By mike Chambers


In my search for an article to interest you all I came across a piece on "relays" and decided it would make a good article, but after reading it, It was no more than a series of wiring diagrams for relays and how to fit connections on cable.

Anyway not to be beaten I've managed to put something together which I hope will be of interest, explaining what a relay is and a bit about electric's in cars, this may be teaching some of you to suck eggs but bear with me. Also some classic car owners are not as old as their cars and they not know how old style relays work.

Before we start on relays and what they are, lets sort out what electricity is ( In cars that is, which is not like house electrics, which is different high power electrics) and what it's measurements are.

First off we have the power supply (This is akin to the pressure in a water system) and that's measured in VOLTS , this is the constant part of the system (Well all most constant).

Next we have the current or flow of electricity and this is measured in AMPS.

Now we come to the energy a piece of equipment in a electrical system will use and that's measured in Watts, (After the famous English inventor James Watt), for example a fog light with a bulb (Modern term lamp, but you know what I mean) rated at 45 watts.

Last but not least is the resistance to the flow of electricity and that is measured in OHM'S. ("Guess what " it's named after another inventor named Ohm would you believe, who was German and I better not to leave out the Italian named Voltaire or the French man called Ampere, So as you can see Electric's is a mixture of English / French / Italian and German ).

The thing about ohms is they are the cause of most of the problems in car electric's, here’s why. If you have a small diameter wire and pass a large current (i.e. high AMPS) through it there is a resistance (OHMS).

This is like trying to pore to much water down a pipe, it over flows, but with electricity it can not over flow, so it heats up the wire, This can melt the covering (Insulation) which will cause a short out if it then touches any metal which in turn would blow the system fuse. In some cases the heat generated will actually melt the wire. (Yes it can even get that hot in a 12 volt system). Basically then when we have high AMPS and want low resistance OHMS i.e. no heat, the way we do that is to increase the wire size i.e. the larger the wire the less resistance. (This only applies for short wire lengths in car electric's )

So you can see resistance, our friend ohms can cause serious trouble in car electric's and we must select the correct wire size to match the amps flowing and have a fuse in the wire to prevent the wire carrying to many Amps, but do not worry because you can now forget about OHMS (You will see why later).



Now how do these electrical measurement relate to each other :-




(This one is more complicated than it looks so forget it)

Do not worry to much about the above formula as we only have to use one part of it.



























What Is A Relay, What Is Its Purpose, Why Use Them and Where To Use Them.

"What Is A Relay",

Quite simply it is an electrically operated switch, and is what in general electrical terms is known as a "relay" (See Fig 1 This is a full size relay), the switch inside a car relay unit is much smaller and of a much simpler design (Relay in a relay,Confused again).

As you can see from figure 1 a relay is a soft Iron core with fine copper wire coil around it which when a "small" electrical current is passed through the copper wire the core becomes magnetic, which in turn draws the armature (Confused again, Yes your right an armature is part of a generator or starter motor) to wards it (The same way a permanent magnet pulls a steel object towards itself ) which intern pushes the contacts 1 & 2 together which are designed for a high current flow [AMPS] and completes the circuit.


"What Is It's Purpose"

, It will allow equipment which requires a large current to operate i.e. Horns with 30 watt rating , 30 x 2 = 60 watts ÷ 12 volts = 5 amps, and to do so without passing the high current (5 amps ) through the driver operated switch or via the ignition system. ( See fig 2 )























This is done by having a direct supply from the battery (The spotted wire in fig 2) to the relay (Passing through connections 1&2 Fig 1inside relay) and on to the equipment (Horns in this case) using a small current from the drivers switch passing through the coil to make contacts 1&2 fig 1 (Remember all this happens in the little black box called a relay). You may be wondering what size the wire should be in fig 2, so lets have a quick go, 5 amps was the current the general guide line is use next size wire, 8 amp and a 8 amp fuse to protect the system.


"Why Use Them"

, Safety is the main reason, why because when a switch is made a spark is likely to occur (This is usually very small unless the switch is worn or damaged ) and the larger the current [AMPS] the larger the spark. Hence it's safer to use low current in switches, Also the amount of wire carrying high current is reduced to a minimum.


"Where To Use Them"

, On classic cars I would recommend all extra lights and add on equipment to be via a relay (Except a radio) and even if spots and fogs are original I would fit relays for safety because wire insulation deteriorates with age (High Resistance OHMS means heat) which means the insulation brakes down easier with the high current flow, (Modern wire insulation uses more advanced plastics) and a short under the dash can take some locating and fixing in a classic car.


Car Light's

A little bit of history on lights now.

When cars first had electric lights the power for the lights came direct from the battery or the generator so switches and all wiring had to be able to carry the full load [AMPS] and you can imagine with those old large head lamps this was a very high load , first off this was not much of a problem as the size of wire used was big and switches big heavy duty ones (The switches had copper contacts, Big current [amps] big spark).

