What Requires a Relay

Hey all, I am thinking of removing the relay's completely from my planned rewiring; I am curious of what actualy requires a relay (for safety of the switch??) and what doesn't...

The accessories I will be hooking up to switches inside the car are:
1) Radiator Fan
2) Fog Lights
3) Neon Undercar Lights
4) Fuel Pump (only as a breaker switch, like a security method - fuel pump wont come on when car is turned on unless switch is turned on)
5) 3 Horns


Previously, I ran the following without any relays:
- Radiator Fan, Fog Lights and Neon Undercar Lights

and they all worked ok (though one of the lights in the switches burnt out?)

Relays can switch any kind of load for any signal in any direction within power limits of course. They can only go on and off, nothing in between. They cannot be switched on and off at high speeds, nor should they be since it is the switching that causes the wear on the relay. They do not wear down while conducting or blocking, unlike solid states switches which can switch very fast and easily, but wear down while they conduct.

Also, if they fail, they fail open rather than short.

So if you need any of these things, that's what a relay is for. What exactly did you want to replace them with? (You probably can't). If you replace them with a wire well, they device will always be on- and if you leave one on while your car isn't running, your battery will die (that's what I assume is the case with most of your devices).

Are you sure something else in the car isn't monitoring the device and the controlling the relay acting as a kind of circuit breaker or smart control?

Well the radiator fan will eventualy be corrected (the sending unit which turned on the fan is shot) - but until then, I operate the fan when ideling at stoplights\stopsigns and when moving, only when the water temp reacher 190'F (the temp at which my thermostat opens - but its not hooked up anymore)

The rest (fog lights, horn, neon lights) don't have any other controls that monitor or controll it (the horn button on the sterring wheel is shot)


Therefore im putting in a switch panel to cover those accessories, and when fixed, those freed switches will be devoted to something else.

Usually you would use a relay if you wanted to switch a high current / high power circuit, but using a small low power swich.

Or if it wasn't safe to route high power circuits to the dashboard.

Or as mentioned because you wanted to be able to switch things off automatically according to a master switch.

I don't know why you want to ditch the relays. Just use the advice I gave you in the other thread to wire them up properly and you'll be ok.

Are you from UK? USA? Somewhere else in Europe? Middle East?

None of the Above?!

Brian

relays will also be used because of the physical gap between the contacts and the physical distance between the coil leads and contact leads - creep and clearance requirements for safety.

dknguyen said:

Relays can switch . . . . .

Also, if they fail, they fail open rather than short.

Click to expand...

FYI,
with regards to relay failure. Failing open is not a guaranteed thing. A failure mode does exist that causes the relay contact to fail closed. It is termed a "welded contact."

Whether or not a relay will fail open or closed depends on the operating conditions that cause the failure (over current/voltage stress, metal migration, oxide formations, temperature etc...)

Also consider a Form C relay where one contact has failed open (for whatever reason) is it not true that the other contact has (practically speaking) failed closed?

Just somethings to consider if the failure mode matters to the application.

They sometimes fail connected to neither contact, this is a welded tertiary mode failure or WTF.

I might have to wire them as before stated, properly; For now I may just route the radiator fan to the inside and rewire the rest when I have the time... doing it properly might be my better course of action since I do want lighted switches.

My only main concerns was what really didn't need a relay? I know the Neon Undercar kit doesn't really need a relay because it only draws 5amps... so coupled with a 5 or 10 amp fuse, I could directly wire them to a switch and bypass the relay completely.

I was just curious if the Fog Lights, Horns, and Radiator Fan really needed to be on a relay.


~Shiin

if you know the current that is required by each load that is to be put through a switch and you have a switch that is rated to carry that current and also with a voltage rating that can withstand the potential across the contacts when the switch is opened, then you can use a manual switch.

Pretty risky trying to remember to manually switch on and off your radiator fan. All the others are only going to run down your battery if you forget... if you get distracted and overheat your engine, that's plenty trouble.

AllVol

I have a switchable radiator fan which I installed in my Land Rover. I also installed one which blows onto the intercooler, although I'm not sure whether that helps performance or not. But my fans are in addition to the viscous fan which is currently fitted, so there's no danger if I don't use the switchable electric ones. I only installed them for better cooling when towing my caravan.

I certainly wouldn't use my main fan through a manual switch as the consequences of forgetting to operate it could be disastrous! You'd be better off leaving it switched on all of the time.

Brian

Well I've rewired the Radiator Fan using a relay, bringing only one wire back from the relay ground to the chassi ground (with a switch in between) - it wont work as a lighted switch (because of the limited current that flows through the coil of the relay) - but will trade off the lighted switch for a normal switch so I only have one wire per switch to worry about. (6 to 7 wires total once I rewire everything)

I rarely forget to turn things off. I'm pretty adament about making sure things are set right before I depart.


The child inside me wants a lot of switches to have to fondal to get my car to do certain things, and since this is a 4-banger for work, im not worried about all the potential hazards of 'forgetfullness'

The catch 22 is how do you remember that you didn't do something that you have forgotten? Everyone makes mistakes.