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Author Topic: Solar kiln, 12v fan wiring thoughts?  (Read 1396 times)

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Offline Delawhere Jack

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Solar kiln, 12v fan wiring thoughts?
« on: November 22, 2016, 05:15:21 PM »
Nearing completion of the portable kiln, and working out the wiring details for the fans. I picked up a CATV Utility box to mount on the outside which will contain all of the controls. A 12v - 400w transformer will provide power. I will install a DPDT switch to allow reversing the fans. This may be useful while loading the kiln since the fans would then draw hot air from the collector and blow it out the top vents. From this switch each fan would be powered through separate fuses. The fans each draw 3.7A max, so I'm figuring 5A fuses(?). I'd also like to install a rotary potentiometer to vary fan speed. Hoping to use only 1 to control all four fans. Would a dimmer switch used for household lighting work for a 12VDC circuit? (I doubt it would, but figured I'd ask). Had considered also adding an on/off switch for each fan individually, but I don't think that will be necessary.

Any thoughts or suggestions would be welcome.

Edit: The fans would be wired in parallel and individually fused.

Thanks,
John

Offline AnthonyW

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Re: Solar kiln, 12v fan wiring thoughts?
« Reply #1 on: November 25, 2016, 09:20:36 AM »
Lots of questions:

What kind of fans are you using? 12V DC or AC?

What is the 12V transformer for? Transformers only work on AC.

Varying the fan speed of DC fans using a potentiometer usually works by dissipating the excess voltage as heat (ie., very inefficient way of varying the supply voltage).

Have you tested the 12VDC fans to run on voltages less than 12V? There may be a limited voltage range (small) on which they will run if they will run on anything less than 12VDC.

If the fans run on less than 12V, I would install different power supplies and a selector switch to vary the fan speeds.
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Offline Den Socling

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Re: Solar kiln, 12v fan wiring thoughts?
« Reply #2 on: November 25, 2016, 10:50:31 AM »
I thought everything sounded good until I got to the dimmer switch. Now days they are - what you would call 15 years ago - solid state. I would look at the rating on the package before I bought one.

Offline Delawhere Jack

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Re: Solar kiln, 12v fan wiring thoughts?
« Reply #3 on: November 25, 2016, 08:06:38 PM »
Anthony. The fans are DC. I have not tried them at lower voltages. Perhaps I meant rheostat rather than potentiometer. What I'm calling a transformer, I suppose has a rectifier built in. 120VAC in - 12VDC out. I guess you'd call it a 12VDC power supply.

Den. The dimmers I was looking at were rated 120VAC - 600W.

Offline GeneWengert-WoodDoc

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Re: Solar kiln, 12v fan wiring thoughts?
« Reply #4 on: November 26, 2016, 07:28:42 AM »
Fans take power to move air.  The power is voltage x current.  So with 12 v. To get the same power as you get at 120 v, you need 10 times the amperage.  With DC carrying high amperage, your wires will get hot.  D.C. Motors will be larger to get the same power as an AC, and 220 v AC would be even smaller.  So, with 120 available, why not use that without going to DC?  A rheostat also generates a lot of heat.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline Delawhere Jack

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Re: Solar kiln, 12v fan wiring thoughts?
« Reply #5 on: November 27, 2016, 04:33:53 PM »
Fans take power to move air.  The power is voltage x current.  So with 12 v. To get the same power as you get at 120 v, you need 10 times the amperage.  With DC carrying high amperage, your wires will get hot.  D.C. Motors will be larger to get the same power as an AC, and 220 v AC would be even smaller.  So, with 120 available, why not use that without going to DC?  A rheostat also generates a lot of heat.

Why not 120V? Short answer, cost. Picked up 4 automotive cooling fans for about $50.

Offline Den Socling

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Re: Solar kiln, 12v fan wiring thoughts?
« Reply #6 on: November 27, 2016, 06:56:08 PM »
I'm with Gene. Just use 120 volt fans. Holy heck! How much is that wood you are putting in there worth? $50?

Offline AnthonyW

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Re: Solar kiln, 12v fan wiring thoughts?
« Reply #7 on: November 27, 2016, 08:03:20 PM »
Anthony. The fans are DC. I have not tried them at lower voltages. Perhaps I meant rheostat rather than potentiometer. What I'm calling a transformer, I suppose has a rectifier built in. 120VAC in - 12VDC out. I guess you'd call it a 12VDC power supply.

Den. The dimmers I was looking at were rated 120VAC - 600W.

There isn't much difference between a rheostat and a potentiometer either way they are a variable resistor, one has two terminals and the other with three.

Reducing the inlet voltage to a 120VAC-12VDC power supply will not reduce the output voltage (12VDC). Power supplies are designed to turn on/off internally based on designed thresholds.

The fans in my cars both require a 15A fuse. So you will need 15A per fan, or a 180W 12VDC power supply per fan.

Initially, I like the idea of using the automotive fans since they are inherently designed to run at high temps. I don't know what the CFM rating is for an auto fan. The ones I have replaced in the passed are larger (in diameter) than I would be looking to use. But I'm not looking to build a very large kiln.
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Offline GeneWengert-WoodDoc

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Re: Solar kiln, 12v fan wiring thoughts?
« Reply #8 on: November 28, 2016, 07:12:46 AM »
How much amperage does you power supply use?  Kiln fans run 24 hours a day when the kiln is on, so the power supply will run the same.

If it is 8 amps, then 8 amps x 120 volts is 0.96 kW x 24 hours x 12 cents per kWH is about $2.50 per day x 30 days per months.  This starts to get expensive, compared to a smaller 120 v fan that gives the same amount of air flow, that might cost more initially but runs for around 33% less money for energy...saves $50 per month.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline Delawhere Jack

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Re: Solar kiln, 12v fan wiring thoughts?
« Reply #9 on: November 29, 2016, 06:49:46 PM »
You all have made some convincing points. Sadly I'm in pretty deep now on the 12V route. Guess I run what I've got until it fails and then convert to 120V.

A big part of my thinking was that the auto fans are built to withstand high temps.

The fans (4) are rated at 80W - 6.6A draw, 1700 CFM ea. The power supply is rated at 480W output.

Thanks for all the input guys.


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