But as cars became more available there was more competition in selling them, and to reduce costs the wire and switch size was reduced (As copper was expensive even then), then the problems started as the designers of the day got there sums wrong (To many AMPS in their wires) hence fires and wiring burning out.

Then as cars moved into larger production numbers the designers sorted things and most of the time all was ok, but still on those dark nights with all the lights on and the wipers a smell of burning could be detected and something had to be switched off.


The problem today is we are used to much brighter lights so we are more likely to be driving our classic with all it's lights on including some extra we may have added and that's when we might be in for a problem. (See selecting correct wire size for enlightenment, "Sorry about the pun")

As a PS if you ever think of upgrading the classics headlights for a brighter drive, “ think first” as classic cars can be using switches that carry the full load (Amps) you need to check the wire can the extra load ( and don’t forget it’s old wire) fitting relays and upgrade the wire from the relay to the lights, is only half the problem you will most likely need to have a new wire from the battery to the relay for the extra load thus the original high load system becomes low (only activates the relay) and the extra current will give you no problems, it may not be original but it will be safe.

Today's cars have just about every thing on relays and all the wiring and switches around the dash is of a very small size. (i.e. Low amperage)


Selecting The Correct Wire Size

Now lets go back to the beginning and the problem of making sure the wire is correct for it's duty.

First you start looking at the relation between VOLTS, AMPS, WATTS and OHMS, now you may find yourself in a muddle, it's most likely.

So as I said before you can forget the OHMS, Why well the calculations have all been done for you because when you buy wire (Cable as it should be called) it's rated for it's use, i.e. in AMPS, 5 amp / 8 amp / 17 amp / 27 amp and so on.

To make it even easier it is also "use" identified along with the AMP rating , i.e. 5 amp Side / Tail Lights, 8 amp Reversing Lights / Horn and so on. See I told "you" you could forget OHMS, (Am I pleased about that only I could see no way of explaining on paper the way to calculate resistance with out getting myself in one hell of a muddle)


So Now We Want To Fit A Relay.

The first thing we do is sort out what cable we will need, say we are fitting a set of twin spot's and each has a 45 watt bulb. (See fig 3)

























= 90 ÷ 12

= 7·5


Due to the resistance in wire we would add some on to this so we round up to 8 amps. Now comes the general guide line which is for safety so you go to the next size up wire, which is 17 amp, (And on that, what does it say, you guessed it spot's ).


Next is the fuse with an 8 amp load our fuse should be larger to allow for changes in voltage and amperage (Voltage regulators are not perfect nor is a alternator system ) we will go for a 10 amp one which is well within our wire amps of 17.


About now you "may" be thinking with two wires going to the spots I can use 5 amp wire as each spot only uses 4 amps, but the problem there is if one spot had a short it would draw up to 10 amps before the fuse blows and 10 amps in 5 amp wire would most likely melt it.

So the only way to protect the wire is to fit two 5 amp fuses one in each wire, Which would mean we finish up with 3 fuses in the system and that's more expensive than wiring in the heavier wire plus the fiddling putting in the two extra fuse fittings.


Next step is a relay, The relay itself is no problem they are marked just like the wire (Most are rated for 35 amps), so we are ready to go and not an OHM insight.( And as I said in the beginning we only use one of the formula)

Installation Of A Relay


This could not be easier, just wire up as per fig 3, using light wire say 5 amp [Plain line] from the head lamp circuit to the switch and on to the relay, and our heavy 17 amp [Spotted line] for the supply to relay and from relay to spots.


One thing you may have noticed in the figures is the line with lines across it that get smaller well it's the sign for an earth and means you secure the wire to the body of the car, this now brings me on to the second biggest problem in car electric's and that's earthing because with out a good earth the spots fogs etc just do not work.


When it comes to earth wire choice it best to use heavy duty wire as this offers the least resistance to the flow, The earth or return to the battery is such very very small current that any dirt / grease / corrosion and the flow stops due to resistance (Ohms is back again), I expect some of you have had this problem and cured it with a quick clean up of the connection and all working again.


I know some one is going to say I've been checking an earth connection and its sparked yes you are right and we are into the way cars are wired i.e. Negative or Positive earth this is a whole new quirk that only applies to car electric's which needs a whole article on it's own so I'm going to finish off with out delving into it, sorry about that, but just remember to use heavy duty wire (Say 17 amp) for earthing then you will be ok no matter if it's a Negative or Positive earthed car.

Well that's all once again folks, I'm looking forward to seeing you all at the shows this year.


All The Best




Have a go at working out what wire and fuse you need for this system it's 40 watt bulbs in each light, also what wire would you use for the warning light.