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How do I knock 12 volts down to 9 volts?

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How do I knock 12 volts down to 9 volts?

Old 12-05-2008, 04:37 PM
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Default How do I knock 12 volts down to 9 volts?

I am working on my AC in my cabin. For the blower I am using a 12 volt DC bilge blower. Problem is that it pumps to much air. The other problem is that the one I have is the lowest CFM made in this particular style.

Just for an experiment I took the trigger out of my Dewalt cordless drill and put it inline. I was able to vary the speed nicely. But I really don't need a varible speed, I just need about 25-30% less fan speed.

The way I understand it is the way you control the speed of a DC motor is to vary the volts you give it. So is there a resistor or something I can put in the line to do that?

Thanks
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Old 12-05-2008, 04:43 PM
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Default Re: How do I knock 12 volts down to 9 volts?

If you don't move enough air you will "ice" the coil. Proceed carefully.
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Old 12-05-2008, 04:54 PM
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Default Re: How do I knock 12 volts down to 9 volts?

I know...That was the reason for the bilge blower, the original blower was not enough and was freezing over. Now this one is to much and sucks the air through before it has a chance to cool, and its loud at full speed. Knocking it down just a pit with the drill trigger was perfect.
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Old 12-05-2008, 04:55 PM
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Default Re: How do I knock 12 volts down to 9 volts?

And, in general, motors aren't happy about running on reduced voltage unless specifically designed to. Maybe you are ok with 9v, maybe not. Get the right size 12v motor/fan for your application.
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Old 12-05-2008, 04:58 PM
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Default Re: How do I knock 12 volts down to 9 volts?

Wouldn't a resitor do the job/
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Old 12-05-2008, 05:07 PM
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Default Re: How do I knock 12 volts down to 9 volts?

Add a control damper to the suction of the blower. It is better to starve the blower than to choke it. This will give you the ability to change air flow without changing voltage. Electric motors do not like lower voltages on an extended basis without an SCR drive and a properly sized dC motor.
MM
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Old 12-05-2008, 05:25 PM
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Default Re: How do I knock 12 volts down to 9 volts?

I think Mark has the right idea, but yes, your trigger is just a variable resistor.

However, how many CFM do you need? There is no shortage of cheap 12v fans out there. You can probably pick up 2-4 12v box fans (think computer fans) for $4-8 apiece and wire them together to get the airflow you need. Added bonus is that they will be much quieter and probably better sized for what you need. I use them in my aquariums, stereo rack, etc.

Back in the day whenever I would junk some piece of commerical computer gear I'd hold on to the cooling fans and any DC wall warts, never know when they'd come in handy.

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Old 12-05-2008, 05:30 PM
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Default Re: How do I knock 12 volts down to 9 volts?

I was thinking a resistor would do it but not sure how to choose one.

I tried restricting the suction and discharge but the motor startd vibrating and making noise when you restrict it.

I have a delema here.

This is the blower I am using. Its the 3" 105 CFM
http://www.westmarine.com/webapp/wcs...classNum=12111

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Old 12-05-2008, 05:32 PM
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Default Re: How do I knock 12 volts down to 9 volts?


Flot - 12/5/2008 8:25 PM

I think Mark has the right idea, but yes, your trigger is just a variable resistor.

However, how many CFM do you need? There is no shortage of cheap 12v fans out there. You can probably pick up 2-4 12v box fans (think computer fans) for $4-8 apiece and wire them together to get the airflow you need. Added bonus is that they will be much quieter and probably better sized for what you need. I use them in my aquariums, stereo rack, etc.

Back in the day whenever I would junk some piece of commerical computer gear I'd hold on to the cooling fans and any DC wall warts, never know when they'd come in handy.
It needs to be a 3" inline squirrel cage type blower to match up with the duct.
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Old 12-05-2008, 06:07 PM
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Default Re: How do I knock 12 volts down to 9 volts?

ScarabChris - 12/5/2008 8:32 PM
It needs to be a 3" inline squirrel cage type blower to match up with the duct.
Ahh, I see, I was thinking of placing them at the vent end and pulling air through the duct..

My suspicion is that bilge blowers are actually designed NOT to be quiet...

Actually now that I think about it - many american cars have used resistor packs to change fan speeds. In fact I had to swap out a couple in my last trucks. Maybe it's not so bad. But they generated a ridiculous amount of heat...
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Old 12-05-2008, 06:09 PM
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Default Re: How do I knock 12 volts down to 9 volts?

I just found this, seems like a pretty good option. Its cheap too incase it doesn't work out.

http://www.electronickits.com/kit/co...or/CKMX033.htm
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Old 12-05-2008, 06:21 PM
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Default Re: How do I knock 12 volts down to 9 volts?

Why not restrict the air flow at the blower? A piece of luanne can be trimmed to fit the intake and various size holes can be made until you get optimum flow.

With two pieces, you could even make it adjustable by cutting "pie slices" in them and sliding one over the other. Or Little hole over the big hole.

Good luck!
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Old 12-05-2008, 06:24 PM
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Default Re: How do I knock 12 volts down to 9 volts?

I tried restricting it already. When I did the motor was making noise and vibrating.
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Old 12-05-2008, 08:17 PM
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Default Re: How do I knock 12 volts down to 9 volts?

