Building a small D cell battery charger?

Question

I am looking to buy/build my own D cell Nimh battery charger with the following requirements: -dual prong AC wall input -charges Nimh D Cell -is safe -contained within 1" x 1.7" x 2.2" enclosure

As far as I can tell there is nothing like this to buy at the moment. I have also searched the web for a schematic and found almost nothing of use.

I have played with bridge rectifiers and basic power supplies in the past and was wandering if building something with these specs would be a possibility? Also are there any ICs that could be safely used instead of actually building the entire bridge (anything to make it smaller)?

1. What would be "desirable" design traits of this supply?
2. Would I need to make it a smart charger? Is a smart charger really all that beneficial for Nimh D cells?
3. Can I get away with a the most basic design of 4 diodes, a cap, a regulator and 2 resistors (similar to this:http://upload.wikimedia.org/wikipedia/commons/1/1d/Diode_bridge_smoothing.svg)?
4. How much current should I want to design at the load?

I realize this is kind of vague, but I have been searching the web for hours and was hoping maybe someone could provide some insight. Possibly just a starting point and then I could start designing my supply.

Answer 1

I would NOT design anything that would require direct AC power if I were you. Really. You can design your circuit to take 12/24VDC or whatever you need from a wall wart and then use that to charge your D cell.

Seriously. Do not go near the AC outlet. There are lots of design safety issues to consider, the first of which is using an isolating transformer.

Oh one more thing. Do not design anything that takes AC power. Don't.

As I said you can get a wall wart, or use any of the adapters you have sitting around if it supplies the DC voltage/amperage you need (which it should be able to) just use that.

There are lots of NiMH charger circuits out there you can use for reference. Your charging current is a function of your battery capacity, often expressed in terms like 0.1C meaning if your battery is 1Ah you want to charge it at 100mA.

You want a smart charger because NiMH are sensitive to charging current and voltage and have differing requirements depending on how discharged they are. If you ignore those you'll damage your battery.

I am fairly certain you should be able to find a commercial charger that can handle your D-cell. It may not be physically compatible in which case a set of alligator clips/wires can help you connect the two.