Relays - why do we need 'em?

[Diesel]

If I pick up a 'switched' circuit - such as my tail light wire (that only has power when the ignition is on) - then why do I need a relay in that circuit considering the power is only there when I want it in the first place - and switches off with the bike?

Cheers, Diesel

[Brock]

The relay is used, when you have a low power circuit, and you want to use it to power a high power device.

For example, you want to turn on a pair of 100 watt spotties when on high beam. the two spotties will draw about 16Amps, the head lights (typically) are 55 wat (may be 60), so total draw is about 8 amps, and would be protected by a 10 Amp fuse. if the driving lights were to be powered directly from the high beam wire, simple maths shows that the draw would be 26 amps, so fuse would blow and then no lights at all. Relays draw very little power, (milli amps) so have no effect on the donor power circuit.

By using a relay, you are able to turn on spotties when high beam is selected with out blowing the fuses, or damaging the high bean switch.

In electronic terms, the relay could be thought of as an amplifier. An amplifier uses low power to control high power.

[Biggles]

No wonder he draws such a high pay!!
We'll have to start calling Brock "The Professor".
Great explanation Brock.
The last line might have been over-kill.  Next thing you'll be hitting us with Transistor physics!

[Brock]

I can do Mag amps as well.... 

Transistors are like an electronic relay, in that it uses a small voltage to control large currents (especially when using 2N3055 power transistors as switches)

[Bikebear]

Another problem with running higher power devices without a relay, is the more current drawn on standard wiring (meaning just large enough for the designed current)  is the wires will heat up. I know people who've just whacked in a bigger fuse and then wondered why they had smoke coming from under their dash and ended up with a mass of melted plastic instead of a wiring loom.

[keith3po]

Another problem with running higher power devices without a relay, is the more current drawn on standard wiring (meaning just large enough for the designed current)  is the wires will heat up. I know people who've just whacked in a bigger fuse and then wondered why they had smoke coming from under their dash and ended up with a mass of melted plastic instead of a wiring loom.

Yep on my Recently bought Wing PO had fitted spot light's and wired them into the sidelight circuit and put in a 30Amp fuse.
The result was a burnt out switch and connector had to rip out all the wiring and fit a relay.

[saaz]

A relay is just a switch capable of powering high powered devices.  What turns the relay on and off can be very low powered, as all it does it tell the relay to be on or off.  By putting a relay into a circuit you can maximise power to a device by using as direct a path as you can from the battery.  So you can wire up heated grips to only power on when the ignition is on, driving lights only come on when high beam is on etc etc.

 I have a relay in the ST1100 that puts power directly from the alternator/battery to the fuse box, rather than all the power going up through the ignition switch then back to the fuse box.

[Malcolm6112]

Great explanation Brock. It's easy to understand how things work when told in simple terms.

 

[keith3po]

Hope this helps i took the feed for the switch from the sidelight circuit.
And the feed for the relay from an aux at the fuse box.

[Desty]

A 'standard' 5-pin automotive relay can have contacts generally rated at between 20 and 50 Amps. The contacts in your headlight/high-beam/accessory/etc switch, are usually physically quite small and rated at around 5A max.

Relays are used to remove high current loads from common switching circuits, reducing the wire size, and thus loom diameter,  and removing heat and arcing issues from the binnacle switches on the bars.

We, the automotive aftermarket modifiers, generally use relays to add load to circuits, indirectly, that weren't deigned to carry such loads.

[Brock]

Thanks for the circuit Kieth, we dont have side/park lights here in Oz, and driving lights must be controlled by the high beam.

By the way, the earth wire off that switch is for a pilot light in the switch and isnt really needed

[keith3po]

Thanks for the circuit Kieth, we dont have side/park lights here in Oz, and driving lights must be controlled by the high beam.

By the way, the earth wire off that switch is for a pilot light in the switch and isnt really needed

Hi Brock i'm a bit confused in the uk we have sidelight/headlight/main beam/fog or spot lights. Are we just using different names?

[Brock]

Nope,

In Oz, as we have headlights on all the time, the side/ park lights arent used so not fitted. the bikes have hi/lo beam head lights. Fog or spot/driving lights are after market. Lights to the rear are combined tail/stop and indicators as usual.

