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Electric fan circuit...is it flawed ?

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Twister
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PostPosted: Sat Jun 14, 2014 17:13    Post subject: Electric fan circuit...is it flawed ? Reply with quote

I've been racking my brains about how to sort this, or even whether it actually needs sorting, so I'm going to put it out there and see what the general consensus is.......

Below is the wiring diagram for the circuit I've built for my 2 speed Elec fan. There are 2 illuminated switches in the circuit, No1 is effectively a kill switch that cuts the voltage to both relays, and No2 switch allows me to control No2 relay when No1 relay is live.....



This works fine in that when the temp sensor switch is closed and No1 switch is "ON" then No1 fan is on and No1 switch is lit. I can then use No2 switch to kick No2 fan in, and both switches are then lit.
Now, here lies the potential issue.....



The way it is the sensor switch in the sender unit (which is just a bi-metallic switch) has a permanent fused +12v supply to it. This wasn't an issue with the original Kenlowe sender unit as it was mounted high up in the engine bay, with the temp sensor wire in the top rad hose, however the new Xeng sender is mounted in the bottom hose, and I'm not sure whether having a permanent live to it is going to be an issue or not in terms of when it goes underwater. If it is then I'm going to have to rework the primary circuit somehow, probably just by reversing it so it's earth switched, although quite how so that I can still get the switches to light accordingly I'm not sure at the moment, but that's just something I'd have to work out.

Will the permanent feed to the sensor be an issue in terms of potential shorting underwater? or will it be fine as it is ?
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PostPosted: Sat Jun 14, 2014 17:57    Post subject: Reply with quote

So you're saying the Bi-Metal strip is not encapsulated??? Two bare contacts????  - This seems odd to me...

In either scenario though - Whether the sensor switched 12v coming via the relay coil to 'earth'  Or (as you have) switched 12v direct via the relay coil to earth... The current passing through it is exactly the same. Personally; I can't imagine an unencapsulated sender being a 'good thing' but if it works...  Swapping wires round on this will make no difference electrically.
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PostPosted: Sat Jun 14, 2014 17:59    Post subject: Reply with quote

The other think I'd add is that I don't understand why you want a 'kill' switch for the fans... Personally I'd be fitting an override switch for the sensor!
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PostPosted: Sat Jun 14, 2014 18:15    Post subject: Reply with quote

I could post loads of pics of mine underwater which works just fine using my half speed for low and both at full for high set up  but  the way to go is to set up the switch in an ole saucepan that you have not weighed in for ally scrap and run 12v in  with two bulbs on the outputs and heat the water and see if it has any problems .

In electrical terms (before Mr B's huge talent diverts the post again ) the rate of conduction at 12v dc  through rainwater is very low, like in millivolts.

If your really fussy use that dielectric grease to waterproof the terminals  .
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PostPosted: Sat Jun 14, 2014 18:23    Post subject: Reply with quote

Yes the bi-metallic switch is encapsulated within the screw-in sender unit in the bottom hose, I'm talking about the electrical connectors that go into it, and whether having a permanent +12v to that would be an issue when wading, but I have since been advised that it won't, and that the circuit is fine.

The alternative would have been to have it so that the kill switch kills the voltage to the sender unit, but that would have made the set up with the illuminated switches and getting them to light for each fan much harder to sort out.

The Kill switch is basically so the fans can't come on when I'm wading, which can wreck the fans by snapping the blades and potentially wreck the radiator.

...and an override is on the to do list, but that's simply a case of bypassing the sender unit.
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PostPosted: Sat Jun 14, 2014 18:25    Post subject: Reply with quote

Water doesn't conduct electricity...   It's the contaminents in it that do!  And we'd be thinking more about the current conducted through it rather than the voltage however.  I think we must assume if these things are actually manufactured and sold for the purpose of living in a hot churning soup of water, anti-freeze and possibly some other chemicals, then any current flow across the open terminals will indeed be irrelevant...  Whatever way its going! Laughing  

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PostPosted: Sat Jun 14, 2014 18:37    Post subject: Reply with quote

As I said in the OP, the original Kenlowe unit was at the top of the engine bay, so it wasn't really an issue,  (until it packed up  Laughing  Rolling Eyes)  . It's just with this new one being much lower down in the engine bay I was simply wondering whether shorting between the "external" spade terminals would become an issue when going through deep water, such as river crossings and the like Wink.

Sorted now anyway Smile.
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PostPosted: Sat Jun 14, 2014 18:37    Post subject: Reply with quote

The terminals - all your electrics in fact that might go under water in fact - should probably be sealed over... Self healing tape, silicone grease and even silicone sealant are used in delicate comms applications; they'd work just as well in the rather more robust environment under a vehicle...   There will be marine sealing solutions too...

In all honesty though; I'd not worry about it... The connectors will have 'boots' on them anway?

As Gribble says; water isn't a great conductor at 12v!  A few milliamps at worst  - Yes; I'd agree, the circuit is fine...
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PostPosted: Sat Jun 14, 2014 20:46    Post subject: Reply with quote

Nobody worries about the starter going underwater do they  Laughing
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PostPosted: Sat Jun 14, 2014 21:00    Post subject: Reply with quote

Fair point  Laughing
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PostPosted: Sun Jun 15, 2014 10:08    Post subject: Reply with quote

I wired my kill switch in before the sensor. Idea
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PostPosted: Sun Jun 15, 2014 10:14    Post subject: Reply with quote

Other than possibly requiring a bit more wire - electrically that makes no significant difference either.
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PostPosted: Sun Jun 15, 2014 10:18    Post subject: Reply with quote

Let me elaborate.  I use the kill switch to energise a 5 pin relay. The n/c circuit being normal feed. When the switch is operated, it lights up, ,energises the relay and breaks powe to everything else.
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PostPosted: Sun Jun 15, 2014 11:40    Post subject: Reply with quote

If by "everything else" you're still just talking about only the fan switching - There is absolutely no point to what you've done as you're not switching load-level currents only relay-switching current. - It's a waste of a relay basically.  And you've added another potential 'fault point' (complexity) to the circuit with no advantage.

In fact... Having a 'kill switch' that requires energising to hold in the 'safe' position is bad practice...  That relay (whatever its purpose) should be wired so that you DE-energise it to make the system 'safe' - That way if the relay fails it won't cause the system to become unintentionally live.
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PostPosted: Sun Jun 15, 2014 12:41    Post subject: Reply with quote

This is getting far too serious.  The kill switch is only there to Rolling Eyes  switch off the engine fans when wading. We are not talking about switching 3 phase industrial motors that have the potential to kill.
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