# need advice - want to power a DC device through AC

Discussion in 'General Electronics Discussion' started by davidcom, Oct 2, 2010.

1. ### davidcom

5
0
Oct 2, 2010
Hello all.

My daughter has a music box she listens to every night while falling asleep. It runs on three D batteries. I'm tired of constantly buying new D batteries! I'd like to wire the music box to be able to plug into the wall. Can you give me some advice. I'm a complete novice at this.

I've got some old cell phone wall chargers. They all seem to give an output of around 5v. The lowest one I have gives an output of 4.9V 450mA.

If the device takes three 1.5V D batteries, does that mean I need to find a AC to DC convertor that will output 4.5V? I'm not sure what the mA needs to be. Is that important?

My idea was to clip the end off the cell phone wall charger, strip the wire and connect the positive and negative leads to the positive and negative battery connectors in the music box. Will that work? Is it that simple?

Any help would be greatly appreciated.

2. ### davidcom

5
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Oct 2, 2010
Sorry, just checked the music box. It takes three 1.5V C batteries (not D). Not sure if that makes a difference.

3. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
2,840
Jan 21, 2010
Its actually reasonably likely that a 5V 1A charger would work fine. new alkaline cells can be up to 1.7V, so 3 are around 5V and thus it is likely that the unit will cope quite well with this voltage.

your plan sounds reasonable. Just make sure you get the polarity right.

4. ### davidcom

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Oct 2, 2010

Should I be concerned about the mA rating. I just read somewhere that if the mA rating on the charger is too low, it could cause melting/fire problems after prolonged use. Is there a way to know what the mA requirements are used in devices taking three C batteries.

I've just looked at my iphone charger, it is at 5v with a 1A output. That seems quite a bit more than the 450mA on the spare phone charger I have.

Thanks!

5. ### Leighcusack

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Sep 9, 2010
How long do the 3 C batteries power the music box for?

Battery
Type Avg. voltage
During discharge milli-Amp
hours (mAh) Watt-hours
Wh Joules
J
Alkaline
Long-life 1.225 7800mah
Carbon-zinc. 1.1. 2172mah
NiMH. 1.2. 4500mah

You mention that you buy new batteries, so let's assume either of the first two in the list above.

If using Alkaline Long-life and they last longer then 17 hours then it's safe to say your phone charger can supply enough current.(this is assuming total discharge which is unlikely but is worse case senario)

Using the carbon zinc you'd be lucky if you got more then 5 hours.

Do you have a multiple meter? Measure the current flow using the various settings of the music box.

However, I'd would be safe to assume that the unit wouldn't draw more the then 450mAh the phone charger can supply. If it did it was poorly designed, as the power reserves are too low.
Hook it up, it it gets hot to the touch then find another power source.

Btw, you sure the batteries are wired in series and not parallel?

Last edited: Oct 3, 2010
6. ### davidcom

5
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Oct 2, 2010
So I checked that the batteries were series, and not parallel. I went ahead an wired it up and it worked! Thanks for all the input!

I've let the music box run for about an hour and checked to make sure nothing was getting warm. All is good.

Thanks again!

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
2,840
Jan 21, 2010
Sounds like a good outcome.

Just to stick a fly in the ointment, here is a potential problem:

Phone chargers are designed to operate as phone chargers, i.e. they supply near their rated current for several hours, then the battery is charged and they need to supply no (or very little) current. This means that they can be designed to overheat if used continuously. The practical implication is that they fail if left connected to some device drawing what may appear to be a moderate load for a long period (say, several days to several months).

I hope you don't get bitten by this, but it sounds like the device does not run continuously either. If nothing gets warm (i.e. cooler than charging a phone with it) then there is much less to worry about.

8. ### davidcom

5
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Oct 2, 2010
Thanks for the input on the phone charger design. Yes, the device, once turned on, has an auto shut off timer. It runs for about 20 mins and shuts itself down. So hopefully that will avoid any issues with the phone charger.

Thanks again!