Q for the electricians

Submitted: Saturday, Feb 16, 2008 at 20:36
ThreadID: 54642 Views:1843 Replies:5 FollowUps:0
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AMPS
To illustrate my question, I will use the example of a Nokia phone charger. (12v cigarette kind)
Fictitious Charger 1 provides up to 2A at 5 volts
Fictitious Charger 2 provides up to 500mA at 5 volts

Now with volts, I can see it is stepped down from 12v to 5v with a transformer (or something).
But with the current draw, is it the charger that dictates how many amps are being provided, or is it the unit being powered that dictates how much is being pulled (up to the maximum rating of the charger)?
E.g. if a GPS navigation unit only required 0.75A @ 5 volts to run, and the 12V cigarette adapter was capable of feeding it 1.5A @ 5 volts, would the GPS unit receive the 1.5A like it or not (and possibly fry something), or only draw 0.75A as required?
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Reply By: oldtrack123 - Saturday, Feb 16, 2008 at 20:58

Saturday, Feb 16, 2008 at 20:58
FOR AC the charger current is the max that can be used without overloading it.
the equipment you use set the amps drawn
you can use a gps drawing 1.5amps on any rated 1.5or higher but not rated less ,but voltage must be correctll
Dc CIGARETTE ADAPTORS ARE TOTALLY DIFFERANT YOU SHOULD ONLY USE ADAPTORS SPECIALLY DESIGNED FOR THE ACTUAL EQUIPMENT
AnswerID: 287832

Reply By: Star Bug - Saturday, Feb 16, 2008 at 21:03

Saturday, Feb 16, 2008 at 21:03
The amp rating is the maximum current the charger can supply if needed. The actual curren draw is dictated by the device being supplied.

so if you GPS only require 0.75A, that all it will draw no mater what the supply rating
AnswerID: 287834

Reply By: lifeisgood - Saturday, Feb 16, 2008 at 21:18

Saturday, Feb 16, 2008 at 21:18
A 40 Amp battery charger will not pump 30-40 amps into a fully charged battery, the load , usually the resistance of the device will govern the amount of amps drawn.
A starter motor has low resistance wiring and invites a large inrush of current. A high resistance device will allow only a small current.
Very high Resistance allows no current to pass (insulator)
AnswerID: 287838

Reply By: Footloose - Saturday, Feb 16, 2008 at 21:27

Saturday, Feb 16, 2008 at 21:27
Current drawn depends entirely upon the requirements of what is drawing it. The source (battery charger) can supply up to a certain amount of current, depending upon its design and build. If this is in excess of what is needed then that's OK.
If demand exceeds this limit, then it's not good :)
AnswerID: 287841

Reply By: Grungle - Sunday, Feb 17, 2008 at 10:00

Sunday, Feb 17, 2008 at 10:00
In the case of a 12V cig charger, the Amps rating will relate to a fuse so this is the only current limiting device fitted. This fuse is the weakest link in the circuit and is designed to blow before the wiring or plug catches fire due to excessive current draw.

Similar to <12V supplies (regulators) which will regulate the voltage and have a low rated fuse fitted.

You can connect a charging lead to a device that is the same voltage but higher current rating but not a charging lead that is a higher/lower voltage than that required with same current rating.

Saying this however, when you buy a device, the charging lead is fused according to the devices maximum current rating so that if something shorted out in the device, the fuse will blow before any of the circuits in the device therefore allowing it to still be repaired. If you do decide to use a same voltage / higher amperage charging lead then be mindful that your device could be totally unrepeairable if something were to go wrong inside.

Very rarly will these devices (the charger themselves) have any form of electronic current limiting. It will be the device accepting the charge (such as the phone, GPS, camera etc) that will limit and control the charging current going to the battery. Electronic current limiting circuits are only seen in devices that replace a battery (power supply) or are a direct connection to a battery (charger) without going through another device.

The battery will determine what current is drawn but to keep heat down and preserve battery life, the device will limit this current.

This is only a basic condensed version and one could go on for pages / hours.

Regards
David
AnswerID: 287894

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