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Old 07-25-2011, 11:41 AM   #1
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AC power to vent fan

So it is hot here in the summer, and we want to have the thermostat controlled fan vent run as designed to help keep the inside of the coach from damage.

It is an older coach, the converter is auto-transformer style, so it operates in full saturation to maintain voltage, so in addition to operating about 1/2 to 1 volt too high that would wipe out the batteries (last owner did) and it draws 5 amps unloaded so the power bill is too much as well.

So we did not want to change converter, works well for when needed, batteryminder keeps batteries up but the fan does not play well with the batteryminder, and the system just does not work right.

We leave the converter off when stored.

So, we went mining in the shop and came up with a solution, works for us, and is easy to do.

It uses the existing fan and controller, but the controller MUST be mechanical in nature, no "active" parts.

We used a laptop computer supply rated at 13 volts@5 amps, and a 7AH Gel battery for supplemental power along with a floater to maintain the aux battery.

Ours was an old "3Ts", forward/reverse and 3 speeds.

The schematic below is straight forward, the existing fan control is modified, the existing thermostat wires are connected together so it is always on and the wires that went to the power switch are also connected together so it also is always on.

Make sure everything is properly fused.

Next a new wire is brought in and connected to the power switch, (this wire goes to fused 12 volts) then from the other side of the power switch it goes to the old thermostat, then from the thermostat it comes out and to the new part. as "control".

We used relays as they are cheap and work well.

The "control wire supplies power to relays K1, K2 and K3.

K1 switches 12 volts to supply the fan via K4.

K2 switches AC power to a 12 volt power supply, make sure relay can handle the voltage, and be careful with the wiring, we used standard computer cord and broke one leg and used insulated lugs, laptop ones work great, you can buy a prewired female connector so it is plug in, then easy to change later.

K3 connects the aux battery to the fan, this assists the laptop power supply, it allows the fan to start running before the power supply gets running.

K4 is powered by the 12 volt power supply.

K4 common goes to the fan, NC goes to K1 output while NO goes to the power supply output.

The gel battery has a separate float charger to keep it topped off (if plugged in)

The output of these relays goes to the original connection for input to the fan control, the switches inside except for the power switch and thermostat are unchanged, so the speed and direction still work.

What happens while UNPLUGGED

When the power switch is "ON" the power is fed to the thermostat, if it is also "ON" then power is fed to the "Control" line activating K1, K2 and K3.

Since there is no AC then the 12 volts flows from K1 "C" to "NO" on to K4 "NC" to "C" then out to the fan control, K3 also connects the aux battery to the fan wire.

Fan speed and direction are controlled in the existing fan control.

When ac power is present the additional things happen.
AC power is fed to the 12 volt power supply.

Output from the power supply activates K4.

Fan power is now connected to the 12 volt power supply via "C" and "NO" connections, so the fan is operating on ac power with supplemental battery.

All of this fit in the space behind our fan control, sorry no photos, but 4 relays and a battery is not that exciting.

Our main batteries are not drained while parked in the driveway, and we are only using the ac power when needed.

Total cost would be less than $50.00 if you shopped well.
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Tony & Lori
1989 Country Coach Savannah SE
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Old 07-25-2011, 03:53 PM   #2
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Seems like a lot of work to achieve what a cheap battery charger from Walmart could do for you. Or do you just enjoy the challenge of cobbling something together?
Gary Brinck
Former owner of 2004 American Tradition and several other RVs
Home is West Palm Beach, FL
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Old 07-26-2011, 09:13 AM   #3
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Battery is important

Yes, we could have a el-cheapo charger to support the fan via the battery, but most of those are poorly regulated, if they are large enough to maintain a dual 8D battery they are large enough to over charge the battery unless you use a higher dollar model.

We also did not want to have another loose item to maintain.

An AC circuit was added to suport batteryminders for each of the 3 battery systems, they are installed as "factory", so the batteries are well cared for, having an additional charger connected to the battery would not be a good solution.

The goal was to have the vent fan run from shore power if available, be fully automatic and have little connection to the coach battery unless no shore power present.

It was fun to come up with the idea, then create the solution, finally to install it is such a manner as to be invisible, other than a fuse holder and tip jacks inside a cabinet for protection and monitoring everything looks as it did when it left the factory.

We posted it here as there may be others who would like to do the same, and remove the additional cost of constant running of their converter and possible advanced aging of their batteries due to a converter that may be overcharging them.
Tony & Lori
1989 Country Coach Savannah SE
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Old 07-26-2011, 12:48 PM   #4
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Tony, how about running the fan only directly from a small battery charger. I have been running an electric automotive fuel pump (12V) in my shop for the purpose of re-circulating cleaning solvent at my self made parts cleaning station, for years with no problem.

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Old 07-26-2011, 01:22 PM   #5
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Exactly...sort of...

The fan is being operated by a seperate power supply.

Basically the same thing as running from a battery charger, but a little bit more elegant.

The computer power supply is switch mode, so small, cool and easy to find, the aux battery helps out, and the control system makes it work with automatic switching from ac power to original battery operation without any action.

The original thought was to simply add a charger to the battery, but then the battery would be impacted, so then a charger or supply just for the fan or adding a small fan on a seperate power supply in the same space as there was plenty of room, but wiring that would be a major pain.

So after looking around in the shop to see what we had to work with the plan that is in the OP was scratched on some paper, then built on the bench and installed.
Tony & Lori
1989 Country Coach Savannah SE
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Old 07-28-2011, 08:40 PM   #6
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Solar powered fan ?
seems like the logical solution...
Rob. 2005 E350 w/ V10
2007 KZ Sportsman 2604
2002 Honda XR650L
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fan, power, vent

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