Source
r/AskElectronics•
Musfuut

Adding wall adapter to battery powered product.

design

A quickie I hope. My mother owns a battery operated light like the one pictured here. It operates on 3 AA batteries. It contains an amber LED and RGB LED as well as a motor, each controlled from a blister IC.

Last year it used about $20 in batteries over the holiday season, and needing to change them daily was inconvenient.

I would like to add an external wall adapter to it but I want to make sure I am doing it correctly and safely so I do not damage the light.

My plan was to connect a 4.5v 300-400ma (power draw of light when motor is running appears to be 200-250ma) wall adapter to the battery leads, using a couple diodes to ensure current from the adapter cannot flow into the batteries.

Does this seem the reasonable method to take?

Things I am trying to avoid on this project:

  1. High cost, if the project exceeds $20 then she will opt to just use batteries.

  2. Needing a switch to select between battery and external, there is little room in the base and it would be hard to mount a switch.

Thanks!

Posted Nov 5, 2013, 12:07 PM UTC

38
classicsat•

My mother has an LED color changing snowman 3 or 4 AAs). I just made a little cord with a pair of dowels, some wire, and a USB B male. A standard USB cable plugs that into a USB wall adapter, or USB on the satellite receiver.

It works well.

If you want to put an actual jack on there, get an interrupting kind.

Don't worry about adapters with poor regulation, batteries aren't much better.

Go to your local thrift store and go through their box of adapters, you likely will find one.

2
MusfuutOP•

Awesome, thanks for the help. :)

1
ChryseusAnalog electronics•

Batteries have far better voltage regulation than your average power adaptor, I've seen adaptors rated for 5V put out 9V which will fry most circuits without internal regulators, of course it depends on the type of adaptor, switch-mode ones generally providing the correct voltage.

1
wolfcry0•

Yep, a diode and a power supply that's similar voltage to a set of charged batteries is the easiest way to do it

I bet you can get away with a mobile phone charger at 5 volts, since they're everywhere and should be a piece of cake to find for free or a couple dollars

And you could always use an LM317 or something to drop down to 4.5 volts, they only cost $1-2 so it wouldn't add much cost

1
MusfuutOP•

Well the LM317 requires resistors and caps of the correct values, as well as either a perfboard or shrink tubing to really assemble it into something that won't short out.

I'll try to find a cheap charger though before I buy anything else as the savings there may pay for the rest of the parts.

I tried buying everything from jameco and it ended up being $32 after shipping, $28 through radioshack. Mostly the adapter cost and the shipping for both places. Once I have the adapter I can figure it out from there.

I was mainly concerned due to some simulations I ran in falstad circuit simulator that showed under a certain condition the total voltage being supplied via the diode method could be ~0.5v higher or lower then expected.

1
ChryseusAnalog electronics•

DC power adaptors are practically free if you look around, people throw them out all the time. Also jameco and radioshack are very well known for being expensive, I suggest taking a look at some proper distributors like digikey, mouser, newark and allied electronics, things like perfboard and heatshrink can be had really cheap from ebay.

Also 0.5V above or below should be just fine.

1
ChryseusAnalog electronics•

Most DC power adaptors have very poor voltage regulation so you really want to include a voltage regulator like a LM317, all you need is a 9V supply, a few capacitors, a 220 ohm resistor, a small 10k pot and a small heatsink for the LM317.

These can be bought for under $10 from any decent supplier.

-1
MusfuutOP•

Why is this answer being downvoted?

1