Welcome, Trent W!
With all due respect to KISS,
A reasonably reliable test of whether a fan is actually turning, in this case, is to monitor its current flow.
12 gauge copper wire has a resistance of about 0.0016 Ohms (1.6m ohm) per foot. The voltage generated across that resistance, at 15A (the stated fan's current draw), would be about 24 mV. Pretty small value. But we can work with that.
Using an OpAmp as a comparator, properly configured, we can take the absence of that 24mV signal to "trigger" the OpAmp to light up an LED, confirming that the fan is NOT drawing current.
I'll post a workable schematic, if you like.
<EDIT>Got it done. Might as well post it:
View attachment 84542
The 12ga. 1 ft wire loop above is in series with fan power lead. Observe polarities.
Yes. For instance, this one ( a "dual" opamp chip - one for each comparator. All other components also available at RS):Thanks for the replies guys you're are awesome! Been scratching my head over this for days.
So from what I understand from your post and correct me if I'm wrong, First I need a Comparator from Radioshack. ...
Correct: only enough to power the TL082 and the LED (<100ma).... I assume i wont be running the load through the device...
No. One 12ga "line" for each fan AND IN THE GROUND LEG!!; I inadvertently misled you on its placement in my original post.... i understand how to wire this I'll just attach a one-line. ...
No. Each "failure" circuit operates independently (hence the seperate sensor lines).... Now will it be an issue when these fans are relayed to run in parallel?...
That's the point of using the LM7805 (5 VDC regulator) so that variations on the 12 VDC line won't affect the operation of the comparator(s).... The second fan/OpAmp wont have its full voltage will that trigger the comparator?
I got to looking at your drawing. Now I am not sure what this project is all about but will take a guess. You are using a pair of thermal switches to control a few fans. Looks like at 180 F. you want low speed and when (if) the temperature reaches 210 F. you want high speed on the fans. Depending on the layout and setup there are a few ways to go about this. One method is to have a single fan turn on at 180F. and the second turn on at 210 F. as needed. The method you have chosen is to run both fans at 180 F. low speed and at 210 F. go to high speed. You are doing this using a matrix of relays. While this should work I think there is a better more simple way to go about it than six relays.
Take a look at the below image. This is a rough drawing of how I would likely go about it. The concept is similar to how the fan speed (heater & A/C) works on my truck.
View attachment 84580
When thermal switch S1 reaches 180 F. it closes and energizes Relay K1. The normally open contacts of K1 close (30 & 87) powering both fans through resistor R1. I labeled R1 as .5 Ohm and that is approximate. Based on these motors being 12 VDC 15 Amp motors their DC resistance would be about .8 Ohm and two motors in parallel would be about .4 Ohm. Therefore if we use about a .5 Ohm resistance in series with the fans they will see a little less than 1/2 their rated voltage. Fan speed control using fans in series or resistance in series with fans is not an exacting science, as long as the fans can start rotation at the lower applied voltage. When and if the temperature reaches 210 F. the normally open contacts of K2 (30 & 87) will close and bypass the K1 closed contacts applying the full 12 Volt power to both fans. Matters not that K1 is closed. Finally as the Ultimate bypass SW1 simply applies full power to both fans.
Resistor R1 at .5 Ohm will obviously be a very high power resistor. What I suggest here is several turns of nichrome wire wrapped over a pencil and the pencil removed leaving an air core. It will look like the heating element in an electric hair dryer. Actually several years ago my own truck lost low speed on the blower. OK, no big deal I still have 3 speeds left. Then a few years ago I lost the next speed but lived with it. Finally a few weeks ago I lost the third speed leaving only the two highest remaining speeds. Had to keep turning the CD player volume up! My truck is an older GMC Yukon XL so I just replaced the resistor matrix for the fan speed control. Damn, that sucker was loud!Not much to the resistor matrix in those units. Just turns of nichrome wire used for three resistors.
