# Resistor Info

Discussion in 'General Electronics Chat' started by kbxc20, Jun 10, 2015.

1. ### kbxc20 Thread Starter New Member

May 27, 2015
11
0
Any one seen a resistor like this.
0.47 ohm 2 watt I assume. However it powers a circuit that is 12 volt DC with a 1.21 amp load flowing thru this resistor.
I can't see how this size resistor can carry this load unless someone knows otherwise.
It blew along with a transistor. I replaced the transistor and this resistor with a wire wound 5 watt which blew from overloading.
Other wise circuit seems to work ok.
from the above values after a short while.

Apr 5, 2008
16,963
2,958
3. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,906
1,789
1.21 amps thru a A .47 ohm resistor is just 1.21*1.21*.44=0.7 watts.

So a 2 watt resistor should be quite fine here.

4. ### kbxc20 Thread Starter New Member

May 27, 2015
11
0
I get 14.52 watts 12Vx1.21A=14.52W

5. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,906
1,789
I don't. Neither should you.

If you put 12V across .47 ohms you get 25.5 amps.

I assumed all of the current was going thru this resistor so the power is I*I*R

6. ### kbxc20 Thread Starter New Member

May 27, 2015
11
0
This is the amount of amperage flowing THRU the resistor, not as if you if you connected each end of the resistor to the power source.

7. ### Veracohr Well-Known Member

Jan 3, 2011
600
86
1.21A*.47Ω = .5687V across the resistor. Not 12V.

8. ### WBahn Moderator

Mar 31, 2012
20,057
5,652
You are making the common mistake of throwing the nearest voltage and the nearest current at an equation just because it needs a voltage and a current.

P = V·I requires the voltage across THAT device and the current through THAT device in order to determine the power dissipated by THAT device.

9. ### kbxc20 Thread Starter New Member

May 27, 2015
11
0
Thanks to all replies.
But back to the original post. The values stated are actual operational measurements. The resistor is just in a series circuit with other components.
All I know is that it now blows 5 watt wire wound ceramic case resistors after about 2 minutes
whereas the one in the picture lasted for 4,745 hours. Again is this one made differently or is the current and voltage stated too
high for the resistor in question. (see pic of disassembled resistor)

10. ### WBahn Moderator

Mar 31, 2012
20,057
5,652
I would say that something has gone wrong with the circuit such that the "other components" that the resistor is in series with is now presenting a much lower effective resistance to the supply so that you are getting more voltage across (and hence current through) that resistor.

What is the voltage measured directly across the resistor when it is in operation?

11. ### kbxc20 Thread Starter New Member

May 27, 2015
11
0
Ill have to get one and then measure