BASIC 
ELECTRONICS COURSE 
Page 14 INDEX
 

OHM’S LAW CALCULATOR

Iamps

=

 Vvolts
_________

 Rohms

          

How to use the Ohm's Law Calculator:
Enter any two values into the equation and the computer will generate the answer. 
Note: the windows only allow up to an 8 digit number to be seen.

 

Ohm's Law

A very simple equation connects AMPS, VOLTS and OHMS. It is called Ohm's Law. It is written: 

 I

 V
_______

 R

 

OR

 I

 E
_______

 R

where:                                       
 I = current in amps,                                          I = current in amps
V = voltage in volts and                                     E = voltage in volts and
R = resistance in Ohms.                                    R = resistance in Ohms

   

To be technically correct, the "volts" term is actually "E" for EMF (ElectroMotive Force) but since the final outcome is exactly the same, we will stick with the symbol "V."

The equation is read as: "EYE" equals "VEE" over "R"  The first value in the equation is not one it is pronounced as (h)igh!

The equation only works in AMPS, VOLTS and OHMS. If the current or voltage in a circuit is less than unity you have to use Sub-multiples (also called decimals). 
Sub-multiples are values less than "1." 
The sub-multiples of AMP are MILLIAMP and MICROAMP(and nano amp and pico amp)
1 milliamp = 1/1,000th AMP.    i.e: 1,000 milliamp = 1 amp.
When putting milliamp into the formula above, decimal values must be used: 
1 milliamp = .001 amp
10 milliamp = .01 amp
100 milliamp = .1 amp

1 microamp (uA) = 1,000,000th amp  i.e: 1,000,000 microamp = 1 amp
When putting microamp  into the equation above, 
1 microamp = .000001 amp
10 microamp = .00001 amp
100 microamp = .0001 amp
1,000microamp = 1 milliamp = .001 amp

Examples: 
250mA = .25 amp
35mA = .035 amp
0.5mA = 500uA = .0005 amp
470uA = .00047 amp
50uA = .00005 amp
5uA = .000005 amp

If you are working with voltages less than 1v, the sub-multiples of  VOLT are MILLIVOLT and MICROVOLT
1 millivolt = 1/1,000th volt.    i.e: 1,000 millivolt = 1 volt.
When putting millivolt into the formula above, decimal values must be used: 
1 millivolt = .001 volt
10 millivolt = .01 volt
100 millivolt = .1 volt

1 microvolt (uV) = 1,000,000th volt  i.e: 1,000,000 microvolt = 1 volt
When putting microvolt  into the equation above, 
1 microvolt = .000001 volt
10 microvolt = .00001 volt
100 microvolt = .0001 volt
1,000microvolt = 1 millivolt  = .001 volt

Examples: 
250mV = .25 volt
35mV = .035 volt
0.5mV = 500uV = .0005 volt
470uV = .00047 volt
50uV = .00005 volt
5uV = .000005 volt

Resistance values must be converted to OHMs 
1k = 1,000 ohms
1M = 1,000,000 ohms

Examples:
1k = 1000 ohms
4k7 = 4700 ohms
47k = 47000 ohms
100k = 100000 ohms
470k = 470000 ohms
1M = 1000000 ohms
2M2 = 2200000 ohms

OHM'S LAW IN A DC CIRCUIT
Ohm's Law works in a DC circuit and the following shows six examples:

The three terms used in Ohm's Law are: I = current, V = Volts, and R = Resistance. 
Current is the movement of electrons through a wire or conductor. Voltage is the pressure or force that causes electrons to move and Resistance is the opposition to electron movement. 
If we take a simple circuit such as (c) above, the current through the circuit is 1 amp when a pressure of 10 volts is applied to a resistance of 10 ohms. If the voltage is increased to 20 volt, the current will increase to 2 amp. It's a simple LINEAR relationship. If the voltage is reduced to 5v, the current will fall to 0.5 amp. 
For electronic circuits, resistance values are generally 1k or higher and current values are less than 1 amp. 
A simple way to remember current-flow is to know that when a voltage of 1v is applied across a resistance of 1k, the current flow is 1mA. 

We have shown above, 1mA = .001A, so that in circuit (a) above, .006A flows when a voltage of 6v is applied across a resistance of 1k. 
In the diagrams below use the statement: "1mA flows when 1v is applied across 1k" to work out the current in each circuit:

Ans: (g) 10mA =  .01A     (h) 20mA = .02A     (i) 100mA  =  0.1A    (j) 1mA =  .001A     
(k) 12mA  = .012A    (l)  6mA =  .006A

FINDING THE CURRENT WHEN LEDs 
and TRANSISTORS ARE INCLUDED

When a LED or transistor is included in the circuit, the current flow can still be worked out. The first thing to do is find out the voltage across the LED or transistor. This voltage is subtracted from the RAIL VOLTAGE and then Ohm's Law is applied. The characteristic voltage across a red LED is 1.7v and the characteristic voltage-drop across a transistor (when it is FULLY turned on) is 0.35v.

In circuit (a) above, the voltage across the 330R dropper resistor is: 6 - 1.7 = 4.3v    Use this value in the calculator above to find the current. 
In circuit (b) above, the voltage across the load is 6v - .35v = 5.65v    Use this value in the calculator above to find the current. 
Ans: (a) .013A  = 13mA     (b) .012A = 12mA


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