Thread: Relays and Terminal blocks oh my

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Most of your relays can mount at your panel with the possible exception of higher current devises (equipment) which can sometimes benefit by having their relays mounted near the devises they control.

Ideally, you would mount your high current relays nearest their high current devises, such as headlights, radiator fans, horns, exc.. Using larger wire sizes and fuses (rated for the current) on the load side of the relay, while operating (switching) the relay with smaller wires on the control side of the relay via another switch such as the ignition switch, exc.. On high current devices, mounting the relay near the devise will take full advantage of the relay’s ability to operate the large current loads while minimizing the length of the larger more expensive wires, translating into an economical control circuit that can handle a high current devise without applying the high current demands through the switch. Just remember to protect your relays if they are exposed.

Normally, one relay is used for each device. However, multiple devices can be controlled from one relay so long as the total load of the combined devices do not exceed the relay's maximum current capacity.

In our car’s “DC” circuits the load capacity can be determined by dividing the wattage of the devise by its voltage, ie., a 60 watt bulb at 12 volts will draw 5 amps (60 / 12 = 5). With the exception of inductive devises such as starter motors and fan motors, these can momentarily draw 25% higher currents at initial start up (a 10 amp motor can momentarily draw 12.5 amps at start). The fuse size should take into consideration the safe limit for protecting the wires, relays and devises, with the fuse size rated slightly higher than the devise's total load, but not more than the wire size and relay's maximum rated capacity.

Also, you can switch multiple relays from one switch or multiple switches (control sources) without them interfering with each other by using diode isolation on the control side of the relays. Diodes can prevent the control of one circuit from activating another circuit when it is not desired, such as using the A/C compressor circuit to operate the radiator fans, while not allowing the fans to operate the A/C.

Hopefully, I have not confused you.

Originally Posted by 36 sedan

.....Hopefully, I have not confused you.

All good stuff, but I had to read through it a couple of times....

I fully understand your approach, but there's no way I would put saving ten feet of heavy gauge wire over the convenience of having all of the relays in one, centralized location. And if mounting them close to the load meant exposing them to splash, spray and weather that would be an immediate pass for me. Especially considering that this is a '34 Coupe with no place to locate stuff out of sight, I'd mount them all together, near the fuse block. Other than the ACC circuit, I cannot think of any group of loads that are served from a common switch, which is the only thing that would support one relay feeding two or more devices but I may be overlooking something. Just my $0.02.

Originally Posted by rspears

And if mounting them close to the load meant exposing them to splash, spray and weather that would be an immediate pass for me.

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