There are lots of formulas in Electronics. The one you must know by heart came to use care of German Physicist Georg Ohm in his paper on the “Galvanisches Kettle”. Basically old Georg figured out that Voltage “Across a Conductor” is related to the Current “Through a Conductor” by the resistance of the conductor. You could say Resistance is the ratio of Current (“I”) To Voltage (“V”) and is represented i he equation:
But you will find Ohms law more helpful if it’s written as:
This is because a Resistor’s purpose is to limit current “I” and since Voltage is usually known and you are trying to produce a specific current, you can find the resistance necessary to produce the correct current by solving for R. But if you already have a circuit you are trying to analyze you will know voltage and resistance and so by dividing Voltage by Resistance you can reduce the current in that node of the circuit.
The defacto “LED Example”:
You have a 9-Volt Battery, a Red LED that wants a .02 Amp Current to function (to little current it won’t light up, too much current and it burns up and catastrophic failure occurs and you now have one less LED). So if I=.02 and V=9-Volts you pop those number in the equation and you get the resistor value necessary to make the LED function perfectly:
.02=9/R > R=9/.02 > R=4.5/.01 > R=450/1
The resistor value necessary is 450 Ohms.
(Resistor Values are measured in Ohms)
If your Resistor was 900 Ohms the LED would have half the current (10 milliAmps vs 20 milliAmps) and not light up. If the resistor Value was 100 Ohms the current would be 90 milliAmps and the LED would burn up aost instantly.
You must get used to this formula. Resistors are everywhere. Every component requires either a specific Voltage or Current to function properly and efficiently otherwise you waste energy, create waste heat and thus fire hazards, and your circuits either don’t work or have a lifetime that is shortened due to the stresses of too high of a current.
That’s enough for today but get a breadboard and connect a 9 volt battery to an LED with a 450, 330, or 220 ohm resistor between the positive battery terminal and the longer lead of the LED and see what happens!
(LEDs have a short lead which you connect to the negative battery terminal and a long lead to the positive terminal. If you don’t have a resistor between either leg and it’s respective terminal you will break the LED)
‘Till Next Time,