12v step down for fan motor - Fiberglass RV


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Old 04-27-2009, 08:59 PM   #1
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My boler came modified with a coleman forced air propane furnace. the previous owner never used it, and she owned it for 3 years. we've used it a few times, but the last time the fan seemed to get slower and slower, until finally the thermal overload killed the propane, and it shut down. upon a bench test of the fan motor, it seemed shot. took it apart and found one of the brushes to be badly worn, and the armature was also pretty shot. rv stores can't seem to get this motor, parts, or an equivalent, so i ended up at a surplus store, and found (what i believe to be) a chevrolet heater motor. the housing is the right size for the clamp, the shaft is perfect for the fan, presto problem solved. not.

the fan sounds like a jet engine, and i'm concerned the windows might shatter if they're all closed when i turn it on. (exaggeration, but you get the point). i want to step it down, to a) reduce the speed of the fan so the furnace isn't cooled to rapidly, and reduce draw on the battery.

finding such a device has proved difficult. some things i'm considering, but looking for input:

- a fan speed switch from a vehicle

- a dimmer switch from a house, or a ceiling fan speed switch (almost same thing), would this work with 12v?

any ideas?
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Old 04-28-2009, 12:39 AM   #2
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Quote:
My boler came modified with a coleman forced air propane furnace. the previous owner never used it, and she owned it for 3 years. we've used it a few times, but the last time the fan seemed to get slower and slower, until finally the thermal overload killed the propane, and it shut down. upon a bench test of the fan motor, it seemed shot. took it apart and found one of the brushes to be badly worn, and the armature was also pretty shot. rv stores can't seem to get this motor, parts, or an equivalent, so i ended up at a surplus store, and found (what i believe to be) a chevrolet heater motor. the housing is the right size for the clamp, the shaft is perfect for the fan, presto problem solved. not.

the fan sounds like a jet engine, and i'm concerned the windows might shatter if they're all closed when i turn it on. (exaggeration, but you get the point). i want to step it down, to a) reduce the speed of the fan so the furnace isn't cooled to rapidly, and reduce draw on the battery.

finding such a device has proved difficult. some things i'm considering, but looking for input:

- a fan speed switch from a vehicle

- a dimmer switch from a house, or a ceiling fan speed switch (almost same thing), would this work with 12v?

any ideas?
Are you trying to step down from 120v to 12v? (I'm guessing not) or do you want to have speed control running a 12v fan on a 12v system?

If you are trying to make your 12vDC fan run off your 120vAC you need a "step down" transformer rated for the amps the fan wants to pull through it. Your camper probably has a 120v>12v converter that can handle the job.

If you want 12v speed control I'd say you want components rated for 12vdc - there are more and less complicated ways of accomplishing what's required, but a surplus 12v heater fan control is a great idea - just double check that it can handle the amps for your particular fan. I'm guessing 18g wire ought to do for hookup.
Don't use the 120vac dimmer switch.

Caveat: I'm just a tinkerer, so take all of this with a dose of salt, but I think the heaterfan switch is an elegant, achievable, straightforward plan. It ought to be cheap and probably even comes with its own mounting tabs!

Cheers,
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Old 04-28-2009, 06:27 AM   #3
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yes I suppose i wasn't clear. I meant step down as reduce the voltage to step down the speed of the fan. It is a heater motor from a chevrolet vehicle, running on 12 volts DC. I am operating it with 12 volts DC. when it gets 12volts, it's running too fast, I want to slow it down. I went to my local surplus store yesterday looking for such a thing you are talking about, but to no avail.

on the bench, I ran it off a ac to dc power source, that has an output of 1 amp. it ran nice, and i think it was a close match to the speed / sound / airflow of the old motor...however when i put it on the battery it was getting all the amps the motor could draw, and was too much. Is there a way to limit the amps going to the fan, to say 1 amp?
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Old 04-28-2009, 06:58 AM   #4
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Sounds like the place for Ohm's Law. V=I*R. Or, I=V/R

There are others with more background than I, but to get one amp from 12 volts you'd need to put a 12 ohm resistor in the line. (You mentioned that you think 1 amp might be a practical upper limit.) I think you'd need something called a power resistor which are beefier to handle being inserted into the line voltage. Maybe a 25 watt resistor for some overdesign (At 1 amp you'd be using 12 watts). Mouser.com has several to choose from, all less than $10.

I don't know if an AC dimmer would work for DC current. Although it would be nice to fine tune the speed.
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Old 04-28-2009, 08:16 AM   #5
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Is there a way to limit the amps going to the fan, to say 1 amp?
I don't think so, Nate. The fan will pull what it pulls at the voltage you give it. You could try going to an auto wrecker for a 3 speed switch. It would perform the function Steve describes - well actually it would give three different resistances for three different speeds. I'd look first at S10s, 80's full size chevs and oldsmobiles (delta 88, parisienne, etc) trucks -- make doesn't matter so long as its easy to get the dash apart and there's lots of room in there to work. I'm sure you could order one at napa if you don't want to mess around. Hey, yeah, napa. let's have a look!

For an '85 Caprice, napa lists (on their US site):
[b]Item#: ECHHC320

[b]Price: $15.29
http://partimages.genpt.com/partimages/275108.jpg
http://partimages.genpt.com/partimages/488358.jpg
Note the rotary operation, face-out mount tangs, and accessible electrical connectors.

