Tech Note 015: An Electrical Question
Submitted by Jorge Menchu, AES
When connecting a light bulb to positive and negative
terminals of vehicle battery, the light lights up, and stays lit up. Yet when connecting a
10 ohm resistor to the positive and negative terminals of battery the resistor wants to
smoke and burn up.
The resistance of the bulb is approx 10 ohms.
Why is the the 10 ohm resistor burning up instead of
carrying the load like the light does?
Jorge's Reply:
The problem is the physical size of the resistor. Get a larger 10 ohm resistor. The
small one can not handle the heat created by the current.
The light for example gets very hot. The filiment gets so
hot that it glows.
A small resistor might get so hot that it burns. So, in the
case of the resistor - get larger. Here is how you figure the size.
First calculate wattage w = v * a
w = 12 * 1.2
w = 14.4
Next, go to the electronics store and get a 10 ohm resistor that has a rating of a
minimum 15 watts. It will be bigger than one that is rated at 1/4 watt!
(Read below for more info about this missing decimal.)
Jorge
Tony asks: YOU CANNOT MEASURE THE
RESISTANCE OF A LIGHT BULB, BECAUSE WHEN POWERED UP IT HEATS UP AND THE RESISTANCE
CHANGES. YOU MUST FIGURE IT OUT WITH OHMS LAW. IT WILL NOT HAVE THE SAME RESISTANCE, IT
WILL BE MUCH MORE.
Jorge Responds: This is correct!
Tony continues: THIS IS WHY THE 10 AMP
RESISTOR SMOKES WHEN HOOKED TO A BATTERY.
Jorge Responds: This statement is not
correct. First, the question being asked is "why does the 10 ohm resistor (that they
selected) burn up when put across the battery?"
The question is not "what is the best resistor value
to simulate the resistance of the light?"
The answer to the question being asked is found in the watt
value of the resistor that is burning up. One of the basic rules of electricity is -
anytime current flows through resistance energy is released as heat. If the power source
is 12 volts and the resistance is 10 ohms then there is 1.2 amps. 12 volts at 1.2 amps
offers a watt (power) rating 14.4. 14.4 represent the amount of energy that the resistor
must dissipate. If the resistor cannot dissipate this energy it will burn up. So, to
prevent a 10-ohm resistor from burning up you must get one that has a watt rating of
greater than 14 watts.
Tony continues: YOU MUST KNOW THE VOLTAGE
AND THE AMPERAGE TO FIGURE OUT THE RESISTANCE.
Jorge Responds: If your goal were to
calculate the resistance of the light bulb when it is lit up then you would want to use a
current probe. Take the current and plug that into an ohms law equation with the
voltage as you state. The answer will be higher than the resistance measured when the bulb
is cold.
Now, if you notice my calculations above are different then
those listed on the site. On the site page I forgot to include the "." in the
amp value so that needs to be fixed!
Tony continues: OK, what if you would
figure the resistance of the light bulb with it on, and get that ohm resistor and hook it
to the battery. Would it smoke
Jorge Responds: If you figured the
resistance of the light when it was warm and got an equal value resistor and then
connected the resistor directly across a 12 volt battery would it smoke?
That depends on the physical size of the resistor. For
example, if you go to Radio Shack and bought a 1/4 watt 15ohm resistor, you would find it
to be physically small and might burn up. A resistor of 1/4 watt used on a 12 volt circuit
can only handle 20mA before it exceedes the 1/4 watt rating. At 15 ohms and 12 volts we
need a (12volts/15ohms=0.8amps....0.8a * 12v=9.6 watt) 9.6watt or greater.
To confirm this go to your local electronics store and get
a 10 (or so) ohm resistor in 1/8 watt, 1/4 watt, and 10 watt versions.
Jorge |