The motor speed controller looks pretty trick, but I don't know enough about 'em to tell you if it will work or not. In theory, DC motors are not fussy about speed like AC motors are. AC motors want to run at their rated RPMs or smoke follows. DC motors will run in response to whatever voltage is available, which is why golf carts and locomotives continue to run.

Speaking of golf carts, many of them use 8 volt batteries and people like to install 12 volt accessories. So, they hook a plate resistor to 16 volts and drop it to 12 volts. That's a drop of 25 percent, which is dead on what you are looking for.

Here's a start: http://www.lvssales.com/store/Golf_C..._Reducers.html
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Old 12-05-2008, 08:24 PM
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Default Re: How do I knock 12 volts down to 9 volts?

Yes a resistor will cut your voltage, but a resistor is going to get hot. A light bulb is really a special purpose resistor. You already know how hot light bulbs get. You will probably need to place the resistor where the air flow from the fan will keep it cool.

I wouldn't even worry about trying to figure out what R value you need. What I'd probably do is get a resistor pack for an automotive A/C blower and use that. It should have 2 or 3 individual resistors built into a self contained unit. Each resistor will give you a different output voltage and therefore a different fan speed. The fan speed selector switch on the dash of your car actually selects which resistor the current is "going thru" at each position of the switch. You could wire up the same circuit here. You could even use the switch from the car too, but be careful not to run the fan voltage thru that switch... you'll melt it in a heartbeat. Use the switch to control a relay. The correct relay will handle the current of the fan motor without melting. You can get all these parts from a auto parts store and they should be relatively cheap. I would probably go in and ask for those 3 parts for maybe, say a 1985 Chevy P/U or something like that.

If you don't want variable speed, leave the switch and relay out of the equation and just try each resistor on the "pack" till you find the one that works for you and just hook it up permanent.

If you want to figure out what value resistor you need, go here, there's a calculator. http://www.angelfire.com/pa/baconbacon/page2.html
You will need to know the current the fan draws in order to get the right resistor and that resistor will be a phyically large unit in order to handle the kind of current that fan is going to draw.... ain't gonna be all that easy to find a resistor like that at your local Radio Shack. That's why I said use the auto stuff. The math is already done, it's cheap and easy to get.
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Old 12-05-2008, 09:23 PM
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Default Re: How do I knock 12 volts down to 9 volts?

Good info B in B. I actually went to Pep Boys and tried to do exactly what you said. But the parts guy wasn't really interested in much blind searching. We looked at a few switches for a few different cars but came up empty. They all seemed to be more of a sensor to tell the computer what to do rather than a switch. I guess we didn't go back far enough. We were looking at Chevy's from the 90s.

I went ahead and ordered that speed control I posted a link for. For 30 bucks I'll give it a shot.
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Old 12-07-2008, 08:11 AM
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Default Re: How do I knock 12 volts down to 9 volts?

Chris, to drop 12v to 9v to run a blower, you will need a wirewound resistor of about 2.2 ohms, and at least 40 watts. The resistor will dissipate the power flowing through it (voltage drop across it), as heat. So, it will get warm. A larger wattage rating (of the same resistance value) will not get as hot, since it will be a physicall larger resistor.

So if you go this route, make sure it is in a ventilated area, and not in contact with anything that shouldn't get hot. Perhaps mount it on an aluminum plate to act as a heatsink.

Here's a link for some wirewound resistors (don't use a carbon resistor).

http://www.surplussales.com/resistors/res-wirewnd.html

And here's a link if you want to do further ohms law calculations the easy way:

http://ourworld.compuserve.com/homep...en/ohmslaw.htm
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Old 12-07-2008, 08:26 AM
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Default Re: How do I knock 12 volts down to 9 volts?

Get a blower resistor from a 1976 to 85 Ford Pickup, it will have 2 setting you can use. For high speed many are just pass through or bypassed to get full voltage. Many older automobiles use these and they are cheap. problem with the current vehicle will be computer controled fan. Try an Isuzu truck, Nissan truck, ford crown vic, many older gm vehicles. should be around 15.00 or less, Make sure they are installed where the air will pass over them to keep them cool>
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Old 12-07-2008, 09:31 AM
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Default Re: How do I knock 12 volts down to 9 volts?

Put a potentiometer (variable output dial switch) on it. DC motors don't care about voltage. It will be just as happy at 3v as it will at 14v. Using a pot, you can get the speed you want without guessing resistor sizes. As said, and you know, gotta haveapproriate airflow over the coil, else it will freeze.

I typed before reading this has already been suggested. An example of the switch being talking about is a Jeep CJ heater fan switch for. It has 3 speeds (non infnitly adjustable), but they are available everywhere--cheap. Check Ebay.
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Old 12-07-2008, 01:33 PM
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Default Re: How do I knock 12 volts down to 9 volts?

I'm not sure of a good option if you want to use that motor but an inline resistor or rheostat (variable resistor) would have to be very large and would potentially be a fire hazard. That motor draws 4.4 amps @ 12volts. If you put a resistor in line the current will drop, but to what? 3 amps? Power = I squared x R. If you use a 10 ohm resistor and it drops the current to 3 amps you are looking at 9 x 10 or 90 watts. That is a rather large resistor or potentiometer. If only a 5 ohm is necessary then you still have 45 watts. Most resistors in electronics are 1/4 watt or less, controls (remember those from the 80's) such as volume controls, bass, treble, balance etc are usually 5 watt.
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