Let's talk about Relays

I have used relays to control and power a number of things including headlights, fans, heater fans, AC, door solenoids etc. Up to this point I have always wired my relays into the positive circuit. 12 volt power from the source to the relay, then to whatever I am powering up. Then using positive signal from a switch to trigger the relay. Lately I have been seeing wiring diagrams where the relay is in the ground side of the device and often times even triggered via a switch in ground side.

How about some ideas of the plusses and minuses :rolleyes:

Pat

well if a sender i used it thru the relay side of the ground i run all my relays thru the ground side to a switch if i can . it is just ez to do you just jump the load side and open and shut the relay with the ground .i started doing them this way just seams to work better for me

Basically, wiring "to a ground" is simpler, 'cause there is a ground almost anywhere close by; saves wire - in theory.. In alternating current work, it is bad juju to wire anything with a standing voltage on the device being operated; too much danger of a catastrophic short. But in a DC system, there is always a standing voltage, or "potential" on any device, but it may be either pos. or neg., so it really doesn't matter, and since an auto chassis and all the stuff attached directly to it constitutes the negative ground system, it usually is easier to wire "to the ground" by the shortest or easiest route. Personally, being an old sparky, and having worked with all sorts of systems in industry, I prefer to wire from the source, positive in DC or "Hot" phase wire in AC, through a fuse and switch, thence to the device to be operated, then to the return; neg. in DC, or Neutral or an opposing phase in AC systems. Saves an occasional brain cramp.

All of the points made by Rrumbler and Pat are good ones. There are times when one needs to have a ground activated Relay. Some remote control devices only provide a switched ground output and your source may need to have a switched power to it. Also on some power window setups, the switch controls the ground and not the hot lead so in these cases, it too makes sense to control the relay with a ground wire.

Like Rrumbler said, sometimes it is just easier to wire the relay using the ground wire as grounds are easier to find. Also, since your relay needs to have power (B+) anyway, you only need to run one power line to the relay. Oh and... ALWAYS... make sure that your B+ line is fused. I once missed that one tidbit and had a wire glowing bright red before I cut it. Then I had to start wiring all over again.

also, some factory and/ or illuminated switches require them to be wired on the ground side for the illumination to work...

I just did the Fog lights in my '98 Ranger. factory lights, but I built the fog light harness.. triggered off of 1 of the park lights for now, but when I get the money for the factory radio bezel w/ fog switch, the switch needs to operate the ground, so the illumination and indicator light function. so for it to function the way I need, there needs to be a diffrent relay to trigger the current 1, so the fog light switch can control the ground circuit

I noticed that relays really started working off of the ground side when computers started running everything... I suppose it is the easiest way for the computer to control things.. doesn't have to supply power much safier to just supply the ground

efi works with power to the injector and the computer turns off and on the ground that's why all the diagnostic test charts have you check powers and grounds at the computer first thing most sys. have 2 powers and 2 grounds....ted

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Originally Posted by atichargr

I noticed that relays really started working off of the ground side when computers started running everything... I suppose it is the easiest way for the computer to control things.. doesn't have to supply power much safier to just supply the ground

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Actually this was happening before computers but it was mostly in industrial appellations and is considered the best way to control devices that use higher amperage's because the device that is being controlled (load)drops the amperage(usually) across the device resulting in the relay handling a lower amperage than it would connected to the positive side. Thus resulting in relays operating cooler,contacts last longer and often times be made smaller especially in computer controlled applications . Most all of the power relays in the aircraft that i use to work on were wired this way.
Kenny