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Old 05-12-2005, 10:02 AM
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Fan Switch on the Ground Leg?

I have noticed in many diagrams of how to wire an electric fan, that the switch is wired into the ground wire rather than the power feed. What difference does it make?

John

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Old 05-12-2005, 10:19 AM
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The only reason I can think of off-hand is not having to run that much more wire into the driver's area and having to use a larger wire. Of course a relay could fix that but it may just be for simplicity? But then again I'm no where near a electrician.
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Old 05-12-2005, 10:42 AM
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The fan needs to be used with a relay. The relay activates with a ground. You put that many amps through a switch without a relay you will smoke it and probably catch something on fire.
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Old 05-12-2005, 01:26 PM
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But it seems like a relay could be used in the power lead or the ground leg. Does the ground leg see less amps due to electrical energy being expended in the motor?

I'm no electrician either.

John
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Old 05-12-2005, 02:01 PM
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The ground is what completes the circut for the relay.

Here is an article with a Schematic

http://www.chevyhiperformance.com/howto/4572/
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Old 05-12-2005, 05:54 PM
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Ground as a control Circuit

Doc here,

Ground switching is most often used as the control circuit for a few reasons, SOME of which are,
  • Many single Wire Senders are internally grounded, and make ground connection when they reach Full operation temp.
  • A Ground wire is LEAST likely to chafe on metal or the hot block and short out and cause a harness melt down. The worst that can happen is the fan will run constantly.
  • Ground USUALLY is available everywhere, thus lowering the wire count.

Running without a relay (or as I have just learned from my good friend here, Alan..2 for safety) Is asking for trouble..Fan motors are a high current draw item...A switch contact may be rated at 5 amps max, and the motor might draw as much as 30 amps (and as much as double that in a rotor lock condition)

When you activate that 5 amp switch, (which may run for a brief period of time) all the current draw will effect the weakest link in the chain (the switch) Causing heat and arc over...since a proper fuse might be 30 amps to get The Fan to run without blowing a lower value fuse...That switch is going to heat up, and eventually arc over, and burn up..and could cause a harness fire..

Relay contacts hooked up to properly fused or linked source Current, can deliver way more current to the motor than trying to run it through an underrated switch, and most likely underrated wiring.

You can not thermostatically Control a fan without a relay, (you'd be hard pressed to successfully try it..) As the Fan motors will draw way more current than the sender can handle internally (as much as 6 times more current..)...hence the need for a relay..the coil draws a small amount of current (from 1/2 an amp up to 3 amps, depending on the type used..) to trigger the magnetic switch, (movable center wiper internal the relay) and make contact on the motor power wires from source current..(N/O, C/W contacts)

ALWAYS use a proper current value relay (s), Proper gauge wire for the task at hand, Find out the total load draw and properly fuse it or link it, Make sure the Switch is rated current wise for the draw of the control circuit..

I usually use Aviation Avionics Grade (TSO'ed) relays & Switches for my projects, they can handle slightly higher current than automotive hardware at about the same sized package, and are better built....but ARE much more expensive (4 to 6 times the cost)..But last almost forever..and operate flawlessly.

For Most applications This is overkill..but how good is best? I'd hate to have my $30,000 Rod reduced to ashes for the want of an overpriced $30 relay...or a $10 Switch..or an undergauge wire...You can run with automotive grade hardware and in 99% of the cases, you'll be just fine..Run without ANY control hardware and you just might want to check your fire insurance...

Doc
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Old 05-12-2005, 06:03 PM
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Quote:
Originally Posted by JohnTN
But it seems like a relay could be used in the power lead or the ground leg. Does the ground leg see less amps due to electrical energy being expended in the motor?

I'm no electrician either.

John
No, there is just as much current in the 'ground' leg as in the 'hot' leg. Electrons aren't consumed in the motor. There is a voltage drop across the device however. Think of electrical current as water flowing through a garden hose and voltage as the pressure of that water. If you run a water turbine with that hose, the same rate of water flows out as flows in but there is a pressure drop through the turbine. The pressure drop does work but so does the mass flow rate of the water. Same for the analogous electrical amps and voltage.
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Old 05-12-2005, 07:17 PM
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Loaded Ground (or neutral)

Doc here,

Willys...

Good Analogy...this is also known as a "Loaded Ground" or in the case of residential wiring, "Loaded Neutral wire" ...

If you have a Ground loss after the Load, and put a meter on that line..you will read a voltage comparable to the input voltage...(although it doesn't have the same potential to do the same amount of work series connected) Until you ground that wire, then the voltage it will go to zero..and the load will be consumed in "Work" by the device behind it..

Two 6 volts lights will function perfectly well on a series connected set of sockets supplied by a 12 volt source..Provided the current delivered is matched to the amount of "Work" consumed.

However 2 ~12 volt lights will operate at much less luminance or none at all, series connected due to the total filament resistance.. In either case is dependent on the source current supplied from the device preceding it..and will not operate at all if the first source is not functional (blown filament) anybody that has ever went looking for the infamous blown bulb on a Christmas tree string of lights is familiar with this malody...

In residential wiring..the loss of a neutral wire ( the white wire) say on a light bulb circuit will disable the light...and If you go messing around with the light wiring with the source power on, and grab that white wire (which should be at zero potential...) you will get one heck of a bite...and wonder why...It's because the filament, without doing any work..is nothing more than a straight wire...looking for a ground or zero potential source..It is then no different than the Black wire (110 volts)..with a little resistance thrown in...instead of doing "Work" to light the bulb and consume current..it WILL do work to make your ears ring..It follows the path of least resistance..even if your 6 foot 3..that would be you... This is regarded as a "Loaded Neutral".

Power is consumed in work...(or current draw to ground..) apply a little ohms law to a coil of insulated wire and you can put 12 volts at "X" amount of amps through that coil to ground and have a magnet..or motor winding...

Make a mistake on your calculations, and turns ratio and you have a short to ground or a resistance so high, it can't do any work..Same with a Filament on a light bulb..the resistance of the filament consumes the current and does the work (lights the filament) and offers a path of resistance to ground . With a properly sized fuse (slightly higher than the draw of the component) The system will operate well.

Doc
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Old 05-15-2005, 09:35 AM
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Many of the Japanese cars have been using cold side switching for decades now. I think, as mentioned, it is safer because the hot can take a direct route to the destination.
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