It'd be a simple matter of figuring out which lead to put the power in and which to ground.
I'm guessing this thing does "on/off" and then three voltages out and that may be what the tabs are about. A quick check on a spec sheet ought to tell.

If it was me I'd use these to hook it up:
http://www.ouros.co.uk/image/dproducts/econnect/5.jpg
then heatshrink over top.


For reference,
here's a unit from a Ford LTD:
[b]Item#: MPEHC301SB

[b]Price: $16.99
http://partimages.genpt.com/partimages/396241.jpg
http://partimages.genpt.com/partimages/501256.jpg

Note the "slidey switch" operation, perpendicular mount tang, and recessed leads - I'd stick with the chev part, but it'll depend on what you need.

I looked up a switch from a dodge K car and it was $45...

You can go into napaonline.com select your potential donor vehicle, and get a look at what you can expect. Try a bunch if you like. I was surfing around in Big 3 offerings circa 1985. Try whatever. Think about how you want to mount it so you can get to the control without bumping into it, and run with it.

You should also put a fuse in your design somewhere. I'd think in the hot lead before the switch would be good, that way if the switch or the motor fry's you're protected.

++++++FUSE+++++SWITCH++++++MOTOR
-------------------------[SWITCH?]--------MOTOR

you dig?
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Old 04-28-2009, 09:03 AM   #6
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AC dimmers do NOT work on DC current.

A 2 or 3 speed switch from a car is only part of the speed control. There are resistors to limit the current. The wiring is usually:


+12 Volts-----Fuse-----Switch(low)------Resistor-----Motor-----Ground

The last one I worked on was a 1990's Chevy Suburban (local school district). The resistors (Resistor wire - like in a toaster) were located in the heater duct work for cooling.

NOTE: The medium setting has less resistance and the high setting has no resistor (straight to the motor).

The resistor is going to get hot - POTENTIALLY SET STUFF ON FIRE HOT - The resistor(s) should be placed carefully.

I suggest using a couple of resistors used in automotive ignitions (before the electronic ignition) They are make to handle higher current and have a provision to be bolted to a metal surface. Wired as follows:

+12 Volts-----Fuse-----Switch(furnace controller)------Resistor-----Resistor-----Motor-----Ground

EDIT: The source of the resistor would be your local parts store or junkyard (breaker?). Ask for an ignition resistor for a 1970 Chevy pickup. I recommend a minimum of 2 resistors in series to limit the current flow (heating of the resistor).
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Old 04-28-2009, 11:36 AM   #7
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thanks for the responses, lots to think about.

tom and colleen, I dig, in fact i'm pickin up what you're puttin down, but i have to agree with russ and marian in that the automotive switches are switches only, and would not actually reduce the voltage, only re-rout the power to the different resistors....also the fact that the resistors generate so much heat is tricky, as there is the power consumption going to generate heat. I wish it was that easy, but this is my dilemma.

Russ and Marian, the ignition resistors intrigue me. i like that i could potentially keep adding them until i get the desired result with the fan. I think i know what you're talking about, but is there any way to post up a picture or a link for a visual reference?

I will be certain that everything is fused properly and installed with the proper connectors as well.
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Old 04-28-2009, 12:05 PM   #8
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Hi Nate,

Since you are getting mixed advice, I felt I needed to say something.

Yes, to what Russ & Marian said" An AC dimmer switch will NOT work in a DC circuit.

Steve summed up the answer best - it's just math.

And yes, the resistor will be inefficient, but there is no way around that unless you want to buy or design a voltage regulator, which will be somewhat inefficient. At least the inefficiency is in the form of heat, and heating your trailer is the idea.

I see your options as:

1) Purchase a high wattage resistor and mount it in a box (ie over design the wattage calulations)
2) Install a high wattage variable resister that could be mounted in a box.
3) Purchase a DC voltage regulator
4) Replace motor

Regards, Dean
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Old 04-28-2009, 04:55 PM   #9
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Try this link for a picture:

shop.oreillyauto.com/ProductDetail.aspx?MfrCode=NIE&MfrPartNumber=DR125 CS&PartType=205&PTSet=A

I looked up an ignition resistor (sometimes called a ballast resistor) of a 1965 Chevy C10 pickup with a 292 CID engine.

EDIT: Had to copy & paste this address

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Old 04-29-2009, 09:41 AM   #10
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Nate,

I understand the noise problem, I must caution you that reducing the air flow increases the temp of the heat exchanger, there is a temperature switch which monitors the heat exchanger temperature, (usually) when it hits a trigger point the switch will shut the gas valve off, protecting the system, the switch is not a "continuous duty" switch, its an emergency system, "intermittent duty" its cycle life is limited. If cycled to many times it will fail, hopefully in the open "burner shut off" position.

If this model uses a "sail switch" a required number of cfm's of air "cubic feet per minuet" is needed...it gets dicey if this is a draft induced model because a required amount of air must be supplied into the burner chamber itself, if to low it will boom on and off like the 1812 overture.

A more modern quieter fan of the same c.f.m. rating is a better solution or checking blower balance or bearing noise.

I did not catch the make/model of your heater...designs vary greatly, be sure what you have.

Coleman was made my the same people as Suburban, they were contracted to build it, some of the parts are still available.

Harry
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