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Old 06-15-2009, 11:51 PM   #1
ThePair
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Default Fridge DC switch question

I'm putting this out to all the experts on this board:

As many others have done, I installed a switch on the outside of the TM to allow me to turn off the power to the fridge when it's on battery, so that while towing, if we stop, I don't have to worry about the fridge draining the battery. One of these days I'll post a pic to show it, but it's not much different than what others have done--I cut the black 12V wire going to the fridge, attached some left over 10G wire and routed it out to a waterproof switch that I mounted on the underside. Works like a charm, tested with a voltmeter.

Here's my question. I once forgot to turn on the fan, and needed to pop the shell back open just to turn the fan on. This got me to thinking: If I splice in a little wire from the "downstream" part of my switch, and connect it to the hot lead of the fan, this would mean that any time my switch is on, the fan will be on, regardless of the interior setting (power tapped off the fridge 12V line). However, if I have the interior switch "on", then the fan will remain on regardless of the exterior switch (power from the normal line). This, I'd think, would be beneficial for a few reasons--still have interior control, if, say, I wanted to try to help the fridge on a hot day, but could still activate the fan from outside if I forgot to turn it on again. Or, if I store the TM in the garage with the fridge set to DC and fan off, I can turn both on 24-48 hours before a trip, to allow the fridge time to cool off without needing to pull the TM out. Or, leave the fridge on AC but unplugged, and then plug in, flip switch so that the fan is on, but with AC power for a faster cool-off.

Anyway, my question is (1) is this feasible, and (2) are there any unforeseen issues? Would tapping the fan's power off the same lead as the 12V to the fridge cause an issue with either? Would having both switches on (power coming from two sources) be an issue?

I'm no electrical engineer, so I put out a plea to those with far more experience than I.

Thanks!

Marc
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Old 06-16-2009, 12:12 AM   #2
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IIRC, on my TM, the fridge and fan share a common hot lead. I wired my switch such that it controls both the fridge and fan, regardless of whether or not the interior fan switch is on or off. That interior switch now serves no function.

The way I look at it, anytime the fridge is on DC, which is when the exterior switch will be on, the fan must be on. Wiring in this way allows me to operate both appliances from the outside, including when the TM is closed.

The switch located on the fridge itself is not bypassed; it is still fully functional. Thus, if I want to use the fan on a hot day when the TM is up and operating on AC or gas, I just go outside and turn the new switch on. That turns on the fan. The only other thing turning that switch on does is feed DC to the fridge; that's not any concern because there was DC power going to the fridge 24/7 before I installed the switch.

I can take a closer look tomorrow if you'd like and be more specific with how I wired it.

Dave
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Old 06-16-2009, 06:37 AM   #3
ThePair
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I could have tied the hot lead into both fan and fridge through that switch, I suppose. That much is pretty straightforward, I think, just by spicing the hot fan lead into the feed off of the switch.

Judging from your experience, then, there is no issue pulling the power for the fan off the fridge hot lead? The additional (albeit tiny) load doesn't detract from the fridge function? That's good to know.

I guess what I'm suggesting would take it one step further. As you point out, I could always turn on the fan from outside if I tied it together like you have. My thinking is that interior override may be desirable.

One downside of what I proposed above is that the fridge DC will always be on if the fan is on inside, unless I put in a diode (I think). Would a diode work to prevent reverse power to the fridge--I would think so.

I guess my next question, to you specifically, is, have you had trouble shutting off the fridge & fan when closed (like at a rest stop). Since the fan's no longer venting, I'd imagine a heat build-up...but if the fridge is off, then there's no additional heat being made. I thought having the fan run even with the DC element off sounded like the best way to minimize heat build-up and maximize the function of the fridge on the road, but maybe I'm overthinking this?
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Old 06-16-2009, 10:20 AM   #4
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Quote:
Originally Posted by ThePair View Post
I could have tied the hot lead into both fan and fridge through that switch, I suppose. That much is pretty straightforward, I think, just by spicing the hot fan lead into the feed off of the switch.
Correct. That is what I did.

Quote:
Originally Posted by ThePair View Post
Judging from your experience, then, there is no issue pulling the power for the fan off the fridge hot lead? The additional (albeit tiny) load doesn't detract from the fridge function?
Correct. The fan does not draw much power....maybe 0.5A or so, compared to the fridge drawing 10A. I have not noticed any adverse effect on the fridge.

Quote:
Originally Posted by ThePair View Post
I guess what I'm suggesting would take it one step further. As you point out, I could always turn on the fan from outside if I tied it together like you have. My thinking is that interior override may be desirable.
Why? The only time I could think this would be desirable is if you are camping already, and already inside, and for some reason you decide the fan needs to be on when it's raining. But the fan can wait till the rain passes.

Quote:
Originally Posted by ThePair View Post
One downside of what I proposed above is that the fridge DC will always be on if the fan is on inside, unless I put in a diode (I think). Would a diode work to prevent reverse power to the fridge--I would think so.
Why is that a downside? The DC at the fridge was ALWAYS on prior to you installing the switch. With the new switch in the on position, you're simply at the same electrical state before you installed the switch.

