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How to Electronically Modify Your Car, Part 7

Relays are much overlooked but they're cheap and effective

by Julian Edgar

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At a glance...

  • What relays are
  • Using relays in electronic car modification
  • Cheap, hard to blow-up and easy to use!
  • Unique uses in complex car systems
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Last week in How to Electronically Modify Your Car, Part 6 we looked at a special way in which pots can be used to alter the output of sensors used in electronic car systems. It’s a very tricky technique that allows you to achieve lots of effective outcomes at nearly zero cost. This week we’re also looking at simple and cheap electronic components that are again often overlooked as ways of achieving excellent car modifications – relays.

What’s a Relay?

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A relay is just a small switch whose movement is caused by the action of an electromagnet inside the box. When power is applied to the relay’s coil, the electromagnet comes alive and pulls across the switching contacts.

Relay Types

  • Single Pole, Single Throw

The simplest relay is a single pole, single throw (SPST) design. This designation refers to the switching part of the relay where when it's activated, one wire (a "single pole") can be connected only one way (a "single throw"). Just like an on/off switch, when you power up the relay's coil, the connection is made; when you un-power the coil, the connection is broken.

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In this diagram the relay's coil is yellow. Near to the coil you can see a switch, which is open. This is called a Normally Open contact - it's open when there's no power being applied to the relay. When power is applied to the relay’s coil, the single contact closes. This is a Single Pole Single Throw relay - SPST.

SPST relays have four terminals - two are to power the coil and the other two are the connections for the internal switch. Look at the diagram and identify which terminals are which. As you can see, there is no electrical connection between the pair of contacts for the coil and the pair of contacts for the switching part of the relay.

Example Car Modification – Auto-Dimming LED Indicator

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In How to Electronically Modify Your Car, Part 3 we talked about using LEDs in a car. As this diagram shows, if the LED is to survive, you need to place a series resistor in the circuit.

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Now we’ve added a pot (wired as a variable resistor) into the circuit. By turning the pot, the brightness of the LED will able to be adjusted. Because the resistor is still there, you can’t blow-up the LED – even you turn the pot fully in the ‘bright’ direction.

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Now we’ve added a switch in parallel with the pot. By closing the switch, the pot is bypassed – as far as the circuit is concerned, it ceases to exist. So we can have two switched light levels for the LED – one that’s bright (switch closed) and one that is dim (switch open). The ‘dim’ setting can be adjusted by the pot.

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And now we’ve replaced the switch with a relay. This SPST relay is a normally closed design – it opens when power is applied to its coil. Here we’ve wired the coil so it is activated whenever the parking lights are switched on. So what have we ended up with? A LED dashboard indicator that is bright in daytime and dims itself whenever you turn on the lights! You can even fine-tune the ‘night’ level with the pot.

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The LED could show turbo boost (just add a boost switch, as shown here), indicate that a fan is running – or a whole host of things.

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Finally, here is the built circuit – from left to right: resistor, pot and relay (the LED and switch are remote mounted).

Heavy Current

In the above example, the relay’s contacts had to take only a very small current – just enough to run the LED. Therefore, a quite light duty relay could be used (more on relay ratings in a moment). But many relays used in cars are heavy duty designs.

In fact, the most common application for a SPST relay is to use a small electrical current to control a large electrical current.

For example, a radiator fan might be triggered by a temperature switch. The temp switch is capable of flowing only 2 amps, but the radiator fan at switch-on takes 15 amps (and then settles back to 8 amps continuous).

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If you wire the radiator fan to the switch like this, after a few weeks the temp switch will fail – its contacts are being hugely overloaded.

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The solution is to add a SPST relay that is wired into the circuit like this. Now the temp switch only has to pass enough current to turn on the relay’s coil – a much easier job than directly running the fan!

Neither the relay coil nor the switching part of the relay has a polarity – both can be connected either way around to 12V and Ground. As we said earlier, relays are very hard to blow-up!

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On automotive SPST relays, the pins are given standardised numbers. The coil connections are 85 and 86, while the two connections for the internal switch are 30 and 87. However, most general purpose relays don’t have any numbers on the pins – instead the functions of the pins are shown on a little diagram on the body of the relay.

  • Single Pole, Double Throw

But wouldn't it be good if we had two contacts inside the relay – one that was opened at the same time as another one was closed? That's what happens in the Single Pole, Double Throw (SPDT) design. (Can’t think of much use for that type of relay? There is – and I’ll show you in a moment.)

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This is what a SPDT relay looks like inside. When the relay is energised, one contact is opened and the other one (the Normally Open contact) is closed. We still have only a single pole to be switched, but now it can be connected two ways - a double throw design. As you can see, it has both Normally Open (NO) and Normally Closed (NC) contacts. (Some people call this a changeover relay.)

A SPDT relay allows you to control two devices, switching one off as the other is switched on. SPDT automotive relays use the following codes for their pins: the coil connections are again 85 and 86, the normally closed output is 87a, the normally open output is 87 and the input is 30.

