Cooling fan circuit re-design

[DMCMW Dave]

Thank you! That's the best news I've read all day.

Yes, I know about the light -- I don't feel like adding another light somewhere, so I will simply use the "fan fail" indicator which is already there.

Farrar

I recall doing this to mine maybe 10 years ago and I built a one-transistor circuit so that both fans had to be powered to make the light go on. If either failed, the light would go off. Any of these circuits of course only tell you that power is being sent to the fans, they don't tell you if the fans are not actually turning.

[Farrar]

The only thing I can think of for a real "fan fail" indicator would be installing a normally-open reed relay between each fan and ground. However, I have never seen a reed relay rated at a high enough amperage to work for a radiator fan.

(Obviously, Dave M has found a solution to this problem, but only for stock circuit configuration.)

Edit: Come to think of it, aren't reed switches very small? Perhaps that is what the original fan fail module is: a reed relay with some resistors to drop the current across the switches. I'm only guessing, though -- I have never taken an electronics course or even read a book on the subject. (... a deficiency soon rectified by a trip to the library.)

Farrar

[Bitsyncmaster]

I recall doing this to mine maybe 10 years ago and I built a one-transistor circuit so that both fans had to be powered to make the light go on. If either failed, the light would go off. Any of these circuits of course only tell you that power is being sent to the fans, they don't tell you if the fans are not actually turning.

Or an open (bad connection) would not be detected.

[Bitsyncmaster]

The only thing I can think of for a real "fan fail" indicator would be installing a normally-open reed relay between each fan and ground. However, I have never seen a reed relay rated at a high enough amperage to work for a radiator fan.

(Obviously, Dave M has found a solution to this problem, but only for stock circuit configuration.)

Edit: Come to think of it, aren't reed switches very small? Perhaps that is what the original fan fail module is: a reed relay with some resistors to drop the current across the switches. I'm only guessing, though -- I have never taken an electronics course or even read a book on the subject. (... a deficiency soon rectified by a trip to the library.)

Farrar

I've never seen an original Fan Fail unit but I would guess it had a relay with two coils. Then if each coil (each fan current) was the same they would cancel and not turn the relay contacts on.

My new unit measures each fan current and software determines if both fans are working properly. So it would detect, an open fuse, an open connection or a high current fan (jammed). A short circuit would blow a fuse.

[Ron]

This is one way to tell you that AT LEAST one fan input wire is being fed (light off = good):
AtLeastOneFed.jpg

This is one way to tell that BOTH fan input wires are being fed (light on = good):
BothFed.jpg

For homework- Combine the different ways of using the diodes-relays to do the latter but light on = BAD...or, I'll show you if you don't want to play

[Farrar]

For homework- Combine the different ways of using the diodes-relays to do the latter but light on = BAD...or, I'll show you if you don't want to play

Oh, boy homework -- and classes just started this week!

I will take a stab at it. Probably wrong, but what else is there to do on a hot Friday afternoon?

fanfailmaybe.jpg

Farrar

[Ron]

Oh, boy homework -- and classes just started this week!

I will take a stab at it. Probably wrong, but what else is there to do on a hot Friday afternoon?

You got!!!

You can do it this way too (save a couple of diodes).

Dave probably has enough jumk laying around to do it a lot easier w/o relays...

FanCircuitFail.jpg

[robvanderveer]

I've been following this thread for a while now. Forgive me for being stupid, but aren't you actually making a lamp to indicate the proper workings of a relay? Wouldn't that be similar to designing a bulb inside a light switch to see that the room is lit?

What we really need is a way to detect that the fans are actually spinning when either the otterstat or the A/C demands it. Or better still: that the fans are NOT spinning even though they are powered.

[Farrar]

Ron -

Okay! So let me make sure I understand WHY that works:

When the fan signal is being sent from the compressor or otterstat, the relay which turns ON the light has current. However, when the fans ARE on, the coil in that relay is energized, breaking the circuit and turning the light OFF. Correct?

Rob -

I'm the stupid one; remember, I'm new at this! Originally, I was planning on having the light come on when the fans were on and simply putting black tape over the word "fail." However, as you can see to implement a "fan fail" indicator as per stock function, even though I have both fans on separate relays and circuit breakers. In either case, you only have proof that the fans are being "told to come on," not proof that they are actually running. It would take someone much better educated than me to figure out how to have a light come on only when the fans have seized.

Farrar

[Ron]

Farrar,
You got it down dude!
To actually confirm it is spinning you could use a lot of things (e.g. led & detector, or counter, air flow detector, or ......

=====

I've been following this thread for a while now. Forgive me for being stupid, but aren't you actually making a lamp to indicate the proper workings of a relay? Wouldn't that be similar to designing a bulb inside a light switch to see that the room is lit?

What we really need is a way to detect that the fans are actually spinning when either the otterstat or the A/C demands it. Or better still: that the fans are NOT spinning even though they are powered.

LOL, yes.
...well, similar, but not the same. Here, the light switch is not in the same room...but yeah I totally see what you are saying.
If you think about it, knowing the fans are/not spinning is also similar.
We only need to know if the engine is getting too warm. If one likes to rely on lights (v/s gauges) or likes a backup (or extra kick when violating DUI laws or Sweet Suzy is being distractive, etc.) then I'd do it like this (and omit the buzzer):
http://dmctalk.org/showthread.php?12...d-w-opt-buzzer