make it conduct.
The current sinking transistor can easily pull the base below that.
sooooooo, i build the circuit as it said in your simulator... an the green led (the one with the NPN) won't go off...
What if you leave out the MAX1555 and simulate it's CHG current sink with a switch directly to GND?