Why is my power supply giving more than it says it should?

[jelly5798]

Hi, I'm new to electronics and exploring the basics so please please forgive me.

Currently I have a simple circuit with a 12V,1A PSU, bulb and mulitmeter.

When using the multimeter to meaure the bulb's resistance I get 4.5 ohms and then 12V for the voltage across the bulb. According to Ohm's law I should get 2.6 A for the current and I do: the multi-meter says 2.6 when set to 200m and the red probe in the 10A connector.

My question is how can I have 2.6 A in my circuit when the PSU says it's output is only 1A? Also, what is the 200m setting for on my multi-meter?

 

[telectrix]

you won't get 2.6A.thre bulb'sresistance you measured is at room temp. once it heats up the resistance increases dramatically, thus reducing the curent accordingly.

 

[jelly5798]

you won't get 2.6A.thre bulb'sresistance you measured is at room temp. once it heats up the resistance increases dramatically, thus reducing the curent accordingly.

If I remove everything from the circuit and just put my meter around the positive and negative from the PSU, I still get more than 1A. Why is that?

 

[James]

When measuring current (A) the meter is shorting out the power supply when you connect from 12v to 0v
the power supply is probably current limiting and reducing the voltage to stop it overloading itself.

 

[DPG]

As above. It's a really common mistake that novices make. An ammeter should be connected in series with the load.

Luckily most ammeters have fuses. Otherwise it can be nasty when someone mistakenly connects one across a car battery to 'see how many amps it has got'.