Using relays

[PotvinGuy]

Here's a nice 8 relay assembly: http://www.ebay.com/itm/250609010929?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

It's good for 10 amps, so should handle most of our needs. I'm using one, maybe two, in my rewire.

 

[sstock]

Don't strap a cooling fan to that thing it will go poof in no time. That board really isn't anything I would use in an automobile, just imho.

 

[RPM]

Here's a nice 8 relay assembly: http://www.ebay.com/itm/250609010929?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

It's good for 10 amps, so should handle most of our needs. I'm using one, maybe two, in my rewire.

10 amps is no where near enough for cooling fans. Mine would melt 30 amp relays in no time, finally found some 40s and no more problems.

 

[madmike8]

Here's a nice 8 relay assembly: http://www.ebay.com/itm/250609010929?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

It's good for 10 amps, so should handle most of our needs. I'm using one, maybe two, in my rewire.

That's for allowing low voltage micro controllers to switch higher amp/voltage circuits than they are capable of handling. Not really designed for cars. There are 30 amp versions of the board.

 

[donsrods]

On a related subject, my Son is using a SPAL fan and SPAL fan controller in his rpu. We love SPAL fans and use them in everything, but the controller has never worked since day one. We called SPAL and as a test they told us to hot wire the purple wire that goes into the unit and that would make it work all the time. It did work, and since my Son is cooling a 468 Olds with a chopped radiator the fan has to run all the time anyway, so it was no big deal.

Last time we were out cruising his fan quit and he had to pull over and restart the car to get it to kick back on. I called SPAL and the guy there said they no longer sell that controller. So there goes $ 160 down the drain ! We are going to rewire the fan and put a 40 amp relay and toggle switch in, just like I have in my 27. That setup has worked for over 20 years and is foolproof. I just watch my gauge and when I come to a long light I just kick the fan on until I am moving again. In all the years I have been doing it that way I have only forgotten to flip the fan on a few times. Sometimes simple is better.

Don

 

[409T]

I'm using a 40 amp for my cooling fan as well after destroying more than one 30 amp.

 

[Chief-T]

I'm using a 40 amp for my cooling fan as well after destroying more than one 30 amp.

Same here.

 

[RazzBarlow7]

Thanks for the "Hot Dot" reference! I've never seen one of those. That's pretty cool and now on my wish list!

 

[sstock]

Thanks for the "Hot Dot" reference! I've never seen one of those. That's pretty cool and now on my wish list!

No problemo, I got it from Wintec

 

[Wild Mango]

Look at a Relay as a Remote Switch. The coil on the average 12v relay should use less that 1/2 an amp (6 watts) The relay contacts will switch loads way greater than that depending on the rating of the contacts. A starter solenoid is a relay, it switches 100's of amps (over a kilowatt). Most ECU's have a number of relay outputs, which is why, together with the fact that electrical loads are coming way down with the advent of say, LED lamps, you will find very few discrete (stand alone) relays in modern auto electrics.
Keep in mind, the higher the switching current on a relay, the higher the coil current needs to ne to achieve the required contact pressure. This is why your starter solenoid requires several amps, and your 30A horn relay requires under an amp. (the starter coil also engages the pinion in some cases, hence "solenoid")
Keep in mind also, a relay (or starter solenoid) has "pull in" current demand that may be very brief but in 10X multiples of the "hold on" current. A classic example of this is the standard SBC starter solenoid which requires a serious jolt of current to pull in, but once cranking drops off to an amp (12 watts) - any volt drop in the wiring thereof and the current rise is too slow and the dreaded click click appears. The cure could well be another relay, in this case called a Pilot Relay, as some other post has told. So, sizing your control wiring for relay coils, keep it a decent size, say rated 10A and avoid the dreaded volt drop.

 

[sstock]

Good post Mango.

 

[Stephen]

Did I just see a video that supports using relays in your electrical system? Watch this and judge for yourself.

 

[sstock]

Hot ignition switch, hmmmmm.

 

[Stephen]

Hot ignition switch, hmmmmm.

Hot fan switch.

 

[Wild Mango]

Hot fan switch.

