# resistor question

Discussion in 'General Electronics Discussion' started by whooshdoo, Sep 11, 2011.

1. ### whooshdoo

2
0
Jul 16, 2011
I have a 1/4 watt resistor rated at 1k ohms in a circuit and it is showing 12 volts at one end and only .4 volts at the other end. Since that is such a drastic difference in voltage levels, I am wondering if that is normal or could the resistor somehow have too much resistance? Is there a way to tell roughly what the difference in voltage should be across a resistor based on its rating? Thanks very much for any help.

2. ### BobK

7,682
1,688
Jan 5, 2010
First, find the current through the resistor:

V = I * R

(12 - 0.4) = I * 1000

11.6 / 1000 = I

So the current through the resistor is 11.6 mA or 0.0116 A

Now the power dissapated in a resistor is:

P = I * I * R

P = 0.0116 * 0.0116 * 1000

P = 0.13456

The power is less than 1/4 Watt (0.25) so your resistor will not go up in smoke.

Whether or not this is the correct resistance for the circuit is dependent on many other factors, but we can safely say that the resistor is not being used in a way that will destroy it.

Bob

Last edited: Sep 11, 2011
3. ### davelectronic

1,087
12
Dec 13, 2010
Resistor power

The resistors power in milliamps is 0.116 amps if its 116 milliamps, not 0.0116. Dave. PS sorry my mistake yes 11.6 milillamps yes 0.0116

Last edited: Sep 11, 2011

250
69
Aug 4, 2011

5. ### davennModerator

13,894
1,969
Sep 5, 2009
A resistor cant have too much resistance as such. its resistance is marked in it using the striped colour code +- a few %. ( a cavet to that. resistors have been known to go hi under fault conditions when it fails)

in a single resistor across a PSU the voltage drop across a resistor will always be whatever the supply voltage is
eg. a 5V supply will have a 5V drop across a given resistor a 12V supply will have the same 12V voltage drop across the resistor. that is on 1 side of the resistor there will be 5V or 12V as per examples and 0V on the other side. doesnt matter is its a 1 Ohm, 100Ohm or 1000 Ohms. What will be a factor is if the resistor can disapate the wattage.

the 12V and 1 Ohm resistor may have 12Amps flowing ( if the PSU is capable of that current) and the resistor, unless it is a hi wattage rated one will vanish in a puff of smoke.

In a circuit that has several resistors in series, a proportion of the voltage will be dropped across each resistor. Each of those voltage drops added up will total the supply voltage

take this circuit for example....

you can add up each resistance and get a total
R1 + R2 + R3 = 10 Ohms + 15 Ohms + 30 Ohms = (you work it out )

with that you can determine the current flowing in the circuit. I = V / R ... 110V / (your resistance total)
(hint it will be the same no matter where it is measured)

once you have the current flow "I" then you can work out the voltage drop across each resistor. V = I x R for each resistor. when you add up the 3 voltage drops they will total 110V ( the supply voltage)

cheers
Dave

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Last edited: Sep 12, 2011