Fog lights are for use only in fog/reduced visibility and should go off when hi beam selected (according to the Standards), Driving/spot lights are only to come on when hi beam selected, not to be used in built up areas (supposedly) Dav time running lights are new(ish), and can be on at all times as they are of a low power. these are after market on the bikes as well.

Some Cars have those bloody hi intensity red rear fog lights as well, a pain in the rear they are as they arent needed 99% of the time and only serve to dazzle the car behind.

[keith3po]

Cheers Brock that's clearer now, the majority of bikers in the uk ride with headlight's on anyway.

Do your headlights come on with the ignition or are they on a switch?

[saaz]

Headlights have been mandated to come on with the ignition for many years. Part of the headlights on laws.  On the ST11 at least we don't have a cutout fitted that switches off the lights while the bike is started.  Seems to be the case elsewhere so don't know why not here.

[alans1100]

Headlights have been mandated to come on with the ignition for many years. Part of the headlights on laws.  On the ST11 at least we don't have a cutout fitted that switches off the lights while the bike is started.  Seems to be the case elsewhere so don't know why not here.

When we start the bike, the starter switch turns the lights off as you press the button so that both lights and start motor are not working together.

[saaz]

Not on my 96 P model non-abs. May be an abs model thing in Australia?  I know that headlight problems can be traced back to that cutout switch if there are dirty contacts.

Headlights have been mandated to come on with the ignition for many years. Part of the headlights on laws.  On the ST11 at least we don't have a cutout fitted that switches off the lights while the bike is started.  Seems to be the case elsewhere so don't know why not here.

When we start the bike, the starter switch turns the lights off as you press the button so that both lights and start motor are not working together.

[Brock]

May be an abs model thing in Australia?

Nope, My 2003 P non abs, the headlight drops out on start.  I dont have any  headlight problems.

[alans1100]

Not on my 96 P model non-abs. May be an abs model thing in Australia?  I know that headlight problems can be traced back to that cutout switch if there are dirty contacts.

Get the same issue if the start switch contacts/spring get dirty and switch sticks halfway on release on mine causing the head lights not to come on after motor starts.

[Biggles]

Yep- I had the same thing on my previous 1300.  Poor old starter motor was running all the way home and I only noticed no headlights as I came in the driveway.  Figured I'd blown both bulbs until next start.  Sprayed silicone into the switch with no further sticking.

[Diesel]

Found this Q&A on Relays - hope it helps......



RELAYS; how they work and how to wire it up...
There seem to be many members who don't know what a relay is, or don't know how to wire one in. this guide is here to help solve that issue...

what is a relay?
-a relay is a device that allows you to control a high-current electrical load with a low-current electrical 'signal'. they are usually electro-magnetic, but are also available in solid-state forms. they can be used with a switch (to allow control of a high-current load with a small switch) or they can be hooked up to a switched power source in the car like the ignition or accessory power circuits (to allow power to be switched on/off automatically with the ignition key).

Why do I need a relay?
-when hooking anything up to a car's factory wiring, it's important to remember that factory wires are designed to carry the load of only the factory installed components. they are not 'general use' power circuits like the power outlets in your house. for example, the ignition (IGN) circuit is designed to power the car's ignition system and nothing else. hooking up a high-current device to this circuit can create a fire hazard. by using a relay, you can use the IGN circuit to control a high-current device without directly powering it from the IGN circuit itself.

Is a relay hard to hook up?
-no... most relays require only 4 wires.

Where can I buy a relay, and how much do they cost?
-you can buy standard automotive relays at radio shack, autozone, walmart, or at almost any electronics store or automotive store. a typical SPST relay will cost about $5(US).

I bought a relay, but I don't know how to hook it up. all I see are a few weird numbers and strange symbols, what do they mean?
-a standard bosch-style relay will have 4 or 5 numbered leads (30, 85, 86, 87, and sometimes 87a). why they picked those numbers, I have no clue; but I can tell what they hook up to.