A good example. 40 watts or so of fan current vs 6 or so of relay coil current.
Mr Switch is hot because his contacts cannot handle the fan current. The switch contacts have resistance, R, which "sees" the current, I, which results in volt drop across the switch, V. The power is, therefore, Vdrop X I fan current = P Watts. (or I2R)
Its the heat that kills electrical stuff the most by far, and of course causes most fires. As things like switches heat up, so the contact resistance goes up, and so the volt drop goes up, and so we go into thermal runaway.

 

[Stephen]

A good example. 40 watts or so of fan current vs 6 or so of relay coil current.
Mr Switch is hot because his contacts cannot handle the fan current. The switch contacts have resistance, R, which "sees" the current, I, which results in volt drop across the switch, V. The power is, therefore, Vdrop X I fan current = P Watts. (or I2R)
Its the heat that kills electrical stuff the most by far, and of course causes most fires. As things like switches heat up, so the contact resistance goes up, and so the volt drop goes up, and so we go into thermal runaway.

Dude your burning my brain... with Vdrop and P watts. No wonder no one invented the electric outhouse.

 

[Wild Mango]

Dude your burning my brain... with Vdrop and P watts. No wonder no one invented the electric outhouse.

Didn't mean to try to fly over any heads. In an automotive application, its handy to think of electrical loads as current, Amps, because thats how we choose the correct conductor (wire) size. However, electrical work is performed by Power, expressed as Watts, and calculated as Volts X Amps.
Electrical work can happen where you don't want it, like in the fan switch example here the Watts appear as heat only, and the work they do is to burn up the switch.
Another example would be a too small starter cable, where instead of most of the electrical work being done in the starter motor, un unacceptable proportion of the work is done in heating up the cable and the power (Watts) is lost to the starter motor.
Whenever electrical work is done, useful evidence is voltage drop. In the case of the starter cable above, with your voltmeter connected to the battery +ve and the starter motor terminal, you can read the volt drop in the cable when cranking, which tells you how much power (Watts) is being lost in the cable.

 

[Stephen]

Didn't mean to try to fly over any heads. In an automotive application, its handy to think of electrical loads as current, Amps, because thats how we choose the correct conductor (wire) size. However, electrical work is performed by Power, expressed as Watts, and calculated as Volts X Amps.
Electrical work can happen where you don't want it, like in the fan switch example here the Watts appear as heat only, and the work they do is to burn up the switch.
Another example would be a too small starter cable, where instead of most of the electrical work being done in the starter motor, un unacceptable proportion of the work is done in heating up the cable and the power (Watts) is lost to the starter motor.
Whenever electrical work is done, useful evidence is voltage drop. In the case of the starter cable above, with your voltmeter connected to the battery +ve and the starter motor terminal, you can read the volt drop in the cable when cranking, which tells you how much power (Watts) is being lost in the cable.

I was just razzing ya... when I start my build I'll be picking a lot a brains bout everything.

 

[rooster57]

I think if your not using relays you have done yourself a disservice. I have 4 relays and two solenoids on my car. I also have a fuse on EVERY circuit. I have a lot of time invested in my car not to mention cash . The use of relays is not only smart but add a level of reliability . As a side note my fan runs right off my alternator it has a relay mounted low on the shroud with an auto reset overload and is simply operated with a 15 dollar temp switch in the block its flawless. No circuit boards or additional stuff needed simple simple simple. Stephan if a drawing would help just drop me a note.

 

[Wild Mango]

I think if your not using relays you have done yourself a disservice. I have 4 relays and two solenoids on my car. I also have a fuse on EVERY circuit. I have a lot of time invested in my car not to mention cash . The use of relays is not only smart but add a level of reliability . As a side note my fan runs right off my alternator it has a relay mounted low on the shroud with an auto reset overload and is simply operated with a 15 dollar temp switch in the block its flawless. No circuit boards or additional stuff needed simple simple simple. Stephan if a drawing would help just drop me a note.

Hey Rooster, does your fan run all the time when the engine is up to operating temperature or does it cut in and out? Thats a neat way to hook the fan up.