backup battery charger circuit help

[babylyn90]

Hey guys, Desiree here from 2265. We're trying to build our own backup battery charger based off the circuit we found posted on another thread. A few questions came to mind since we can't seem to find some components:

1.) I was wondering if we'd be able to do series connections with some of the resistors in a way that would add up to the needed voltage. In example, combine 1K, 200ohm, and 40 ohms resistors to make the 1.24K in R3.

2.) Are precise values necessary? In example, could I use 220 ohms instead of 221 ohms for R2?

3.) I can't seem to find the zener diodes either

4.) Does it matter what I build the circuit on? Recommendations?

I'd really appreciate it if you guys could post either with links or tips to other threads/posts i might have missed. Thanks and sorry if this was already posted, i probably missed it!

[GrammarNazi]

1) yes, i forgot that you could
2) no, try to get it as close as possible though
3) try mouser electronoics of even radioshack they shouldnt be hard to find
4) i would just build in on a prefab general purpose circuit board, making your own would take to much time and energy

[GrammarNazi]

also you can just buy a charger for like 20 buck thats guarenteed to work and take it apart and use that that might be easier

[kevin.li.rit]

Originally Posted by babylyn90

Hey guys, Desiree here from 2265. We're trying to build our own backup battery charger based off the circuit we found posted on another thread. A few questions came to mind since we can't seem to find some components:

1.) I was wondering if we'd be able to do series connections with some of the resistors in a way that would add up to the needed voltage. In example, combine 1K, 200ohm, and 40 ohms resistors to make the 1.24K in R3.

2.) Are precise values necessary? In example, could I use 220 ohms instead of 221 ohms for R2?

3.) I can't seem to find the zener diodes either

4.) Does it matter what I build the circuit on? Recommendations?

I'd really appreciate it if you guys could post either with links or tips to other threads/posts i might have missed. Thanks and sorry if this was already posted, i probably missed it!

1) You can connect them in series, it'll have the same effect. (You might want to measure it with a ohm meter to see how close you are to 1.24k)
2) The schematic doesn't mention anything about the percision so I doubt 1 ohm will kill you.
3)It's a digikey part number for the Zener 1N5345BMSCT-ND. It gives you a substitute. You can also try mouser like grammar said.
http://search.digikey.com/scripts/Dk...1N5345BMSCT-ND
This one should be good too.
http://parts.digikey.com/1/parts/421...8-1n5345b.html

4) I would probably build it to a solder on break board.

[RyanN]

For the resistors, I used a potentiometer and set it to 7.5 volts and skipped the Zener Diode and the battery will just slowly charge using the resistors that go around the diode connected to the battery. I did not include the resistor from the 12V to the backup battery as I saw no real reason to include it. What I have right now supplies a steady 7.5V and I think it will work reliably. I also added a diode so it would not try to power the whole robot once the breaker was switched off.