-30 = constant [positive (+)] power (usually wired directly to car battery)
-85 = coil ground (wired to the negative (-) battery terminal or any grounded metal panel in the car)
-86 = coil power (wired to the control source. could be a switch, or it could be the car's IGN or ACC circuit.)
-87 = switched [positive (+)] power output. (when the relay coil is powered, lead/pin 87 is connected to lead/pin 30)
-87a = [on 5 lead/pin relays only] this lead/pin is connected to lead/pin 30 when the coil is NOT powered.

here is the bottom view of a standard 4-lead Bosch-style automotive SPST relay




here is what happens inside the 4-lead/pin SPST relay. on the left, the coil is NOT powered. on the right, the coil IS powered. notice the switch changes positions when the coil is powered. when the coil is powered, pins 30 and 87 are connected. when the coil is NOT powered, then pin 30 is not connected to anything, therefore it is in the 'off' position.



here is the bottom view of a standard 5-lead Bosch-style automotive SPDT relay. you'll notice that the only difference is the addition of the 87a lead/pin.



here is what happens inside the 5-lead/pin SPDT relay. you'll notice that instead of an 'off' position, there is an electrical contact (87a). on the left the coil is NOT powered, and pins 30 and 87a are connected. on the right the coil IS powered, and pins 30 and 87 are connected.



here is a typical setup to control power to a carPC via the car's ACC/IGN circuits. this is generally use for laptop installs or 'no-write' setups (not good for normal carPC because abruptly cutting power without proper shutdown can cause data corruption);



I want to use the relay to turn on/off with the car. how do I hook that up?
-connect lead/pin 86 to the car's IGN, or ACC circuit. these circuits can be tapped into in the wiring harness that goes to your car's key switch. the ACC circuit can also be tapped into at the fuse box or in the stock radio harness.

What is the difference between the ignition (IGN) and the accessory (ACC) circuit?
-they are both powered when they key is in the 'RUN' position, and they are both not-powered when they key is in the 'OFF' position.

-the 'ACC' circuit is powered when the key is in the 'ACC' position, but is not powered when the key is in the 'START' position. when the key is turned, power to the relay will turn on as the key passes the 'ACC' and 'RUN' positions, then turn off in the 'START' position, then turn back on as the key is released (springs back to the 'RUN' position. this is not an issue if you're using a manual on/off switch, or have a delayed on startup controller.

-the IGN circuit is powered when the key is in the 'START' position, but is not powered when the key is in the 'ACC' position. this circuit avoids the issue stated above, but requires that the key is left in the 'RU' position if you want to use the carputer with the engine off. on some cars, this can burn out the ignition coil.

I bought a 5-pin relay, can I still use it as a simple on/off switch instead of a changeover switch?
-yes. simply leave pin 87a disconnected.

My relay has more than 5-leads/pins?
if your relay has more than 5-leads/pins then is is most likely a DPDT, MPDT, or MPMP relay (M=multiple). it will work the same way, it simply has two or more separate switched inputs/outputs inside it. regardless of the number of switched contacts, it will still use a single coil, so it will still be controlled by a single power source.

Are there others types of relays other than the standard 'bosch style automotive relay'?
yes, just like regular switches, there are several different types, sizes, and styles of relays. some have multiple switched contacts, some are circuit-board mountable, some have high-current capacity, some have low-current capacity, some use higher or lower voltage coil power (generally you would only use a 12v coil in a car).

What is a 'valet switch'?
-the 'valet switch' is a standard SPST switch wired between the coil power source and pin 86. it allows you to leave the relay turned off, so nobody (including a 'valet' driver) can turn your carPC on. how effective this will be depends on how well you hide the switch (it should be accessible, but hidden from plain sight).

does a relay take the place of a fuse?
-no! a relay provides no protection from overload or short-circuits.

do I need to use a fuse if I use a relay?
yes! you must still fuse your power wires!         

 

Cheers, Diesel

[Brock]

 

[saaz]

Good information. Although not that common, there are relays out there that are on (ie main power goes from pin 30 to 87) until activated. Most tend to be always off (ie need to be powered to enable power to flow from 30 to 87)  Where is the original source, as I would not mind saving that.

[Diesel]

PM sent

[terrydj]

Headlights have been mandated to come on with the ignition for many years. Part of the headlights on laws. 

Pretty sure headlights on is not a law