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Fan Circuit Bug

Skandocious

Post Whores Make Me Sick
19,076
655
California
So I've discovered a small bug in my fan circuit. The way I've got it wired is with a 3 position switch, as follows:

~top = automatic; turn on low-speed when temp probe reaches xxx degrees OR if I turn on the A/C
~middle = force everything off
~bottom = force high speed on


So let me lay out the scenario for when the bug rears its ugly little head. Driving around on a hot day with enough wind to keep the engine cool without fan; no a/c on either, so the fan is NOT running at this point. Pull up to a really long stop light and the engine temp climbs a bit -- normal -- hits a certain temp and the low speed turns on. All is good at this point EXCEPT that when the fan kicked on, I notice that the RPMs had a slight dip and then climbed back up. That's strange... The fan shouldn't mess with the engine RPMs-- did my compressor just turn on? Suddenly I notice that my vents are starting to blow ice cold air even though I'm on the VENT setting... Ehhh... WHAT?!

So clearly my fan circuit is backfeeding power into my truck's A/C circuit and turning on my compressor and accumulator (I tapped the power wire at the accumulator). I've only got 1 wire tapped in my A/C circuit so I know just exactly WHERE the power is backfeeding but I'm just trying to figure out how to fix it. My current circuit (which I designed) is shown below:



newfancircuitwithlabels.png




Right now I've got the third (center) relay there so that when the A/C kicks on, it will bypass my temp switch and send power to the inductive circuit on the low-speed relay, which will close the switch and send +12V out to the low speed windings on the fan. But it appears that when my A/C is OFF, and the temp switch gets closed, it's sending +12V backwards through the switched circuit on the center relay, and the switched circuit is already closed because there is already +12V going through the inductive circuit.

So I've already figured why it's happened, clearly the circuit design is at fault. But I've been through SO many different designs and I just don't think there's any other way to do this exactly the way I want while maintaining proper circuit protection. Would it be best to just put a diode on the wire that is backfeeding into the center relay's switched circuit?
 
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Skandocious

Post Whores Make Me Sick
19,076
655
California
Just realized I should have been a bit more clear... My question is:

Is my circuit faulty and in need of being redesigned, or do I just need to throw a diode in and be done with it? I'm fairly sure that a diode would fix this problem and I'd never have issues again, but if the circuit is designed improperly then I'd like to fix it.

Also-- I know of the little tiny diodes you get at radio shack for using on bread-boards, etc... But can I buy automotive diodes that can be used with, say, crimp-on spade connectors?
 

Bob Ayers

North Carolina Chapter member
1,474
111
Durham, NC
The AC compressor relay is going to pick along with the low speed relay, so I would expect the AC compressor to run when the fan comes on low speed. The circuit needs to be
changed so whenever the AC compressor gets power, the fan will come on. Right now, the fan turning on low speed causes the AC compressor to get power.
 
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Skandocious

Post Whores Make Me Sick
19,076
655
California
The circuit needs to be
changed so whenever the AC compressor gets power, the fan will come on.
It runs like that already. The problem is that when the temp switch closes it backfeeds power into the A/C circuit.

Bob Ayers said:
Right now, the fan turning on low speed causes the AC compressor to get power.
I know that, lol. That's the point of this whole thread. The low speed relay is backfeeding 12v to the A/C circuit and turning the compressor on.
 

89frankenford

Grabber Green Consultant
4,547
147
NH
hmm this is a good question. looking at the diagram i believe you can get away with putting a diode in the circuit and calling it a day
 

Skandocious

Post Whores Make Me Sick
19,076
655
California
hmm this is a good question. looking at the diagram i believe you can get away with putting a diode in the circuit and calling it a day
Yeah it seems to me that there's really nothing wrong with the circuit except that it backfeeds across that 1 wire... A diode should fix the problem. I simulated it in my circuit simulator and nothing burned up...
 

89frankenford

Grabber Green Consultant
4,547
147
NH
bwahaha yeah thats what they did with the big dig in boston and hey look ceiling tiles fell and killed people because the glue wasnt good enough.....(it worked in theory)
 

Bob Ayers

North Carolina Chapter member
1,474
111
Durham, NC
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Skandocious

Post Whores Make Me Sick
19,076
655
California
Actually that makes a lot of sense... And it's so simple! 'smiliedoh' Except I'd want it on the low speed not the high speed. I originally had it wired to the high speed and it was overkill. The low speed pulls TONS of air; the high speed pulls a ton of juice and so I only like using it in case of emergencies or under heavy load, like when I was running high RPMs at low speed for long periods of time on the snow trip.

