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How to Electronically Modify Your Car, Part 3

Volt, amps and ohms

by Julian Edgar

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At a glance...

  • Volts
  • Ohms
  • Amps
  • Watts
  • Relationships between all of the above!
  • Example car modification - dashboard monitoring LED
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This article was first published in 2008.

Last week in How to Electronically Modify Your Car, Part 2 we looked at circuits – including series circuits, parallel circuits, short circuits and fuses. This week it’s time to examine some of what is happening in a circuit.


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This circuit, that we looked at last week, consists of only a battery and a light. The battery is marked as being ‘12V’ or 12 volts. But what does this mean?

Like many electrical terms, it’s easiest to understand if an analogy is used - the voltage of electricity is a bit like pressure, for example, the pressure of fuel in a fuel-line. A fuel pump in an EFI system pressurises petrol, pushing it through the fuel line to the injectors. A battery produces an electrical pressure, causing an electric current to flow through a circuit.

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The higher the voltage, the greater the distance that an electrical spark will jump. The ignition system produces a voltage of more than 20,000 volts, and this high voltage allows the spark to jump across the plug’s electrodes.

Electrical pressure is measured in Volts.


Current is the amount of electricity flowing past a point. Using the fuel line example, it’s like measuring how many litres per second are passing along the pipe.

Current is measured in Amps.

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Wires that need to take a lot of current (like the one to the starter motor) are thick. In contrast, a fuse uses very thin wire inside it – so thin that if it is required to flow a current greater than its rating, the wire burns out.

A starter motor might take 100 amps while an interior light might draw only half an amp.


Resistance is a measurement of how hard or easy it is for a current to flow through a substance.

Something with a really high resistance is called an insulator - it lets almost no current through it. On the other hand, anything which allows current to flow very easily is called a conductor.

The normal wires within a car loom are good conductors, while the plastic covering around them is a good insulator - stopping the current from going where it’s not intended to.

As the resistance goes up, the flow of electricity is reduced (more on this relationship in a moment). There are lots of graduations between good conductors and good insulators, and the exact value of the resistance posed to the flow of electricity is measured in Ohms.

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Many engine management sensors work by varying their resistance. For example, this intake air temp sensor is a resistor that varies in electrical resistance with temperature.

Resistors can also be thought of as being a bit like a restrictor on a pipe. We mentioned above that you can think of voltage as being rather like fuel pressure, and current flow as being like fuel flow along the pipe. If you put a restrictor in a pipe, there will be a pressure drop across the restrictor. In the same way, if a resistor is placed in a circuit, there will be a voltage drop across the resistor. And, again like the flow of fuel along a pipe, the greater the current flow, the greater the voltage drop across the resistor.


There is a strict mathematical relationship between voltage, current and resistance. (It’s called Ohms law.) For example, if you know the voltage drop across a known value of resistor, you can work out how much current must be passing through the resistor.

The equation is:

  • amps = volts divided by ohms

This can be re-arranged to be:

  • voltage = amps x ohms


  • ohms = volts divided by amps

I don’t think it is worth memorising these (or any other equations we cover – just look them up as needed). What is important to remember is that there is a relationship between volts, amps and ohms. Therefore, change one variable and the others will change.

There is also another relationship with which you should be familiar.

It’s this:

  • volts x amps = watts.

‘Watts’ in electrical terms has exactly the same meaning as ‘watts’ (or kilowatts) applied to car engines – it’s the rate at which work is being done.

This equation can also be expressed as:

  • amps = watts divided by volts


  • volts = watts divided by amps

Again, unless it’s easy for you, don’t bother memorising all these – just remember that there is a relationship between volts, amps and watts.

Real World Stuff

OK, let’s look at an example involving watts.

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You decide you want to put some driving lights on the front of your car. They’re rated at 50 watts each, and because you are using two, you know you’ll need to supply enough juice to run 100 watts of extra lighting. You go off to chase some wire but you find that automotive wire isn’t rated in watts, it’s rated in amps. So how many amps will need to flow in this new wire?

Watts divided by volts = amps. We know the wattage is 100 watts. We know that car systems run on 12 volts. So what is 100 watts divided by 12 volts? It’s 8.3 amps. Use 10 amp cable and you’ll be fine.

Exactly the same idea applies to more powerful systems. A hybrid petrol/electric car might have a 30 kilowatt electric motor and a battery pack that provides 288 volts. So how much current does the wiring need to handle? Watts divided by volts = amps, so that works out to 30,000 watts (ie 30kW) divided by 288, which is over 104 amps! No wonder the cables are so thick...

Now what about a trickier example?

We’ll assume here that you know how to use multimeter (although we won’t cover that until next week!) so if you’re unsure of how a multimeter is used, that’s fine – at this stage just pretend you do.

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Let’s say that someone has told you that your trailer brake lights are very dim. You check and they’re working – but the person was right, they are hard to see. You get out the multimeter and probe the trailer socket on your car. When someone puts their foot on the brake pedal, there’s a measured 12V at the socket – so that’s OK.

