Why relays?

[KTurner]

I have a kind of dumb question. I'm no stranger to simple electronics, but I'm relatively inexperienced with automotive electronics, and I'm trying to figure out the proper usage of a relay. My understanding is that a relay is used when the current draw of the load is more than the switch that controls the load would be rated to handle. But what's confusing me is, I see switches all over the Internet that are rated to handle 15-20 amps at 12 volts, which is plenty for many applications. So why not just run the hot wire directly through the switch and be done with? And then I look under the hood of my tractors, and there's relays all over the place. So there must be something I'm missing. There must be some reason to use a relay other than that the switch isn't rated to handle the current draw of the load.

The example I'm pondering right now is, I'm thinking about putting a light set on my ROPS. I was thinking, if I wired a relay into the hot wire running to the headlights, then I could control the ROPS-mounted lights with the headlight switch. But wait. Why not just splice into the headlight wire and run my lights directly off that? Assuming, that is, that the headlight wire could handle the current draw.

Real life example - I have a '96 Dodge Dakota. From the factory, power to the headlights goes through the switch in the passenger compartment, no relay involved. It's a common upgrade to add a relay and a short power wire, with fuse, from the battery to the headlights. You still use the same light bulb. There's less of a voltage drop in this arrangement so the bulb gets more power. I haven't done it yet (it's on the list), but it's supposed to be a big improvement in the amount of light the bulbs put out. Could you do this without a relay? Yes, but it would involve using bigger gauge wire, even bigger than the short power wire you'd use in the relay arrangement, from the battery to the switch to the lights.

Also, some switches are not easily upgraded to handle the current needed by the devices they control (ie, ignition switch).

Keith

 

[the old grind]

12v @ 20a is a mere 240 watts, as is 120v @ 2a, so 12v switches must handle a lot of current to control a decent amount of power. A 30w bulb draws 2.5a @ 12v and you'd need the capacity to light a pair or a quad of bright ones. One switch can control several relays, and with a few well-placed diodes different load/relay combos can be grouped to specific switches. (watch the v & a ratings) This can also provide alternate/back-up current paths to relays if a switch fails at the worst time. More to test/maintain too, but one either gets wiring or should find help easy to attain. Swapping relays is a trouble-shooting shortcut ...

 

[joshuabardwell]

There's less of a voltage drop in this arrangement so the bulb gets more power.

So, the point is that because the power wire to the headlights is shorter and can be thicker, more power is delivered? Whereas if you had to wire directly to the switch, it would be longer, and would need to be thinner, and you couldn't deliver as much power.

 

[joshuabardwell]

12v @ 20a is a mere 240 watts

That's a good point. The applications I'm considering are well under 20 amps, but that's not always the case.

 

[dstig1]

Joshua, Count me in the same boat as you - to a degree. I get the reasons for relays in many cases. You don't want to run 30a or 50a current through switches as the switches would be very expensive to be rated for that. If you add in that many automotive switches are customized for aesthetics to look nice with the interior, this could get really pricey. On the flip side, relays are hidden and standard - just pick the rating you want - so they are cheap and easy to change if they fail (plus they are less likely to fail by design). Then the short current path some discussed is also a good reason for heavy current draws (shorter wire = less resistance, or allows them to use a smaller wire due to the shorter run to save $$). All that makes sense. But for the little 5-10A loads? Those I just don't get, but they still use relays. Perhaps it just became so routine that they do it out of habit. Dunno.

The other item cited - control a high voltage circuit with low voltage controls is very common but does not apply to automotive applications where everything is 12v. That is a very common use in industrial equipment where you may have a computer controlling 460v/3 phase loads, or even higher current 120V or 220V loads in a house. A forced air furnace or central air conditioned would be a very good example of this at home. The thermostat is typically low voltage 24v but control 25A+ compressor motor starting loads at 220V.

 

[mikehaugen]

I'd rather have any switch that I have to touch in an outdoor environment be low current.

The amount of current going through the circuit has very little/no affect on how much it will hurt you. Voltage is what matters, most of the time 12v is not enough to break the resistance of your skin. That is why it is nearly impossible to be electricuted by a car. Now the amount of current that flows through "you" does matter. But that is dependent on the resistance of "you" and the voltage of the circuit, not the current in the circuit that is there. I know that this is a terrible explanation but I am not a teacher.

Another reason for relays is that their action is nearly instantaneous, which means much less internal arcing while the switch is being thrown. This is especially important when controlling motors. The contacts also should theoretically last longer in a relay because of this.

 

[joshuabardwell]

Upon further thought, I can see that relays would also be very important if your switches were very small--in other words, if you were using DIP switches or had a microcontroller (like an Arduino board) or something like that. These switches have much less current-carrying capacity than full-sized switches.

 

[sparc]

The amount of current going through the circuit has very little/no affect on how much it will hurt you. Voltage is what matters, most of the time 12v is not enough to break the resistance of your skin. That is why it is nearly impossible to be electricuted by a car. Now the amount of current that flows through "you" does matter. But that is dependent on the resistance of "you" and the voltage of the circuit, not the current in the circuit that is there. I know that this is a terrible explanation but I am not a teacher.

Another reason for relays is that their action is nearly instantaneous, which means much less internal arcing while the switch is being thrown. This is especially important when controlling motors. The contacts also should theoretically last longer in a relay because of this.

Circuit current may not be as important, but the available current if you were to get shocked does have relevance. If the circuit only handles low current (1/10 amp) say, it poses little threat, but the same voltage in a 30 amp circuit has more potential to do harm.

 

[mikehaugen]

not really, available current is only limited by your fuse protection. If you have wires ran straight from a battery to a switch (as a lot of people do) there is no limit (short of wires melting open or just the capacity of your battery) to the amount of current that has the potential to go through your body, even if the switch was only powering a little LED light.

 

[pat32rf]

By using relays you can design circuits that would take a very complcated switch to duplicate. One of my favorites is a circuit that won't let you switch off an engine while the lights are still turned on. Turn off the key and remove it, the engine still runs until you turn off the lights, at which time everything shuts down and won't restart without the key. On an old 4x4 I could push a button which would let the engine run without the key, but shut everything down as soon as anyone touched the brakes-great for leaving it running with the heater on while you went in for coffee in an area where someone else might try to drive away.