Leave it to me to over complicate everything :headbang:
 

Skandocious

Post Whores Make Me Sick
19,076
655
California
Nevermind I just found the flaw in your circuit Bob. Say the switch is set on manual High, and now +12V is being sent out to the high-speed windings in the fan. Now I turn on the A/C, this will trigger the low speed relay and ALSO send +12V to the low speed windings in the fan. BAD NEWS! This fan has separate windings and is not designed to get power to both at the same time. I'm now remembering that this is why I had to add a third relay-- to ensure that the first two relays were NEVER engaged at the same time.
 

Bob Ayers

North Carolina Chapter member
1,474
111
Durham, NC
Nevermind I just found the flaw in your circuit Bob. Say the switch is set on manual High, and now +12V is being sent out to the high-speed windings in the fan. Now I turn on the A/C, this will trigger the low speed relay and ALSO send +12V to the low speed windings in the fan. BAD NEWS! This fan has separate windings and is not designed to get power to both at the same time. I'm now remembering that this is why I had to add a third relay-- to ensure that the first two relays were NEVER engaged at the same time.


Connect the diode like I suggested!!!!!!! If the switch is set to manual HIGH, the relay for the low speed will never pick!! If the fan is working properly, the temp switch shouldn't close while the high speed is running.....
 

Skandocious

Post Whores Make Me Sick
19,076
655
California
Bob like I said, I don't want the A/C connected to the high speed on the fan. It just pulls too much current to be running for extended periods of time (I often run my A/C for several hours on long trips). So lets move on from that idea. Since I'd like to connect my A/C to the low speed, the best way would be to leave my current circuit intact but to insert a diode on the wire that is backfeeding electricity?
 

Bob Ayers

North Carolina Chapter member
1,474
111
Durham, NC
Bob like I said, I don't want the A/C connected to the high speed on the fan. It just pulls too much current to be running for extended periods of time (I often run my A/C for several hours on long trips). So lets move on from that idea. Since I'd like to connect my A/C to the low speed, the best way would be to leave my current circuit intact but to insert a diode on the wire that is backfeeding electricity?

The way your circuit is done, tha AC will not turn the fan on unless you have the switch set to give power to the low speed relay (through the temp sensor). This is not a good design, which I've previoulsy told you about!
 
I guess I am wondering why your switch doesn't just jump the thermostat...
One position for "ON", one to manually Kill the fan and center for normal ops? -use a "normally closed" relay to open the AC/low line when your hi speed kicks in?
I think, in my limited knowledge, you're making this harder than it is, Chris, but I've been wrong before.
 

Bob Ayers

North Carolina Chapter member
1,474
111
Durham, NC
ChrisFan.jpg


This is what it should look like!!!


Both low and high speed windings can be powered together with this circuit, if the manual switch is in the high speed position......
 

Skandocious

Post Whores Make Me Sick
19,076
655
California
The way your circuit is done, tha AC will not turn the fan on unless you have the switch set to give power to the low speed relay (through the temp sensor). This is not a good design, which I've previoulsy told you about!
This is how I'd like it designed. The center position on the switch forces the fan to be NO MATTER WHAT. That's how I want it. This thread is not about discussion how you'd like my fan to be wired, it's about fixing my bug and keeping my circuit operating the way that it is. So drop it!

mrfixit said:
I guess I am wondering why your switch doesn't just jump the thermostat...
One position for "ON", one to manually Kill the fan and center for normal ops? -use a "normally closed" relay to open the AC/low line when your hi speed kicks in?
I think, in my limited knowledge, you're making this harder than it is, Chris, but I've been wrong before.
I'm not sure I understand what you mean by all this...

Bob Ayers said:
Both low and high speed windings can be powered together with this circuit, if the manual switch is in the high speed position......
You are correct.
 

Bob Ayers

North Carolina Chapter member
1,474
111
Durham, NC
This is how I'd like it designed. The center position on the switch forces the fan to be NO MATTER WHAT. That's how I want it. This thread is not about discussion how you'd like my fan to be wired, it's about fixing my bug and keeping my circuit operating the way that it is. So drop it!

I'm not sure I understand what you mean by all this...

You are correct.

One other major problem your circuit has, the temp sensor will supply the current for the AC compressor clutch winding. It is not designed for that much current.
 

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