Hello everyone - this looks like a fantastic forum. I know that this is a topic that comes up somewhat frequently however I have searched and couldn't find anything specific to my needs. I'm a hobbiest with no formal training but have fooled and built more than my fair share of circuits.
I have a design right now that uses 2xCR123A rechargeable batteries in series to power a big array of LEDs. I've figured out so far that what appears to be my best/cheapest bet is to use the MC34063A regulator in step-up configuration to power 4-led-series in parallel (for example a grid of 4x12 LEDs). The output voltage is 12.8V and Vin is up to 8.3V.
What I'd like to do is cutoff the power from the cells once they hit about 6.4V. I know that I could go lower but it's a tradeoff and besides from my testing and research there's only about 5% total power left between 3.2V and 3.0V on these cells - but the power supply design is easier if I cut off higher.
I was looking at several different circuit types to accomplish this, preferably without impacting the efficiency of the system at all. The basic idea seems to be to use some sort of voltage supervisor with a transistor. I could use a TL431 and a mosfet (from
here) but that incurs a loss through the mosfet. Small but I don't think necessary.
The MC34063A regulator (
datasheet here) is a cheap and efficient step-up converter that I would like to use.
Here's a sample circuit:
**broken link removed**
Now here's where my ignorance kicks in.
What I'm wondering is if I could use a similar circuit to TL431/mosfet but instead use a simple NPN transistor on either pin 4 (which is the ground connection for the reference regulator) or between R1 and ground. Alternately could I actually replace the 1N5819 with a mosfet instead?
The MC34063A was chosen because it's cheap and efficient - and this is for something that will be sold at small profit margins. However if there is something else equally as inexpensive that would also work I am open to suggestions!
Thanks everyone!