### Author Topic: Resistor math check  (Read 2162 times)

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#### krich

• Robot Overlord
• Posts: 165
##### Resistor math check
« on: March 03, 2008, 07:04:15 PM »
Okay, so I hooked up a circuit on my breadboard.  One of my resistors started to smoke, a clear sign I screwed something up.  I checked and double checked my circuit and it was hooked up correctly.  The only thing I could think of is that there is too much current going through that resistor.  I decided to do the math.  This is where I'd like to make sure I'm doing the simple calculations right.

voltage = 16v
resistor = 100ohm

I=V/R = 18v/100ohm = 0.18A
W=V*I = 18 * 0.18 = 3.24W

So, 3.24 Watts going through a 1/4Watt resistor.  No wonder.  Adding additional resistance to the circuit eliminated the smoking.

For a little background, this resistor is part of a circuit going to the base of a transistor (2n3904).  In "production", there will be more resistance than the 100ohms introduced by way of separation of contacts in a liquid (non-flammable, resistance yet to be calculated).  Basically, this part of the circuit is a water level detector.  Liquid is present, all okay.  Liquid not present, sound an alarm.

A second related question.  It seems reasonable to me that this circuit should protect itself from having the probe contacts shorted.  I'm thinking I need to figure out how much base current the transistor needs in order to switch on and try to find a happy medium between base current, resistor value, the rated wattage of the resistor, and the expected resistance of the liquid.  Am I going down the right road here?

• Supreme Robot
• Posts: 11,696
##### Re: Resistor math check
« Reply #1 on: March 15, 2008, 04:55:21 PM »
Yeap, you needed a resistor that can handle more watts.

The equation is:

P=IV, or P=(V^2)/R

solving . . .
P=16^2/100 = 2.56 Watts

Quote
I'm thinking I need to figure out how much base current the transistor needs in order to switch on and try to find a happy medium between base current, resistor value, the rated wattage of the resistor, and the expected resistance of the liquid.  Am I going down the right road here?
Sounds fine. Or you can be lazy and use a resettable fuse