The above drawing omits additional fusing and I didn't call out the wire gauges. The main wire for both fans should be AWG 10 since two fans running full power will draw 30 Amps. You need to give consideration to the wire gauges used. Anyway, if this isn't built yet you may want to consider the above method.
Ron
Yes. For instance, this one ( a "dual" opamp chip - one for each comparator. All other components also available at RS):
https://www.radioshack.com/product/index.jsp?productId=2062594
I'd also recommend this LED:
https://www.radioshack.com/product/index.jsp?productId=3096133&filterName=Type&filterValue=>7 mm lens
Correct: only enough to power the TL082 and the LED (<100ma).
No. One 12ga "line" for each fan AND IN THE GROUND LEG!!; I inadvertently misled you on its placement in my original post.
Like so:
View attachment 84570
No. Each "failure" circuit operates independently (hence the seperate sensor lines).
<EDIT> You will need a seperate, complete "failure" ckt for each fan.
That's the point of using the LM7805 (5 VDC regulator) so that variations on the 12 VDC line won't affect the operation of the comparator(s).
Here is a modified schematic to accommodate the TL082 dual OpAmp chip that Radio Shack sells (note that R3 has been changed to 270 ohms):
View attachment 84567
Probably the best way to arrange the 1 ft 12ga "resistor" is to make a coil (like a spring) of it.
Ron thanks for looking at it, and no I was about to attempt to build this setup in the next few hours. I went and got all my parts today. I appreciate the tip I wasn't looking forward to wiring that rats nest of relays! Where would you get the nichrome wire? I was trying to keep everything redundant as possible so no single point of failure existed and using your method I can eliminate a lot of them especially keeping both fans running parallel. I would need 1 more relay for a total # of 3 because my bank of switches which one would be used for the override switch mounted in my dash are only rated at 10 amps.
Ron, you've got a point with the fan noise and its effect on the comparator input (power filtering caps also - forgot to put those in. Thanks).
I'll sim the comparator ckt with some noise on that line and see what happens. Since, however, the ckt only trips "on" at a reduction to around 8 - 10 mV, I'm thinking the noise will have a negligible effect. We'll see.
CBB
Yes. And a salient point.View attachment 84588
I was just curious about it. Something else I am not getting is the shunts. Using the shunt to sense the current through the motors I am fine with. When the motors run on high speed all well and fine as each motor has its low side to ground and each shunt will produce a voltage proportional to the motor current. That voltage will be referenced to ground and the op amp comparator will work. However, and here is where I may be wrong, when the motors switch to low speed the motors are placed in series as I understand it. Now if I am correct this far things will look like the above less the relay contacts ect. Each motor will see about 6 volts less the preceding shunt drops. The first motor's shunt is fine referenced to ground. However, the shunt for the second motor low side is about 6 volts with the high side being 6.012 volts. Am I looking at this correctly?
Ron
Yes. And a salient point.
I hadn't considered that the half speed condition was the result of a series fan configuration.
What would happen (ideally) would be the lead (positive side) fan's shunt generates 6.01v, more than enough to keep the first comparator off. The downstream fan shunt would generate 12mv, which is still high enough to keep that comparator in the "off" state (anything >8mv).
Now, with the fans in series, if either or both fan motors fail, the entire circuit will cease to draw current. As a result, both comparators will go high, lighting up both LEDs, a pretty obvious failure indication. It would not, of course, indicate which fan failed.
If need be, the comparator circuits could be modified for a slightly lower trigger voltage by reducing R3.
Noise tests in SIM (12V [24v P-P] sine, triangle and square waves, sweeping 1 to 1kHz over 1s, injected at sensor input to comparator input) had no significant discernable effect on comparator function. Only the negative going transitions had marginal effect and were generally too low to trigger an LED response. I think that this largely due to the shunt(s) essentially shorting the noise to ground. This may not reflect real-world conditions.
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