Measurement question- how appliances draw needed current

a measurement question

Alright, lets say I want to measure an appliance getting it's power through a 240Vac mains.

My question is.. how does an appliance (lets say a cell phone charger) draw out the amount of current it needs?
What makes it not susceptible to the high voltage that is 240Vac which could ruin it?
How can a measurement be done (with respect to the Live/Neutral/Earth of the mains and the appliance)?

Re: Measurement Question

The point is that when an appliance is connected to AC mains say 230 Volts AC, it has some internal resistance. If the internal resistance is 460 ohms, the current that will pass through that appliance will be given by Ohms law I = V/R which in the above example is 230/460 = 0.5 (Ampere or 500 mA). The appliance is designed to stand that current and hence that burdon or load that is called the power rating of the appliance.
The Next issue is about mobile charger like appliances, it has a circuitary called SMPS (Switch Mode Power Supply), earlier it use to contain transformers for that purpose. Both of these Isolate the output of the power supply from the AC mains. Hope this helps resolve the issue.
Raoof

Measurement Question

I see, that makes a lot of sense.

Now, how would you do a measurement with respect to the Live/Neutral/Earth of the mains and the appliance?

Or shall I phrase it like this:
Live, Neutral and Earth - how do they function?
How do they connect between mains and appliance?

Re: Measurement Question

Hi Fiann,

There are essentially two types of 'connections' between Mains and Device when you are reducing the output voltage dramatically (in for example a charger).

The old way to do it would be via a transformer which actually electrically isolates the mains from the device - i.e. there is a magnetic coupling, but no electrical connection.

The more recent way of doing it is using a switched mode power supply (as ark5230 was descripbing) which doesn't have to be isolated, but can be; depending on the design.

If you wanted to measure the output with respect to mains, I guess you would measure against neutral, but I ceratinly wouldn't reccomend it =P

BoB

Re: Measurement Question

There are two things.
1. The input mains supply
2. The output from appliance say mobile charger.
So far as the input mains go, the voltage to be measured is AC
It is to be measured between Neutral and Phase.
In fact neutral is nothing but ground, at times in wiring it may not be connected, else you can measure between ground and phase as well.
There is sligt difference between ground and neutral, if it is of consequence, can be discussed.
The second thing is the output from the charger, it is a DC voltage in the range of 4 to 6 volts. This is to be measuered between the positive and negative terminals of the charger (the output pin).
The point to be noted is that one is AC the other is DC and both are isolated. the measurement of the two are independent. Mostly the input AC and output DC are isolated (not directly connected), however at times, one the output may be connected to ground which does not change what all is discussed above.
Hope this resolves the issue.
Raoof