Ohms law | Electrical calculations | Current measurement

Useful product and technical information guides

Disclaimer: The power guides on this page are given as an explaination of the principles of electronics and are not published for experimentations with high voltage, you can purchase electronic learning kits which run on low voltage domestic batteries to give students and enthusiasts a working example of the below principles in operation,TheToolBoxShop Team

Ohm's Law explained

The history of the terminology

Each of the units of measurement are named after famous experimenters in electricity and energy

Voltage, Volts (V) named after the Italian Alessandro Volta known especially for the development of the first electric cell or modern translation "the battery" in 1800.

Amps (A) named after the Frenchman Andre M.Ampere one of the main discoverers of electromagnetism.

Ohms (Ω) named after the German Georg Simon Ohm a physicist who began his research with the recently invented electrochemical cell (invented by the Italian Alessandro Volta). Using equipment of his own creation, Ohm determined that there is a direct proportionality between the potential difference (Voltage) applied across a conductor and the resultant electric current now known as Ohms Law.

Watts (W) named after the Scotish inventer and mechanical engineer James Watts who's name was adopted as a measurement by the second congress of the British association for the advancement of science in 1889.

Joule (J) named after the English physicist James Prescott Joule in recognition of his work on heat and it's relationship to mechanical work or energy.


Voltage = Current x Resistance	Current = Voltage over Resistance	Resistance = Voltage over Current

As depicted in the above three boxes Ohm's law states that the electrical current (I) flowing in a circuit is proportional to the voltage (V) and inversely proportional to the resistance (R). Therefore, if the voltage is increased, the current will increase provided the resistance of the circuit does not change. Similarly, increasing the resistance of the circuit will lower the current flow if the voltage is not changed. The formula can be reorganized so that the relationship can easily be seen for all of the three variables.

V= Voltage expressed as Volts (V)
I= Current expressed as Amps (A)
R= Resistance expressed as Ohms(Ω)

For many electronic circuits the amp is too large and the ohms are too low therefore we often measure the current in milliamps mA (where 1mA = 0.001A) and kilohms kΩ (where 1kΩ = 1000Ω) this would translate to:

V= Voltage expressed as Volts (V)
I= Current expressed as milliamps (mA)
R= Resistance expressed as kilohms(kΩ)

The calculation for Ohms law must follow the format of one or other of the two translations to work correctly, in other words: V, A and Ω or V, mA and kΩ

The VIR triangle is still to this day the best way to show the relationship between voltage, current and resistance and the beauty of it is in its simplicity, armed with only your thumb it is an instant reminder of the three explanations of ohms law, students should print out the triangle, laminate it and put it in their wallets, a technician should already have it burned into their memory,


Current = V over R	Voltage = I x R	Resistance = V over I

Ohms law calculation

See the following examples for understanding the calculation of Ohms Law:

	Voltage (V), a common calculation used in domestic application could go like this, we have a 1.2kΩ resistor passing a current of 0.2A, The calculation for voltage goes: 0.2 (being the Amps) x 1200 (being the resistance), NB. when using (A )Amps you must convert Kilohms (kΩ) to ohms (Ω ) which concludes the voltage accross the resistor = 240V
	Current (I), a common calculation used in application could go like this, 6V is applied accross a 12Ω resistor, the calculation for the current goes: 6 (being the Voltage) divided by 12 (being the resistance) gives us a current = 0.5A
	Resistance (R), common calculations used in application could go like this, we have a lamp connected to a 6V battery passing a current of 60mA, the calculation for resistance goes: 6 (being the Voltage) divided by 60 (being the mA) = 0.1kΩ which = 100Ω, which gives us the lamps resistance, NB. using the mA for current means the resulting calculation gives the resistance in Kilohms.

The Ohms Law simplified

Each of the following statements are correct the first is a simplified explaination of Ohms Law the second is an analogy for teachers explaining to students.

Simplified 1, When we have a steady increase of voltage (V) in a circuit with constant resistance (R) it results in a constant linear rise in the current (I)

Simplified 2 ( for teachers explaining to students), Using the analogy of a stream or river, think about rocks and boulders in the river being the resistance (R), the current (I) or flow of electrons is like the flow of the water in the river, potential difference (V) is like the difference of the rivers height, the current (I) or flow is directly related to the potential difference (v) or height, when the height difference of the river is greater more water flows through which increases the current (I) , when there are rocks and boulders in the river or resistance (R) the water slows down therefore reducing the flow or current (I)

The Power Triangle or Joules Law

The Power (PIV) Triangle Explained

For calculating using Power (W) Current (I) and voltage (V)

Used in the same way as the Ohms Law triangle we can use the same method to see the relationship between power (W), current (I) and Voltage (V).