Or is it?

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Here’s the circuit - or at least the bit of it that matters. The multimeter is plugged into the trailer socket brake light connections, and is showing 12V. And if the meter reads 12V, then it must be 12V that gets to the trailer brake lights, right? Now you might be thinking: “Not if the wires that connect the trailer plug to the trailer lights are bad,” but let’s state that all the bits on the trailer (eg the plug, wiring, bulbs, reflectors, etc) are fine.

So why on earth would the trailer brake lights be dim?

Here’s a clue: remember we said above that a resistance in a circuit is a bit like a restriction in a fuel pipe? If the current flow (ie amps) is very low, the voltage drop across the resistance is also very low. A multimeter takes only a tiny amount of current from the circuit it is measuring, so if there is any resistance in the circuit, measuring voltage won’t show it. You need to have lots of current flowing to see what really happens.

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So what we’ve done here is to plug in the trailer brake lights (they draw lots of current) and then again measure the voltage at the socket with the brake pedal pressed. As can be seen, the voltage has dropped to 8V, explaining the dim trailer brake lights. Therefore, there must be a resistance (ie lots of ohms) in the circuit between the car brake lights and the socket.

So without the correct current (amps) flow, the voltage drop across the resistance was not visible - there’s another example of that relationship between volts, amps and ohms.

Example Car Modification - Dashboard Monitoring LED

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LEDs are often used as dashboard indicators – to show when an intercooler water spray pump is working, or even when an Exhaust Gas Recirculation (EGR) valve is switched on. (Running lots of EGR is good for part-throttle fuel economy, so it’s useful to know when an electronically controlled EGR valve is activated.)

But LEDs operate on very small currents – they are devices that die if too much current flows through them. Connect a normal LED across 12V and the LED will immediately be destroyed.

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The easy way of using LEDs with 12V car systems is to place a series resistor in the circuit. The series resistor limits the current flow through the LED.

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Resistors are electronic components that are very cheap (a few cents each) and are available in an enormous range of types. They have two key specs – their resistance (measured in ohms) and the power that they can dissipate (measured in watts).

So what type of resistor do we need to allow the LED to be run off 12V?

In addition to colour, intensity and package size, LEDs have two other important specs. One is what is called the “forward voltage drop” and the other is the LED’s “maximum current”. With these two bits of information (that are available from where you buy the LED), the required resistor can be calculated.

A bright orange LED might have these specs: voltage drop of 2.2V and a current of 75 milliamps (ie 0.075 amps).

From Ohms Law above, ohms = voltage drop (across the resistor) divided by amps

If we are supply 12V, and we only want 2.2V at the LED, we want the resistor to drop (12V – 2.2V =) 9.8V.

So, ohms of the required resistor = 9.8V divided by 0.075 amps (the required current flow through the LED)

9.8 divided by 0.075 = 131 ohms.

Therefore a 131 ohm resistor will limit the current flow to 0.075 amps through the LED.

The other spec of a resistor is its required power dissipation in watts.

Watts = amps x volts, so that’s 0.075 amps x the voltage drop across the resistor, which is 9.8V.

Watts = 0.075 x 9.8V = 0.7 watts.

So we’ve worked out we need a resistor with 131 ohms of resistance and a power handling of 0.7 watts. The nearest off-the-shelf design to this is a spec of 120 ohms and 1 watt.

(However, with a running car voltage that is higher than the nominal 12V, and with the fact that you probably don't want a dashboard monitoring LED to be super-bright, a 620 ohm, 1 watt resistor will drop LED current and still result in a LED bright enough to be easily seen.)


Now if you recoil from maths, you might be looking at the above breakout box and seeing just mumbo jumbo.

But don’t worry about it!

Take just this information from it: we decreased the current through the LED by putting a resistor in series with the LED. The greater the ohms value of the resistor, the less current that will flow through the LED (and, incidentally, the dimmer it will be).

As I have kept stressing in this article, it’s the idea that there is a strict relationship between volts, ohms and amps that is the critical thing to remember. If one is changed in a circuit, then the others are changed as well. The same applies for watts, amps and volts: if one is changed, then the others must change too.

If you simply remember (volts, ohms, amps) as one bunch of inter-relating variables, and (watts, amps, volts) as another bunch of inter-relating variables, you’ll be streets ahead of where you were at the beginning of this article.

Next week, we’ll use a multimeter to directly measure these things.

The parts in this series:

Part 1 - background and tools

Part 2 - understanding electrical circuits.

Part 3 - volts, amps and ohms

Part 4 - using a multimeter

Part 5 - modifying car systems with resistors and pots

Part 6 - shifting input signals using pots

Part 7 - using relays

Part 8 - using pre-built electronic modules

Part 9 - building electronic kits

Part 10 - understanding analog and digital signals

Part 11 - measuring analog and digital signals

Part 12 - intercepting analog and digital signals

Part 13 - the best approaches to modifying car electronics and the series conclusion

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