Drop amperage question.

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dl_evans
 
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Drop amperage question.

Post by dl_evans »

I picked this up from Amazon

http://www.amazon.com/Belkin-F8Z492-P-B ... b_title_ce

and it's 5V at 180mA (from what I've found about .2 amps right?)

My cell phone car charger is 5v at 2 amps.

I want to try and make some kind of android car-puter and would like to use a single power supply. How could I drop the cell phone charger voltage? Would I even need to? I've tried to find a calculator but couldn't figure out all the info I needed.

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easternstargeek
 
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Re: Drop amperage question.

Post by easternstargeek »

You don't need to "drop amperage"

As long as the power supply you want to use:
a. Has an output voltage lying within the rated range of operating voltage required by your device, and
b. Is capable of supplying at least the amount of current ("amperage") required by your device

Your device will only draw as much current as it needs. Unless a power supply is the wrong (higher) voltage, it will not "force-feed" more current into your gadget than it wants.

Just hook it up and have fun!

Agent24
 
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Re: Drop amperage question.

Post by Agent24 »

What EasternStarGeek said is correct, but there is another angle to this.

An unregulated power supply is designed for a certain voltage at a certain current. If you draw less current than it is designed to supply, the output voltage can increase beyond what it is rated for, and could cause problems depending on what you are powering from it.

It's unlikely that your car charger will be unregulated, but if we were talking about a linear power supply, it could be a problem, since 180 mA is a fair bit less than 2 Amps.

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dl_evans
 
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Re: Drop amperage question.

Post by dl_evans »

Agent24 wrote:What EasternStarGeek said is correct, but there is another angle to this.

An unregulated power supply is designed for a certain voltage at a certain current. If you draw less current than it is designed to supply, the output voltage can increase beyond what it is rated for, and could cause problems depending on what you are powering from it.

It's unlikely that your car charger will be unregulated, but if we were talking about a linear power supply, it could be a problem, since 180 mA is a fair bit less than 2 Amps.

yeah the 180 mA to 2 Amps is what I was worried about. Thanks a lot guys! This forum rocks.

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easternstargeek
 
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Re: Drop amperage question.

Post by easternstargeek »

Power supplies for consumer goods probably will not be subject to the kinds of minimum-load problems described up-thread, but just to be sure, you can take a voltage reading at the output of your power supply when it is not connected to your gadget. If the voltage is not higher than your gadget's rating, you'll definitely be OK when you finally do hook it up.

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dl_evans
 
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Re: Drop amperage question.

Post by dl_evans »

Wish I would have read that last post about an hour ago... I hooked everything up. Cut an old USB cable and wired up the positive / negative of the wall wart to the red / black wires of the USB cable. Did some research and found out the white stripe on the wall wart was positive (http://forums.qrz.com/archive/index.php/t-77685.html) and after I got everything wired up it didn't turn on. Put the wires back to the wall wart and it still didn't turn on. I think I fried it. :(

Agent24
 
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Re: Drop amperage question.

Post by Agent24 »

Do you have a voltmeter so you can check the output of the adapter?

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easternstargeek
 
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Re: Drop amperage question.

Post by easternstargeek »

Ouch!
That's a tough lesson- I'm really sorry to hear that you may have let out the Magic Smoke.
Playing with 'tronics is great fun, but it is important to have good tools. A multimeter is absolutely essential (as well as the knowledge necessary to use it properly), but the good news is that there are many more choices available today for the hobbyist than there were even 20 years ago.
See the Lady's tutorial: http://www.ladyada.net/learn/multimeter/
This is a good introduction, but it doesn't talk about one of the most important and vexing causes of multimeter failure, which is improper current measurement. Be sure you know exactly how to do it and understand well the relationship between Voltage, Current and Resistance before you attempt current measurement. Current measurements usually requires moving the positive probe to a different jack (or jacks, depending on the current ranges of your meter). An almost zen-like concentration is required to never measure anything but current when the probes are in that configuration. Newbies often forget, and wind up blowing the fuse in the meter. Really cheap meters may not even have a fuse, in which case the meter becomes toast. (Avoid those!)
Lastly, don't beat yourself up too badly over this- we were all beginners once, and I have certainly smoked my share of things over the years. Take it as a learning experience, get back on the horse, and have fun experimenting and building things!

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dl_evans
 
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Re: Drop amperage question.

Post by dl_evans »

Thanks guys. Sorry I didn't get back sooner... darn holidays... I've been meaning to get a good multimeter anyways so it sounds like this is my chance. Will report back once I get another adapter and the proper tools.

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