• Welcome to ClassicBroncos! - You are currently viewing the forums as a GUEST. To take advantage of all the site features, please take a moment to register. It's fast, simple and absolutely free. So please join our community today!
    If you have problems registering or can't log into your account, please contact Admin.

Relay Wiring

lonesouth

Contributor
Bronco Guru
Joined
Dec 18, 2003
Messages
5,045
What is the better way to wire a relay? Top or bottom.
 

Attachments

  • relay.JPG
    relay.JPG
    13.7 KB · Views: 131

DuneBuster

Sr. Member
Joined
Dec 14, 2004
Messages
423
Loc.
Canton
Hooking up a relay

Normally, you'd hook the switch directly to ground and the other side to the bottom of the relay, but that way the relay is always "hot", that is the way most electronic controllers do it.

I'd look over the routing and figure which gives you the shortest run from power to the relay and the switch so you use the least wire.


How about the power side? Do you plan to attach the load to battery and then to the relay or, conect the relay to VBatt and run the wire to the load?

The biggest question your schematic had was do you have a fuse in that power circuit??
 

RnrdTheFox

Sr. Member
Joined
Dec 4, 2005
Messages
727
Loc.
Superior, CO
If I were doing it and didn't care about the temp sending unit turning the fan on and off once you have started the truck, I would do it as follows:

1. Find a high current bosch relay with a built in diode. Get one that's rated about 50A. The inrush current is higher, but once the fan get's started it's fine.

2. Wire it so that it comes on when you start the engine. So use a switched 12V to trigger the realy.

3. Place a fuse at the battery lead that goes to the relay that is connected to the fan when the realy is switched on.

The bosch relay wires as follows:

pin 85 - 12V switched
pin 30 - 12V battery
pin 87 - +V on fan
pin 86 - gnd

Couple of things to consider. Do you want a switch that's in the cab that will turn off the switched 12V in times where you don't want the fan running (think water crossing)? If you do this, I'd also add an indicator light on the fan side of the in cab switch that would tell me if the relay was getting 12V so I didn't forget about it off once I turned it off.
 
Last edited:

broncow72

Village Idiot
Joined
Jun 5, 2005
Messages
2,174
Loc.
Spring,TX.
I prefer the top. I like to disconnect the power at the closest point of origin.
Keep in mind that the relay power needs are minimal. I have 8 relays and breakers on order. I Plan to run one power source for the dash lights, guage power, and switch (both acc. and run) power. One 20 amp circuit daisey chained amongst all these will be more than enough. A 10 amp circuit would probably do it, but this gives me room for add ons.

I'm planning on using relays for anything @ 20 amp or more. No fuses, but auto reset breakers for everything.

Seems we're at the same stage in our rebuilds... Too bad we're not closer to each other. Collectively we could be dangerous.;D
 

broncow72

Village Idiot
Joined
Jun 5, 2005
Messages
2,174
Loc.
Spring,TX.
When the power is cut to an inductive load (fan) it creates a current spike in reverse. The diode give this current a path to flow so nothing gets damaged.
Does this mean the fan, when shut off, acts like a generator while still rotating?
And... as I understand, the diode is sort of a check valve for electricity.
You'll have to excuse my ignorance, I is a bit unedumacated in the arts of electrical engineering.
 

RnrdTheFox

Sr. Member
Joined
Dec 4, 2005
Messages
727
Loc.
Superior, CO
Does this mean the fan, when shut off, acts like a generator while still rotating?
And... as I understand, the diode is sort of a check valve for electricity.
You'll have to excuse my ignorance, I is a bit unedumacated in the arts of electrical engineering.

Not exactly. When you try to change the current flowing through an inductive load it creates what's called an inductive kickback. The diode is meant to clamp that.

I haven't looked at the bosch relays in a while, but some of the high current ones had diodes built into them. And you would notice that in the price and spec sheet. You could use a cheaper relay and add the diode to it also. Also, many of the relays have diodes across the coils that allow them to open better, but those are different than what I'm talking about.
 

broncow72

Village Idiot
Joined
Jun 5, 2005
Messages
2,174
Loc.
Spring,TX.
Alright. I googled the term "inductive load" (warned ya I is a bit slow) and now am under the impression that a relay with diode eleiviates the load on the contacts (when swotched on or off) reducing the "spark" thus increasinng the life of the relay.

Am I right? Or a boob who will never understand this?
 
Last edited:

Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
34,835
As for the original question, either way is fine. Sometimes the curcit didcitates what has to be done. Like a temp switch that grounds puts the switch on the ground side of the relay. Other times you want the relay to come on when power is applied to the curcit, so the switch is on the power side.

I like the ground side switch for the fact that if it chafes to ground (I would have to have done a blunder to do that, but I have done blunders before), the curcit turns on. Also when trouble shooting, it is easy to get a jumper to ground to test it. I can keep the fuses and relays all together.

With the power switched relay, you risk shorting the wire going to and from the switch. That better be on a fused curcit (probably a shared curcit). If it shorts, you blow fuses and chase it. And it is stuck off until you fix it.
 

