Ohm's Law

Authored by: Phil - Orvac Electronics

Now we have this thing called OHM's LAW and it really isn't as difficult as it may seem.

Ohm's law is made up of three components; Voltage (E), Current (I) and Resistance.  You can also use these components to calculate Watts or Power (W or P)

If you understand OHM's Law and can apply it you will more comfortable working with; LED's, wall warts, transformers and components in general.

To start, let's look at a simple garden hose connected to the faucet.  The water pressure available is FIXED and determined by the utility company, this applies to electricity too.  In electricity this could be called POWER or WATTS.  Have you ever been in the shower when someone flushed the toilet? For that instant, you demanded more pressure (WATTS / POWER) than was available.

To translate to electricity, let's call VOLTAGE (E) volume and CURRENT (I) pressure.  Along the same lines let’s call the diameter of the hose RESISTANCE (R ).  

Let's say that you have 1,000 gallons of water available and you can pick how fast you pump it through the hose to water the grass.  You can take the hose off of the faucet and turn the handle all the way open to create a MESS or you can attach a hose.  If you pick a 3" hose (minimum resistance) and turn the faucet all the way on (Voltage) you will have a smaller mess.  Now select a 1" hose (more resistance) and turn the faucet down half way (less VOLTAGE) and the pressure (Current) will follow along creating an even flow to accomplish your task based upon the TOATAL water pressure available (Power and Watts)

Now to the math and you will need a calculator.  The formula is E (Volts) = I (Current) x R (Resistance); this is OHM's Law.  The formula to calculate POWER (Watts) is P = I x E and POWER is the foundation.

Let's assume that you have 200 WATTS available (total water pressure, if you will) This will allow you to generate 200 VOLTS  (E/ Volume) at 1 AMP (I/ Pressure) if the RESISTANCE is ZERO.  This just does NOT happen in a real world because EVERYTHING has RESISTANCE.

Let's assume that you need to light an LED with a 12 VDC automotive system.

Assume that it is a 3mm ultra-bright to act as a fake alarm. The LED's we offer take 3-4 VDC and require 30 milliamps or 0.030 Amps.  If you apply 12 VDC to the LED it will just melt; so what do you do with 9 VOLTS that you don't need?

Electricity is ENERGY and you can use various means to dissipate it or use it up, if you will.  In this case you would use a resistor.  Resistors convert voltage and current to HEAT and use up the power.  

In this application you need to burn 9 VOLTS at 30 ma before the LED.  Use OHM's LAW and divide 9 by 0.03.  Use your calculator and that is the value of resistor that you need; SIMPLE.

Resistors come in POWER or WATTAGE ratings up to 100 plus WATTS; the smallest is 1/8 Watt and you need to determine what wattage your customer will need.  If you UNDERSIZE a resistor it will try to convert more WATTS to heat than it has the surface area to accommodate and will go UP IN FLAMES; perhaps causing a "carbecue"; BAD DOG!  Simply calculate the WATTAGE (P=IxE) or 9 VDC x 0.030 AMPS.  As a general rule just sell a half watt resistor and be finished with it.  

Current (AMPS) is a funny beast!  It kind of hangs out until it is needed; it takes around 700 AMPS to start a car but takes considerably less to turn on the vanity mirror.  POWER (Total WATTS) determines how much current / voltage is available.  If you flip down the vanity mirror is just DRAWS the necessary current and the rest just hangs until it is needed.

The whole deal about electronics is about CONTROLLING POWER (Voltage and Current) in the most efficient way to get the job done.

Please keep in mind that OHM's LAW ONLY applies to DC!  AC is a whole different deal and capacitors and coils are used for resistance.

DC electricity is what you get out of a battery; it is just a constant voltage and easy to deal with.  AC electricity is constantly changing in voltage and is a BEAR to deal with.  If you look at the lights in your house, they are actually turning off and on 60 times a second.  All of your home appliances plug into the wall and then that POWER is generally converted from AC to DC so that it is easier to work with; toasters and the like are the exception.

HEAVY industrial machines are NOISE intensive because of the magnetics.  Relays can cause the same type of noise too.   IT also creates a TON of EMI/RFI and thus the need for shielded wire!

In almost ANY situation you NEED to know "What VOLTAGE and CURRENT" before you can move to the next step.  

You can NOT visually see electricity unless you short something and see sparks BAD DOG!  On the other hand, electricity can be precisely controlled.  OHM's LAW only applies to DC electronics in a useful way.  

Then you have DIGITAL electronics and it is a whole New World!