Measuring current using Multimeter

[Badar]

hi.
I have a DMM and want to measure current from my outlet that is having voltage 220-240.
If e.g. i connect a 40 watt bulb and install the meter in series it'll be giving me the current that a 40 watt bulb draws and will show a greater reading if i use something of greater watts.
Am i true?If so then what is the way of measuring the ratings of any power supply.We'll get voltage but how do i know how much current this power supply can supply if it isn't labeled.For exampled my sound system supply shows 12 volts 5 A.

 

[dknguyen]

Yes, a higher wattage light bulb will have lower impedance and allow more current to flow (that's how a higher power device at the same voltage works).

But you cannot measure a wall outlet with any multi-meter. You have to use special probes (and maybe a special meter) so you don't electrocute yourself.

That method will not let you determine the rating of a power supply. If you have a 10A power supply, and a 20A power supply, they will both measure the same if you connect something that only draws 5A.

If it's not labelled, all you can do is SLOWLY increase the load (decrease the load impedance) until the power supply voltage begins to drop too much, or until it starts to overheat- whichever comes first. Make sure the loads you attach can dissipate the heat.

 

[Badar]

i m unfamiliar of the fact that increasing power of something like bulb e.g. from 40 to 100 watts will decrease the impedence SO it draws more current.
that seems surprising to a beginner like me.
But will you explain it to me.I think 100 watt bulb will have more resistance hence the impedence of tungstun filament than the one having 40 watts so that current or charges have to face more difficulty to pass through the filament hence more power is consumed.
Am i true?If so then how impedence decreases.

 

[Nigel Goodwin]

You need to learn ohms law - it's dead simple!.

Volts = Amps x Ohms (V = I x R)

Apply that to your bulbs, bearing in mind you know 'V' (your mains voltage), and you can calculate 'I' from the formula 'Watts = Volts x Amps (W = V x I)'. Do this for a 40W bulb and for a 100W one, then calculate their resistances.

These two simple formulas are the most important, and most often used, have them tattooed on your forearm!!

 

[Hero999]

I've made a plug for measuring the current concumption of domestic appliances. All it is is a plug which the fuse removed and three small hole drilled in the back connected to another socket. All I do is plug the appliunce into the socket, stick the multimeter's probes across the fuse section and measure the current.