Why does my LED circuit draw 120–150 mA instead of 41 mA?

[anj16]

As the title states I am really doubting Ohm's law. This is why: On a breadboard I placed a white LED and a 220Ω resistor with the 9v battery all in series. The total current flowing through the circuit shouldn't be more than 41mA by V=IR, but my multimeter points between the range of 150-120(Edit: mA). Can someone tell me why?

Thank you.

 

[Integral]

We can't help without knowing what you are doing. A circiut diagram including how the meter is connected would help. Try to diagram what you have on the breadboard.

120-150 whats? What kind of meter are you using?

 

[davenn]

what sort of white LED is it give us a part number ?
it may be one that draws more current

Hint... If you ever want to doubt a known physical law, assume it's something you are doing wrong, not the law ;)

Dave

 

[anj16]

120-150 whats? What kind of meter are you using?

Sorry about that I have edited it in the original post.

We can't help without knowing what you are doing. A circiut diagram including how the meter is connected would help. Try to diagram what you have on the breadboard.

Just so you know the correct way of connecting a meter in a circuit to measure the current is to connect it in series. which I did.

 

[anj16]

what sort of white LED is it give us a part number ?
it may be one that draws more current

Hint... If you ever want to doubt a known physical law, assume it's something you are doing wrong, not the law ;)

Dave

I agree with you which is why I thought someone could point out where I am wrong. And about drawing more current wouldn't the current be restricted by the resistor?

 

[BackEMF]

As the title states I am really doubting Ohm's law. This is why: On a breadboard I placed a white LED and a 220Ω resistor with the 9v battery all in series. The total current flowing through the circuit shouldn't be more than 41mA by V=IR, but my multimeter points between the range of 150-120(Edit: mA). Can someone tell me why?

Thank you.

1) Measure the actual resistance of the "220 ohm" resistor.
2) Check the accuracy of the meter (use another meter - digital might be better as it would probably change the circuit less than and analgoue one)

 

[anj16]

The resistance of the 220 ohm resistor comes to about 218 ohm.

 

[BackEMF]

Can we verify the battery's voltage?

 

[davenn]

I agree with you which is why I thought someone could point out where I am wrong. And about drawing more current wouldn't the current be restricted by the resistor?

yes, but the calculated total current through the circuit is a combination of the total resistance of the resistor and of the forward resistance of the LED

you still didnt tell us what sort of LED ??

Dave

 

[K^2]

yes, but the calculated total current through the circuit is a combination of the total resistance of the resistor and of the forward resistance of the LED

Unless LED develops a negative voltage drop, that seems kind of irrelevant, no? 9V / 220 Ohm = 41mA.

Any LED will actually reduce voltage by about 1 - 2 volts. So the current with an LED should be even less than 41mA. 150+ mA cannot be explained by any kind of LED.


anj16, can you take a picture of your setup?

 

[anj16]

@Dave about the part# for the LED, I have no clue. I bought in bulk off Ebay.
I have attached a picture of the setup

 

[anj16]

Can we verify the battery's voltage?

The voltage of the battery is 9v

 

[K^2]

Isn't the entire + line connected on the breadboard? If so, your resistor is shorted out. Try checking resistance while it is in the breadboard.

 

[anj16]

Isn't the entire + line connected on the breadboard? If so, your resistor is shorted out. Try checking resistance while it is in the breadboard.

Thank you so much! I never thought about that. I think I need a break from this ;)

EDIT: Also the doubt I had is gone because I re-measured the current and it comes to about 41 mA.

 

[davenn]

prob solved ...

the old saying ... a pic is worth 1000 words ;)

Dave

 

[davenn]

Unless LED develops a negative voltage drop, that seems kind of irrelevant, no? 9V / 220 Ohm = 41mA.

Any LED will actually reduce voltage by about 1 - 2 volts. So the current with an LED should be even less than 41mA. 150+ mA cannot be explained by any kind of LED.

yup fair comment :)

D