# watts

Hello. How do you find out how much watts, there will be in a 150 resistor with 5v and 2mA. And how do i find out for any restistor or any thing?

Pie.

Power (P) = Current (I) x Voltage (E)

thanks:D how do i find out the resistor rating?

Is that ohm's law??? or?
Is that how you would right it: Power (P) = Current (I) x Voltage (E)??

A 150 ohm resistor with 5V across it will not have a current of 2mA. the current would be V/R=5/150=33mA. Alternatively, the 150 ohm resistor with 2mA flowing through it will have a voltage of V=150*.002=0.3V.
But to determine the power dissipation of a resistor you can use either P = VI or P = IIR. The 150 ohm resistor with 33mA would dissipate .033.033*150 = .163W = 163mW. The resistor's rating should be higher than this - double will be comfortable.

Pete
 - corrected the above - Grumpy_Mike's post reminded me that what the resistor dissipates and its rating aren't the same thing.

Is that ohm's law

No.

Is that how you would right it: Power (P) = Current (I) x Voltage (E)

No but it is how you would write it.
P = I E
or W = I E or any other product where the two are defined as current and voltage with the resulting power in watts.

how do i find out the resistor rating?

A rating is the amount of power a resistor can dissipate it is not the resistor's value in ohms.
To find the resistor's value in ohms given the voltage across it and the current through it use
R = E / I
or
resistance in ohms = voltage in volts divided by current in amps

Most resistors in the market are rated like : 1/8 Watt, 1/4 watts, 1/2 watt, 1 watt, 2 watt...etc.

You need : P= V * I
V = R * I

I hope you are good in math and algebra.

Let take an example : A led example. Vin = 5 V Vled = 1.2 V I led = 50 mA Find Resistance and Power.

1. Find the voltage drop across the resistor. 5 - 1.2 = 3.8 V Voltage in serie add up. V t = V1 + V2 + ... You have only 2 drop.
5 is Vt, V1 is Vled and V2 is V r drop.

2. R = V / I --> 3.8 V / 0.05 A = 76 ohms. Closest market resistor : 75 ohms.

3. P = V * I --> 3.8 V * 0.05 A = 0.19 Watt So a 1/4 Watt is 0.25 Watt. 0.25 is over 0.19. Safe to use 1/4 W

4. Answer : 76 ohms at 0.19 Watt. Commercial value available : 75 ohms at 1/4 Watt. A 1 ohm difference is "no big deal" !

i under stand that serta what is p= V * I called?

mrdove500:
i under stand that serta what is p= V * I called?

Algebra.

what is p= V * I called?

The power equation or Joule's law

God i am 13 this hearts my brain but i got it! thank you all so so so so much!

1 more thing how do i find out the resistor raiting?

The people who well it to you should know

You look at the data sheet for the resistor or look at the suppliers description. The power rating is very rarely marked on the resistor.

If it is through-hole, you might be able to tell by the physical size of the resistor. 1/4W and 1/8W are generally about the same size, for example.

The power rating isn't generally marked on resistor itself.

oK:) how much watts could i go over on a 10k resistor prob?

ON R i ment to put 190 ohm's

Let's try and debunk this a little bit for you. First off, some definitions of the various single characters that have been posted here.

E = Electromotive force. Volts in plain English which is the measurement unit.
I = Current flowing through any given part of a circuit. Measured in Amps
R = Resistance in Ohms
P = Power or Watts if you wish which is the measurement unit.

So, you can express watts as Watts = Volts x Current or, Watts = Current squared x Resitance in Ohms.

This link is the Wikipedia entry for Ohms Law which is essential if you are to work with circuits. As Grumpy_Mike showed, you also need a small amount of algebra to move the formulas about.

Resistors have several colored bands around them and knowing what these mean will allow you to know their values in Ohms. Here's a link to the code Electronic color code - Wikipedia

The codes are not immediately obvious but you can have some fun by learning one or more of these little mnemonics in this link List of electronic color code mnemonics - Wikipedia

If you have not yet got yourself a multimeter, then you need one quick.

I hope this helps.

how much watts could i go over on a 10k resistor prob?

Any amount of power that the resistor is rated for.
A rating is how much power you can use without destroying it.

This can be as little as 0.02 Watts to hundreds of thousands of watts, it depends on how the resistor is physically constructed. It is not something that is related to the voltage or current or resistance value in ohms.