Understanding relays

So over the years I’ve read a lot on this web site about how relays can improve or protect an MGB’s electrical components. I decided it was time to understand what they do.

The Relay is an electromagnetic switch where an lower current can be used to trigger the relay which can carry a larger (stronger?) current.

So in the case of an MGB’s light switch that carries enough current to heat up the use of a relay reroutes the stronger current through the relay and the light switch really only activates the relay.

Is that right or too simple of an explanation?

Is there anywhere I could find a before and after circuit diagram for a relay on the MGB’s lights? (here's a link to the simple explanation that's helped me to this point http://electronics.howstuffworks.com/relay1.htm )
Thanks

Pretty much bang on. The electromagnetic switch or contactor routes current straight from a power source directly to the user (lights, horn, etc.)

Bob

Yup, and instead of having to travel through the light switch and the dimmer switch, the heavy current would then travel only thru the relay. Each switch introduces resistance, every foot of wire, more resistance. Result of the higher resistance is, of course, dimmer lights. The relays are lower resistance and rated for heavier loads than the regular light and dimmer switches. The lowered current passing thru the old switches increases the life of the switches also.

And, if I might add, it works like a charm. For only a few bucks you have headlights that rival any new car. Search on this forum and you'll find out exactly how to do it. An easy couple of hours work. One of the best things I ever did with the B.

You are spot on in your understanding, Steve. In a well designed automotive electrical system, the idea is to offer full battery/alternator power with the shortest possible route to the load you select. Relays assist us in doing this. That the switches we use to trigger those relays benefit from substantially lowered current is, in a sense, a bonus.

The standard electrical system in our MGBs was not designed that way and far too many circuits use long runs of wiring to route the full current needed to their ultimate destinations and in doing so, introduce not only voltage drops but place more stress on the switches and connections used than would be necessary had the harnesses been designed in the first place to use relays. Modern cars take advantage of relays to make their systems not only more efficient, but increase their reliability by a factor of several points.

What had been acceptable practice in designing automotive electrical systems in the first 50 yrs of automotive history is pretty much what our cars came with. But, as more and more demands have been placed upon electrical systems, the need to make them much more efficient has required a re-think of how to accomplish this and that has benefited the cars we are offered today. In the end, by adding a few relays, you are not only making our vintage cars simply work better, but are raising their ability to perform at their best to something closer to what we demand out of the cars we design today.

I would like to hear Limey's response to the excellent question, too. I keep learning a lot from this Forum.

I used 2 relays for the lights. One turns on the light circuit, and the other selects low or high beam.

And remember when these cars were first built (1963) the life of the new cars was after 4 years or 50,000 miles expect to start replacing parts such as generators, starters, pulling the head and do a valve job etc. Now a days a Toyoda can go for over 200,000 miles without any major part replacements.

Absolutely correct. Not only can the wires be shorter, but also thicker. It may not be feasible to route thick wires through the instrument panel or to column switches. If the relay is under the hood you can use as thick of wire as you like.

The only reason that they didn't use relays to start with was to save money.

Here is another site about upgrading your lights with relays

http://www.danielsternlighting.com/tech/relays/relays.html

Everyone; Thanks for the info.

And Kent thanks for the link, it has a useful before and after diagram.

Many of the relays also have a center 87a connection that completes the circuit to #30 when the relay is not energized and breaks it when the relay is turned on. This makes all sorts of switching possibilities using a couple of relays. I use just such a circuit to set off a buzzer to warn me that I just shifted out of 3rd or 4th gear and the OD switch is still engaged. Its helping me save my OD cone clutch.

What everybody said is pretty much on the money. Ideally, we would go to solid-state switching devices as they do on modern cars, but that is more difficult to reverse-engineer into an older vehicle without completely re-wiring the car (and losing some of its authenticity).

Retrofitting with relays simply reduces the current through the switching system, while allowing a shorter wire run to the device you want to switch - thus the device receives full voltage and works better, and the switch wiring sees lower current - and so lasts longer! Size of wire is a tricky subject - ideally you should not use any greater diameter than is necessary and, in fact, most of the wiring on an MGB is plenty thick enough for the application - except when you have long wire runs, and add switches in the system that can add resistance!

The headlights on our cars are a classic case where adding a couple of relays makes a HUGE difference. I think everybody that installs them is surprised at the improvement!

Eric,

I understand your simple explaination and it helps. As a matter of fact, this whole thread has been very good. I was wondering, what size wire do you use in the kits to supply the 12 volts to your aux relay panel and what size do you use to the headlights from your relays?

I have a 1979 and 1975 and I also whould like to know where you would have me mount your kit? Should it be closer to the headlights or to the fuse area? Please answer considering also that it will be my desire to order your new fuse panel also. My electrical on both cars is stock but seems messy.

Have you given any thought towards a kit that combines both your headlight relays and the new fuse / relay panel?

Thanks,

Scot