SaddleUp

Bronco Guru
Joined
May 23, 2004
Messages
9,655
Loc.
Vancouver, WA
I run a big relay (80 Amps) on my fan. The 40 amp one I had installed originally didn't last very long. As far as the choice between them I don't think it really matters. Most of your wiring will be switched on the power side so for consistency that is how I would wire it. The power (switch) side itself isn't going to need much amperage. A 5 amp circuit will probably support double the number of relays you will ever install. (That's why we run relays)
The diode simply stops power from back flowing through the system (Only allows it to run in one direction). If for instance you run the fan off a thermal switch then it will get power whenever the temperature is high enough. This can actually backflow through the system and power up any circuit it is connected to. A while back I inadvertantly cut the diode out of mine and hooked it up directly. I had the fan set up on 2 circuits. One when the ignition was off that only operated off the thermal switch and then another on the accessory side of the ignition. I set it up that way so I could bypass the thermal switch and have the fan on all of the time if I wanted. Anyway once I cut the diode out the fan would come on when I shut it off (Which was how it was supposed to be) but in addition to this it powered up anything that was turned on that was on the accessory side of my ignition. For instance the radio would turn on and stay on until it cooled down and then would shut off. A few minutes later it would come back on when the fan turned on again. Without the diode it will also act like a generator as you mentioned earlier. I.E. When you turn the ignition off then it will take a few seconds before it shuts off. In that case your putting the full amperage of anything running through that wire for a few seconds.
 

broncow72

Village Idiot
Joined
Jun 5, 2005
Messages
2,174
Loc.
Spring,TX.
OK, now you all are messing me up. Saddle, you are confirming what my understandig is. Rnrd, is, I think, going into more detail than my feeble mind can comprehend.
Since all my relayed items are under 15 amps. I feel safe running the 40 amp bosch relays. When I decide to run an electric fan, I will run a larger relay, w/ diode.
 
OP
OP
lonesouth

lonesouth

Contributor
Bronco Guru
Joined
Dec 18, 2003
Messages
5,045
i was planning on just running a standard 40 amp relay without a diode on the fan. The way a relay works, there shouldn't be any reason for backflow...all a relay does is open or close a circuit, on or off, that should be it. So, if your switch is off, the relay should be open, effectively cutting off the output from the rest of the car.
 

RnrdTheFox

Sr. Member
Joined
Dec 4, 2005
Messages
727
Loc.
Superior, CO
For what it's worth, here's an example of what I was trying to explain. http://www.fordmuscle.com/archives/2003/02/electricfan/index.php
You will notice that there is a diode across the fan called a freewheeling diode. It is there to handle the inductive kickback when you break the circuit although they don't explain that in the article. I've seen power relays with these built into them. The example the Saddleup gives is diferent than what I'm trying (very poorly I might add) to say. Sorry for the confusion.
 

SaddleUp

Bronco Guru
Joined
May 23, 2004
Messages
9,655
Loc.
Vancouver, WA
i was planning on just running a standard 40 amp relay without a diode on the fan. The way a relay works, there shouldn't be any reason for backflow...all a relay does is open or close a circuit, on or off, that should be it. So, if your switch is off, the relay should be open, effectively cutting off the output from the rest of the car.

FWIW, When I had the problem I mentioned above I cut the wires running to the thermal switch and haven't used it since. I never got around to adding a diode in and mine now has a delay before it shuts off. (Just enough to notice) I know it's coming from the fan because it stopped doing it when the fan was trashed. (It got shoved into the engine and the fan motor fried) The only explanation I would have is that apparently the relay will stay energized as long as there are amps flowing backwards through it. Might be because the relay is actually a large solenoid instead of the solid state relays I use for everything other than the fan.
 

RnrdTheFox

Sr. Member
Joined
Dec 4, 2005
Messages
727
Loc.
Superior, CO
FWIW, When I had the problem I mentioned above I cut the wires running to the thermal switch and haven't used it since. I never got around to adding a diode in and mine now has a delay before it shuts off. (Just enough to notice) I know it's coming from the fan because it stopped doing it when the fan was trashed. (It got shoved into the engine and the fan motor fried) The only explanation I would have is that apparently the relay will stay energized as long as there are amps flowing backwards through it. Might be because the relay is actually a large solenoid instead of the solid state relays I use for everything other than the fan.

The delay is probably the brief voltage spike arcing across the terminals when it's breaking the circuit. As mentioned, it will work without the diode. But with the diode, you will significantlly increase the life of the relay. And yes, the larger the relay, the more abuse it will take. If you look at the schematics for any of the mustangs that have electric fans, the diode mentioned is built into the Constant Control Relay Modlue. Those fans cycle many more times when it's tied to a thermostate trigger. The less times the fan/relay cycles and the large the relay, the longer it will survive under abuse without a freewheeling diode.
 
OP
OP
lonesouth

lonesouth

Contributor
Bronco Guru
Joined
Dec 18, 2003
Messages
5,045
Working out my wiring now, and i'm looking for a normally closed latching relay that will reset when input power is turned off. Anyone know where to find something like that?

Something intersting I came across in my search

The goal of the fan circuit if for the temp switch to turn the fan on, the speed switch to allow me to choose high or low and the momentary button to allow me to turn the fan off. The resetting latch relay would reset after turning the engine off to ensure i didn't forget to turn it back on.
 

Attachments

  • fan circuit.JPG
    fan circuit.JPG
    34 KB · Views: 32
Last edited:
Top