Power = Current x Voltage	Current = Power over Voltage	Voltage = Power over Current

As depicted in the above three boxes we can express the values as follows:

P= Power expressed as Watts (W)
I= Current expressed as Amps (A)
V= Voltage expressed as Volts(V)

For many electronic circuits the Amp is too large therefore we often measure the current in milliamps mA (where 1mA = 0.001A) and Power in milliwatts (where 1mW = 0.001W) this would translate to:

P= Power expressed as milliwatts (mW)
I= Current expressed as milliamps (mA)
V= Voltage expressed as Volts(V)

The calculation for Power must follow the format of one or other of the two translations to work correctly, in other words: V, A and W or V, mA and mW

The PIV triangle like the Ohms Law triangle is still to this day the best way to show the relationship between Power, Current and Voltage, armed with only your thumb it is an instant reminder of the three ways to write an equation for Power (W), Current (I) and Voltage (V),


Current = P over V	Power = I x V	Voltage = P over I

Horsepower (hp) it would be worthy of note at this point that some electrical devices such as electric motors have a power (W) rating in horsepower to which a conversion of 1hp=746W would be required for calculations.

Batteries used in Series and Parallel

Batteries used in series

Batteries joined together in Series have the effect of doubling the voltage but the Amps stay constant as the diagram above using identical batteries (of the same voltage and Ampere-hours) shows,

Configuration: 2 x 12V 60Ah connected in series = 24V 60Ah output,

Ampere-Hours: The time that a battery can deliver (in hours) the stated current (in Amperes), or the electric charge transferred by a steady current of one Ampere for one hour

Batteries used in Parallel

Batteries joined together in Parallel have the effect of doubling the Amps but the Voltage stays constant as the diagram above using identical batteries (of the same voltage and Ampere-hours) shows,

Configuration: 2 x 12V 60Ah connected in parallel = 12V 120Ah output,

Batteries used in series and Parallel example 1

Batteries joined together in Series and Parallel: the above diagram shows how we start to create a bank of batteries as we would use in principle on an EV (electric vehicle), by joining two battery banks (already linked in series) and connecting them in parallel we increase the battery banks voltage and Ampere-hours,

Configuration: 4 x 12V 60Ah connected in series then connected in parallel = 24V 120Ah output,

Batteries used in series and Parallel example 2

Batteries joined together in Parallel and Series: the above diagram shows another way to create a bank of batteries, by joining two battery banks (already linked in parallel) and connecting them in series we increase the battery banks voltage and Ampere-hours,

Configuration: 4 x 12V 60Ah connected in parallel and then in series = 24V 120Ah output,

Emergency Bulb Kit Guide

Emergency bulb kits are a requirement by law in some european countries and the legislation is due for expansion accross the whole of the European Union, the below kit guides are as stated "guides only" and do not take into consideration vehicle modifications, if you are unsure as to which bulbs are in your vehicle please contact your nearest dealer who may advise you.


Alpha R to Audi	Audi to BMW	BMW to Chrysler	Chrysler to Citroen	Citroen to Daihatsu

Diahatsu to Fiat	Fiat to Ford	Ford	Ford to Honda	Honda

Honda to Hyundai	Hyundai to Jeep	Jeep to Land Rover	Lexus to Mazda	Mazda to Mercedes

Mercedes	Mercedes to MG	MG to Mitsubishi	Mitsubishi to Nissan	Nissan

Nissan to Peugeot	Pergeot to Porche	Porche to Renault	Renault	Saab to Seat

Seat to Suburu	Suburu to Suzuki	Suzuki to Toyota	Toyota	Toyota to Vauxhall

Vauxhall to Volks.	Volkswagen	Volks. to Volvo	Volvo

Glow Plug, Heater Plug and Controller Guides

This section is the glow plug and controller application guides which is to be used to identify which plug or controller could be in your vehicle and specifications, a better option would be to get the manufacturers reference from your controller or plug and input it into the product search section, If you require assistance please email us your vehicle type/year and any references you may have to: sales001@thetoolboxshop.com we can only advise which plug or controller is recommended, if you are still unsure you should contact your nearest dealer who may be able to help.




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