Powering & Illuminating a Clock

[kewrock]

Hello forum. Noob here, pleased to meet you.

I have a battery operated clock that I want to adapt for AC, and illuminate it with two constant running LEDs.

The clock was designed for two AA batteries, so I figured I'd get a 3 volt wall adapter, cut the end off, and wire it to the batt terms. They're on ebay as cheap as a dollar. What's involved with wiring in two high intensity LEDs? I've found 3 volt LEDs, but it seems they're sensitive to mA. most run at 250, and most wall adapters put out 400-1000mA.(the switchable multi-volt adapters put out 1000). I read the MODS sticky on LEDs, but it's way over my head. I do understand that they need to be either in series with one resistor, or parallel with 2 resistors. Which is better for my application?

What LEDs should I use, and what resistors? And will the mA rating on the adaptor effect the operation of the clock itself?

The LEDs will be on 24/7, so should I look for a higher capacity LED, like 4 volt? or underpower the amperage?

Any help would be greatly appreciated.

 

[(*steve*)]

Check the sticky on driving LEDs here again, and tell me what you don't understand. You only need to read the first 2 sections. One of the first thing you should note is that LEDs DO NOT have voltage ratings per se. 250mA would be enough to illuminate a room, way to much for a backlight on a clock.

Note that at 3V you're going to have trouble with any colours other than red, green, or yellow. And 20mA is the current you'd want to use.

A 3V adapter would require lots of filtering (say 1000uF) so that you don't see too much ripple (since the clock circuit is expecting DC from a battery and may be quite sensitive to noise).

The output voltage may also be way higher than 3V. Anything higher than about 3.4V would be a bad thing.

Personally, I would go for a 5V switchmode plugpack (typical of most cellphone chargers these days. 5V is fine for driving any colour LED, and a 3.3V regulator would make the voltage suitable for driving the clock. The amount of ripple on these power supplies is likely to be fairly low, but I'd still go with some filter capacitors (maybe 100uF)

 

[kewrock]

Steve. Thanks for your help.

The clock and adapter were delivered today.

The wall adapter output is 3V DC 100mA(sorry, I added an extra zero to the specs in the previous post). That should be fine for the clock.

The LEDs I want to use are coming out of a 5.3V USB driven keyboard light. The light assy has 3 very bright clear LEDs connected in parallel with a 20ohm resistor. I want to cut out one or two of these bulbs(haven't decided yet), and install them in the clock with the new adapter. Can it be done? What resistor should I use? Or is there a different resistor/LED combo I should use. It needs to be pretty bright though. thanks again for any help you can provide.