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How can I drop 12V to about 8.4V?

apples

Jul 1, 2012
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Hi,

I want to use my r/c electric plane motor and prop to make a fan that I can use to run off a car battery. The battery is 12V. The battery pack that is for the planes motor is 8.4V.

What is the best way to drop it from 12v to 8.4v?

I did read up about voltage regulators, but there is this thing called "vdrop" I think it was. That is like a dead spot in the voltage range where you need to have a minium of say a 2v gap between the two voltages otherwise it will not work.

So that is one way I can think of to bring the voltage down. The other way would be to get an arduino and set up a PWM circuit. I wonder how much buzzing there will be, and and also if that would run my electric motor or not.

Any ideas?

Peter
Australia
 

CocaCola

Apr 7, 2012
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A regulator is easy, you have the overhead voltage...

IMO forget about the Arduino it's like using a bazooka to shot a mosquito in this application...

Another way that I would likely do because it can be done real clean wrapped in some heat shrink is a series of 5 diodes like the 1N400x series... You get about .7 volts of drop per diode...
 

apples

Jul 1, 2012
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Okay then once the power is down to th 8.4v. How can I from that voltage go about having a knob I can turn to increase or decrease the fan speed?

Do you just hook a potentiometer to the line?
 

duke37

Jan 9, 2011
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Use a LM317T regulator with a variable resistor to control the output. You should be able to set the voltage anywhere between 1.3V and 10V.

There will be power loss so heat will be generated, you may need to heat sink the 317 if it gets hot to the touch.
 

apples

Jul 1, 2012
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Yeah ok, I can do that. Now what about the motor, what if it draws too many amps? How do we find out if that LM317T will handle the power?
 

BobK

Jan 5, 2010
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What is the current rating of the motor? If it is not marked, use an ammeter to determine how much current it draws from your 8.4V battery.

Bob
 

apples

Jul 1, 2012
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UPDATE:

Hi again. Well I have scrapped the idea of using the R/C plane motor at 8.4 volts. Instead I went to a car wreckers and picked up a good used car fan heater motor/blower from under the dash for the AC and heating.

Made up a quick housing for the fan, and man it really blows some air out hey. Throws as much air, than a normal cheap pedestal fan.

Now I have a motor to work with, I really need to slow it down to make it usable. I am not sure how many amps it draws. All I can say is that a similar looking motor on ebay out of a car that was 20 years old said 11amps and maybe I think was 300watts. So if that is 11amps continuous or that it what it draws at start up I don't know.

Maybe my motor will have similar specs that we can work with? btw, I do not have an ammeter.

So I need a pot, a regulator then what?

The pot will adjust the resistance on the leg of the regulator which in turn lets more or less voltage pass. yes?

Then obviously that regulator will have a current rating. yes? What if I can only find one that is good for say 5 amps but I might need one for say 15 amps. What do you do then?

Have include a pic of the blower.
 

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apples

Jul 1, 2012
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Well I just found out that my meter will read amps. But it blew the fuse, which is 10amps. So it is at least 10 amps then.

I also watched a video on youtube of a guy using a turnigy 130Aamp watt meter. It shows voltage watts and amerage on the lcd screen.

And you can get these servo test things that you plug in and twist a knob to adjust motor speed as well. I don't know how may amps they are good for though. This might be an easier/ cheaper route.

My local electronics shop sell dc motor speed contollers but are only good for 10 amps though, sigh.
 

rogerk8

Jul 28, 2011
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Love your simple suggestion!

A regulator is easy, you have the overhead voltage...

IMO forget about the Arduino it's like using a bazooka to shot a mosquito in this application...

Another way that I would likely do because it can be done real clean wrapped in some heat shrink is a series of 5 diodes like the 1N400x series... You get about .7 volts of drop per diode...

Just want to add that 1N400x voltage drop is closer to 1V at currents close to their limit (1A). But your suggestion is simple and brilliant!
 

apples

Jul 1, 2012
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Hi guys, I managed to find a 20 amp 12volt motor spped control kit that uses PWM.

I have assembled the kit and it is all working, but.

For some reason I can not get the motor to spin at maximum rpm, the adjustable pot will get to say 70-80% then the motor just stops. I think it must have to do with the frequency of the pulse train and how the motor works. So I hooked up a 12v 5w light bulb and it worked fine, I was able to go from 0volts up to my battery max of about 12.33volts and it worked fine. Yet the motor is funny.

Maybe I need to filter the output power. I think you use a low pass filter?
 

rogerk8

Jul 28, 2011
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Hi! You have a strange problem. My thoughts are these. You can run a lamp without any problem. This kind of load is mainly resistive. When your PWM puts out close to its maximum the negative pulse is very narrow (the ratio of ON to total is high). The DC of the output of a PWM is always the ratio of ON to ON+OFF. (or Ton to T). While it works with a simple lamp and not your motor I am thinking that the averaging output filter of the PWM is badly designed. I am however uncertain of how to solve this. Maybe incresing the inductance of the output filter might work (this should mean that narrow spikes will be taken care of). But I am really bad at inductor theories. Anyway, I think your problem is somewhere around the output LC-filter. Best regards, Roger
 

apples

Jul 1, 2012
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You know what, one of the little 10uf capacitors is / or has been squashed. So instead of being round along its length is it round at the top, nearly rectangle in the middle and round a the bottom, lol.

Maybe this might be something? Will a capacitor still function if it has been squashed?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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Generally speaking, physical damage is not a good thing.

It may be perfectly OK, it may be completely dead, it may be somewhere in between. It may also be about to fail, or fail worse.

If you can replace it, replace it.

Just make sure you're looking at damage as opposed to a funny manufactured shape.

It's also worth considering whether the deformation was caused by an external force, or some internal failure of the device. In the first case, try to protect it; in the second, check to see that it is the right component for the job.
 

apples

Jul 1, 2012
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No it was defiantly my doing as you can see that it was been squashed onto something with a square edge. Can't remember doing it myself, though there was a little helper rustling about when I was putting it all together.

I will go and spend $0.50 and get a new one and try the motor running issue again.
 

duke37

Jan 9, 2011
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The motor can have considerable inductance which could generate high voltages with a rough switched supply. Perhaps this is what affected your capacitor.
You could try:-
1. Putting a diode across the motor.
2. Putting a large capacitor across the motor.
3. Putting a resistor across the motor (wasting power).
 

apples

Jul 1, 2012
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The instructions do say to put on the supplied capacitor, but it is a very small capacitor though.
 

apples

Jul 1, 2012
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Well I replaced that capacitor and guess what I now get full speed. I get a loss of about .30 to .35 volts as compared to running directly off the battery.

Also the motor will not start spinning on it's own. Once I give it a flick it will though. What can I do to fix this?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Once it is spinning, is the torque of the motor lower than you expect?

Does the PWM controller get warm or hot?

If the motor is always operating from the same polarity (i.e. you don't reverse the polarity to run it in reverse) then I would advise placing a reverse biased diode across the motor. It needs to be rated for the full motor current and preferably be quite fast.
 
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