Quote:
Originally Posted by ThePair View Post
I guess my next question, to you specifically, is, have you had trouble shutting off the fridge & fan when closed (like at a rest stop).
No. The fridge and fan turn on and off with the exterior switch just as easily closed vs. when open. That's precisely the reason I installed the switch -- to conserve battery power when turning off the TV, at gas stations, restaurants, while setting up at camp, etc.

Quote:
Originally Posted by ThePair View Post
Since the fan's no longer venting, I'd imagine a heat build-up...but if the fridge is off, then there's no additional heat being made. I thought having the fan run even with the DC element off sounded like the best way to minimize heat build-up and maximize the function of the fridge on the road, but maybe I'm overthinking this?
I'm not sure what you mean. The fan is venting -- it vents downward, pulling hot air from the fridge down through the hose and out the bottom of the trailer, not vice versa. I'm sure there is some additional heat generated behind the fridge when the TM is closed, but I have not had a problem. I think probably the only adverse effect from inadequate cooling of that area would be a warmer fridge, since the absorptive capacity of the coils is less (because they are warm). Thus, I have a wireless thermometer that measures the fridge and freezer temps, with the monitor being in the TV. Sometimes I can get a signal in the TV, but I can always get a signal if I just put the monitor next to where the fridge is. That's what I do when I stop for gas. The freezer actually is where all the cooling takes place. It will cool first, and warm up last.

Dave
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Old 06-16-2009, 11:06 AM   #5
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I guess my last two questions were the same question, poorly worded. When I said "trouble shutting off" I meant, were there any issues with having the fridge and fan shut off together, rather than leaving the fan running. Right now, I turn off the fridge and leave the fan running, which will continue to vent any residual heat even with the fridge off. In your setup, the fan is off when the fridge is off, meaning any (theoretical) residual heat is not being vented out. But, it sounds like that hasn't been any kind of issue for you. I would think, if the contents are cold and the door stays closed, that the fridge could stay cold for quite some time without power, perhaps a little longer with the fan on, maybe.

Hmm. If I do go this route, I could use the extra (now unused) fan switch for the radio. Any tricks on getting that out to get to the wiring, or is it model specific?
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Old 06-16-2009, 11:24 AM   #6
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Hmm....I think I understand what you're saying. When I pull into the gas station or restaurant and flip the switch to turn off the fridge AND fan, you're saying there is an advantage to having the fan continue to run to cool the fridge for the 15-60 minutes we're stopped. I agree -- that would be better, but I'm not sure if it would make a difference or not. If you wanted to do that, you could just install a second exterior switch for the fan. That's cheap and easy to do.

The interior fan switch should be easy to remove from the panel. There are probably 2 plastic tabs on the switch behind the panel that, when pressed in, should allow the switch to pull right out. I think TM installed different control panels in the newer models like yours, which I have not seen in person, but from the pictures I've seen, it looks like the switches are probably mounted the same way. Stick a mirror under the sink, and you'll find out. With the SL models, you certainly can't stick your head under there!

And I bet the wiring for the radio is right there behind the fridge, easily accessible from outside and right next to the wiring that leads to that switch. Good thinking!

Dave
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Old 06-16-2009, 11:52 AM   #7
ThePair
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Heh. The hardest part would be figuring out which wire is radio power... I could pull out the radio, I figure, I just wonder how hard that is to do, too.

As to the fan, another exterior switch = another box. I had a hard enough time finding a good place for the one, I think I'll try to find another solution. If a diode will allow the fridge to turn off while the fan stays on (from the interior switch) that would be simplest, because the two wires in question are already right next to each other. Isn't that what diodes are meant to do? Allow current in only one direction, right? It would be a minute's job to splice in a diode and be done.

Otherwise, I may just wire it like you did, and stop worrying so much about it!
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Old 06-16-2009, 12:15 PM   #8
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Yes, that is what diodes do, but it's not obvious to me how you're suggesting you use one to accomplish your goal. Bill is an EE, so perhaps he will chime in here. Otherwise, perhaps you can draw a schematic.

An easy way to determine which of those wires leads to the radio is to pull all the fuses to the DC circuits except the radio. According to the wiring diagram in the TM blue binder, the radio is the only appliance on that circuit. Turn the radio on, and turn the volume up to a decent listening level.....might as well enjoy some tunes while you're working. Using a clamp-on DC ammeter, check each of the wires back there for current. The only wires that should have current going through them are the ones that supply the radio.

Sears sells such a meter (which is also a multimeter) for about $50, which is a small fraction of what all the others ones I've seen out there that measure DC current. It also comes with a thermocouple, which is cool, although I haven't used it yet. Thanks to Wayne / harveyrv for pointing it out to me.

Dave
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Old 06-16-2009, 03:00 PM   #9
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Wow, off to Sears to get that meter. Thanks for the tip. DC ammeter = happy troubleshooting.
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Old 06-16-2009, 04:27 PM   #10
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Default Double pole switch

How about using a double pole switch. They are like 2 switches wiyh one lever. Run the fridge thru one and the fan thru the other. That way nothing can back feed and when the outside switch is off, you can still control the fan from inside!
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