Example Car Modification – Switchable Fuel Pressure

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An example of where I used a SPDT relay was in a fuel system that needed to be switched between two different fuel pressures. To raise the fuel pressure, a solenoid valve had to be turned off and at the same time, a fuel pump needed to be switched on. Both devices draw a fair amount of current so a heavy duty automotive relay was used – one of the pictured relays did this function.

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The circuit diagram for the fuel system relay looked like this. Power was normally supplied to the solenoid through the Normally Closed (NC) relay contact, energising the solenoid. But when the relay’s coil was activated (by closing the High/Low Fuel Pressure switch), the relay pulled the contact across, switching off the solenoid and switching on the fuel pump.

(If you get lost, follow the circuit in two parts, starting off from either of the ‘12V’ power supplies.)

The High/Low Fuel Pressure switch had to handle only enough current to switch the relay’s coil, so this could have been be a light-duty switch (eg a boost pressure switch or a microswitch).

  • Double Pole, Double Throw

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A Double Pole, Double Throw relay allows you to switch two different circuits simultaneously. The 'Double Pole' bit just means that it has two separate inputs that can be switched - and we now know the 'double throw' means that one contact gets opened as the other is closed. With this type of relay you can:

  • turn on two completely independent circuits

  • turn one off and one on

  • turn off two completely independent circuits

These relays are less common in automotive aftermarket use and so don’t have coded numbers for the pins.

Example Car Modification – Switching Out Oxy Sensors

So what use is a DPDT relay, then?

Again, I’ll use an example from a car modification I’ve done. What was needed was the on-demand disconnection of two oxygen sensor input signals from the ECU.

The two signal wires from the oxy sensors to the ECU needed to be kept completely separate; this meant they couldn’t be joined together and a SPST relay used. Instead a DPDT relay was used. (It didn’t actually have to be a double throw design, but DPDT relays are more common than SPDT designs.)

When the relay’s coil was energised, both oxy sensors were simultaneously disconnected from the ECU.

Using Relays

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Using a relay is made a lot simpler if you follow these steps.

Draw a circuit diagram. The first step is to draw a simple circuit diagram showing where the wires go. Which wires go to the relay coil, which to the Normally Open and Normally Closed contacts of the relay?

Decide what type of relay is needed. If just one connection needs to be switched on and off, you’ll use a SPST design. If two connections need to be switched, a DPST or (more commonly) a DPDT design will be the one to use. A changeover (where one device is switched off and the other switched on) can use a SPDT or a DPDT design.

Work out the functions of each pin. If it’s a standard automotive relay, read the numbers. If it’s a general purpose relay, look for the diagram on the relay body. If neither of these apply, by careful use of a short-circuit protected power supply and a multimeter, you can work out the functions of each pin. (Unless you use too high a test voltage, you can’t damage the relay!)

Wire the relay coil first. If you wire the relay’s coil first, you’ll be able to check that the relay is working by listening to its click.

Example Car Modification – Disabling Traction Control

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It’s easy to think of relays as being suitable for just simple car modifications, but that’s not always the case.

This circuit shows the use of two relays that deactivate traction control without affecting ABS or stability control.

The system works by connecting the un-driven wheel ABS sensor outputs to the driven wheel ECU inputs, so that the ECU cannot see a speed difference between the undriven and driven wheels. The modification is automatically switched off whenever the brakes are applied, or by a manual on/off switch. This diagram shows only half of the system - the complete the system mirror-images the wiring for the other side of the car.

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The total cost of the modification was well under AUD$30 – relays are cheap! For more on this approach, see Modifying Electronic Car Handling Systems, Part 3.

Relay Specifications

In addition to its contact configuration (SPST, DPDT, etc) there are at least three other specifications that are important.

  • Coil voltage

This refers to the voltage which the relay is designed to have its coil triggered by. A nominally 12V relay is fine on car voltages, even though they can extend as high as 13.8V. However, you shouldn’t use a 5V coil relay on a 12V system.

  • Coil current

This is the amount of current the relay coil will draw when energised. This can be expressed directly in milliamps, or indirectly as a coil resistance. A very sensitive relay might have a coil resistance of 360 ohms. 13.8 volts divided by 360 ohms gives a coil current of 0.038 amps, or 38 milliamps. In other words, the switch that you’re using to operate the relay has to handle just 38 milliamps. That is a very low value of required current.

A typical automotive relay is more likely to have a coil resistance of 80 ohms, giving a coil current flow of 170 milliamps. (13.8/80 = 0.17 amps). That’s still low – most switches will handle this without problems.

  • Maximum Contact Current

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This spec refers to the max current that a relay’s contacts can handle. To avoid arcing, you should use a factor of safety where the max current of your switched circuit is less than the relay’s spec.

Automotive relays are available with current ratings like 25, 30 and even 60 amps. Be careful when checking max current specs that the listing is for the DC at or above the voltage you’ll be using – ie, in cars, 13.8V. For example, a relay rated at 10 amps at 240V AC is not the same as one rated at 10 amps at 12V DC.


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Relays can be utilised in nearly every electrical or electronic car modification. Get your head around their use and you’ll never regret having spent the time to find out how they work.

Next week we’ll look at using an off-the-shelf electronic module.

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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