Thread: Convert battery prop to plug-in

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Help and sanity check needed please.....

I want to convert a battery powered LED candle to be plugged into the wall. I first thought this was easy....just Google and WALLA! I get an answer. Well...no

Through much research I have learned that batteries are DC powered...not AC like what comes out of your wall. So I need an AC to DC adapter. OK, got that. The prop uses a 9-volt battery. I also learned that you must match the voltage needed. OK, so I need a DC adapter that provides 9 volts of power.

Here's where I am stuck. I do not know how much current the LED light uses. I know that a 9 volt battery provides 565 mAh of current. But, that means for 565 milliamps per hour, it will provide 9 volts of power. But...UGH! how much current does the LED candle draw? What current (milliamps) does the DC adapter that I buy need?

I have a few guesses...I think that battery driven devices use about 600 mA. Am I right?

Bonus question: What if I wanted to string 4 of these LED candles together and use one DC adapter? Do I multiply current for each candle needed times 4?

Thank you so much for helping me here

I believe Monster Guts has what you're looking for.

Should be able to run just about all the LEDs you want + many more

Lighting, Strobes, LED's - 9V Battery Eliminator - Halloween Prop Building Supplies For The Home Haunter - Monster Guts

Terra, a typical LED will draw between 20 and 30 Ma of current. If you want to run 4 of them for instance you would take lets say, 20Ma times 4 equals 80Ma or .08 Amps. A typical "wall wart" supply will give you more than enough. I am assuming that, since you are merely eliminating the battery connections and adding the wall wart that the current limiting resistors are already in place to run the LED from a 9V source. (They typically run on between 2-3 volts) Hope this helps, Love all your props they are an inspiration to me just starting out. Keith

It would be a safe guess to say that the device doesn't use over 50mA constant current draw. If the candle is just a single LED it most likely uses around 20-30mA. The way to test for sure is to use a multimeter to test the current, but its not a big deal. We will call it 50mA to be safe.

The wallwart andisnw supplied would be fine to power 2 (maybe 3). However this one will get you a bit more:

SparkFun Electronics - Wall Adapter Power Supply - 9VDC 650mA

650mA/50mA (per prop) = 13. You could safely run 13 props with the wall wart from sparkfun. (if the props use 50mA or less, that is). After shipping it would probably cost you $10 total.

If you go with something else make sure you get REGULATED power supply. Some walwarts are not regulated and will actually have an AC wave form, this will break your stuff. Dont trust the guys behind the counter at ratshack

SOME BORING SCIENCE:
The wall wart andisnw linked is probably transformer based which is fine but has a couple down sides. One being its not as size efficient. Both that wallwart and the one I linked are about the same size but the one I linked can source 6 times more current. That is because it is a switch mode power supply (SMPS), which for the same size, can most often source more current than a transformer based one. Another advantage is they are more power efficient. Transformer based power supply are around ~80% efficient, that is 80% of the power being taken from the wall is used and converted to the DC current used to power your props, the other 20% is being wasted as heat. Switch mode power supplies are somewhere around 90% efficient. This doesn't really matter much for such low power devices. We're talking power bill savings of pennies in the differences.

Don't feel bad - it is confusing. A standard LED needs about 25mA for full brightness. You have to use a resistor in the circuit to prevent the current from getting much higher than this or the LED will just burn up. (you don't need to worry about it in this case as your LED candle already has the resistor in it. But just for reference, if you were using a 9V battery and you wanted to connect an LED to it you can use the equation R=V/I to calculate what resistance you need where V = the voltage and I=current so in this case you would have R=9V/.025A = 360 ohms. Resistors only come in certain values so just buy whatever you can find close to this value.

For the 9V battery you have is rated at 565mAhrs. This means that it can supply 565mA for one hour and then its dead. It also means that it can supply half that current for twice as long ie., 282mA for 2 hours. For a single LED, it only needs to provide 25mA so 565/25= 22.6 hours of operation.

So how do you connect 4 of these leds? You could string them end-to-end but this is not a good idea as each LED will drop the 9V down a little for the next LED. If you were only going to do 2 LEDs then you could probably do it without a problem but more than that and you are pushing it. The other way to do it is to connect each LED in parallel meaning each LED is connected to both the + and - sides of you power supply individually. In this case the current of each LED is added so your power supply would need to supply 4 x 25ma = 100ma. In reality you should look for a power supply that has at least 25% more capacity than this or about 125ma. If you find one that is more than that than thats even better. Relatively speaking this is a small amount of current so you should have no problem finding a 9V power supply that will work.

Thank you so much everyone. I'm going to spend the day munching on this info. (That's how I learn this complicated engineering stuff).

Just to let you all know what I am up to: I ran across the most realistic fake pillar candle and finally tracked down the manufacturer. Sadly, they no longer make them. UGH! But, there are many different versions of flickering candles and so I thought I'd buy one of each and test them out. They all use a variety of battery configurations and bulbs (incandescent and LED). To make it simpler for testing and use, I want to be able to have them be plugged into the wall. I will be making a video showing all the different varieties out there. There's one that I love so far (expensive - of course!) but I'm hoping the less expensive ones will give it a run for the money

Thanks again, this info was really helpful.

I've played around with my tea lights and a 12v wart with and without resistors given the 3v x 4 = 12v principle. I could get the lights to come on but none would flicker. I tried them in a daisy chain, each direct to current, added a 5th, removed one and tried 3 with all of the above combinations. Each and every try they would light but not flicker. I gave up (for now).

I must have been missing something?

Just as a side note, as azdude was pointing out, the ratings of a power supply is how much current draw that it can handle without it failing . A power supply that puts out 2 amps will not be a problem because the draw will be much less. Hope this helps...

Does this help the cause?

9-Volt Battery Eliminator

Dan

Terra,
I to am interested in this subject. I want to make several single bulb blinding LED blue spot lights for my tombstones in my graveyard. What I don't want is big extension cord running throughout the graveyard (over kill for LED lights). The guys in our IT department at work suggested using CAT5 cable to each LED light, then bringing them back to a single punch block. Then powering the punch block with the 3.3 v side of an old computer power supply.

I love this idea because you could build a little box with the powersupply and punch block and make the box look like a stone with foam, place it in the center of the grave yard and spider your lights from there. They would easily be plug and play (light). Plus the CAT5 cable is small and easy to handle. I would be interested in anyones opinion on the suggestion.