Question about connecting cell phone charger to transformer

In article , snipped-for-privacy@SPAMNOT.screaminet.com says... | IF I wanted to take my cell phone car charger and plug it in to a 12 volt | transformer instead of the car battery, what amp rating to you think the | transformer would have to be? I know it would have to be an output of 12V | DC and know it would need to put out sufficient current but don't know what | is enough for my Lucky Goldstar V111 Verizon cell phone. | |

12 to 14.2 DC would be fine. Check the mA hour rating marked on the battery, time how long it takes to charge it from flat to fully charged.

Divide the hours into the mA hour rating, then multiply that figure by

3 (to allow a bit of spare capacity). That last figure should be an adequate mA rating guesstimate for your PSU.

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Yikes. I need an example to make sure I understand.

So it my 1200 ma hr battery takes 4 hours to fully charge, I get 300, then multiply by 3 to get 900. So you say I should use a 12V DC power supply that outputs at least 900 ma. Right???

Um, I assume no damage will be done if my PS is rated at higher than that, correct?

Also, what happens if it is much lower, say 100 ma? My guess is that all will work fine with no damage (do you notice that is a big concern :) ) although it will take much longer to charge the battery. Am I correct on this?

Thanks for all the help!

Ted

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Darn. That's a good idea. Check the output on the wall charger (if it has one) and use at least that ma rating? I ask again, am I correct that there will be no damage if my PS is rated for a higher ma value?

On a different subject, how does one tell if an output is AC or DC? I have a multitester. Is it enough to set it on DC Volts and see if switching the leads makes it go from positive to negative? This would mean DC, right? Can this hurt the tester? Is there another way? There must be.

And you are correct, I was sloppy with my wording when I called the wall plug DC power supply a 'transformer'. My knowledge of electronics is basic and rusty so I really appreciate the help you guys give.

Ted

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Your supply will probably die and certainly overheat if your phone tries to take more than the supply can give. No damage at all will occur if the supply can supply more than the charger needs. This assumes that you are using the charger that you used to plug into the cigar lighter of your car. If you intend to do it without the car charger then the problem is a lot more complex and I wouldn't recommend it particularly if you have LiIon batteries in your phone.

Thanks! You all are sure nice to lend your knowledge to others. (I do too, but not in electronics :) )

Firstly, yes, I'd absolutely be using the car charger.

Secondly, I just noticed something else. My battery says 3.7 volts. (doesn't tell amps) The house charger I use says it supplies 5.2 VDC.. BUT, the terminals from it attach to a different place than does the plug for the car charger and the other type of house charger which I do not have. (With the house charger, I have to set the phone in a base to charge and cannot use it while charging. The car charger can talk and charge at the same time.)

So, this kills the other idea of looking at the output of my wall charger (5.2V) since it connects to a different place on the phone.

Well, I do have some DC power supplies from 12 to 14V that are from .6 to

2A. Do you think I could try the lower ones and just keep watching (feeling) the PSU to see if it is getting too warm? It would take a few minutes to overheat, wouldn't it?

Any informative web sites I could read (and understand) on this topic?

Thanks again for all the help. If I do decide to try this, you've all be really helpful.

Ted

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In article , snipped-for-privacy@SPAMNOT.screaminet.com says... | Well, I do have some DC power supplies from 12 to 14V that are from .6 to | 2A. Do you think I could try the lower ones and just keep watching | (feeling) the PSU to see if it is getting too warm? It would take a few | minutes to overheat, wouldn't it? | |

Start with the heavier current PSU's and work down from there. Make sure the battery is is need of a charge each time and check the temperature of the PSU case after 15 minutes. They should get appreciatively warm to the touch (guess 50 deg C internal), but not hot.

A vehicle power supply will normally vary between 11.5v and 14.2v, so it should be perfectly happy with any of your described supplies.

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Regards,
         Harry (M1BYT)...

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The way to tell is to measure it when you have it plugged into the car, the battery is flattish and it is making a phone call, that should be maximum current requirement. If you can make it a weak signal as well then that would be more accurate. Multiply that figure by 3 or 4 and use that as a minimum.