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electronic motor speed control

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jared dunn

New Member
Basically i want to turn 6 volts and very little amperage into a large wattage to turn a motor in the 20-30,000 rpm range. for a model airplane if that helps. want to make a small can motor turn a prop at tens of thousands of revolutions. maybe a MOSFET? does not need to be fully proportional. maybe just an off-low-high settings. i have seen projects involving an old futaba servo and 500 ohms resistance and 2 or 3 mosfets. PLease HElp.
 

Hero999

Banned
It's impossible, you can't generate energy from nothing. Increasing the voltage always reduces the ampage, if this was possible then why don't you think people are powering cars from button cells or entire cities from 9V batteries.
 

gramo

New Member
You may require an additional battery pack, but if your looking at driving FET from lower voltages (not too sure about your application) maybe this diagram could help

**broken link removed**

Watch the circuit in action here

The drive circuit (NPN/PNP transistors) is only necessary for logic driven FET's that are switching large amounts of current.. If you not using a logic device to switch the motor on/off, then its not required
 

audioguru

Well-Known Member
Most Helpful Member
My city has a population more than 700,000 and it is powered by only three little alkaline button cells that are about 5 years old and are nearly ready for replacement.:D

I made an electric powered model airplane. It uses a 8.4V Ni-MH battery (seven AAA cells) that powers a geared motor. The motor spins pretty fast and the gears drive a pretty big prop at a slower speed but the torque is increased. I use a Mosfet and PWM to contol the speed. The max current is 8A.
 

ikalogic

Member
electronic motor speed control..hum.. isn't PWM the most obvious answer..

i've tried many ways to control the speed of DC motors.. however PWM is still the best for medium and small size motors..
 

jared dunn

New Member
My Hero,
but i dont want to make something from nothing. LARGE WATTAGE, small voltage. if perhaps you CAN help, feel free.
 

dknguyen

Well-Known Member
Most Helpful Member
Actually, you do want something from nothing.

Wattage = Voltage * Current

Wattage always stays the same (it's the conservation of energy). You can ever get more wattage than what you started with. From that equation, you can decrease current to increase voltage, or you can decrease voltage to increase current. You can never increase both because if you do, wattage will change.

It's like making cookies from a bunch of dough (power). The size of the cookies is like the voltage, and the number of cookies is like the current. You can either make a million tiny cookies from that same dough, or you can make one really big cookie. But you can't make a million really big cookies from that dough because you only have so much. The only way to make more larger cookies is to add more dough.

You are mistaking current and voltage for wattage and voltage. The only thing you can do is add on more batteries (power) at the start. Hooking the batteries in parallell will increase the power by increasing the current. Hooking up the batteries in series will increase the power by increasing the voltage.
 
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audioguru

Well-Known Member
Most Helpful Member
I wonder why his 6V battery has very little amperage. Maybe it is a dead battery.
 

jared dunn

New Member
i thank you all for your help and i guess my terminology leaves something to be desired but your right on track. i do need huge current and small voltage. i fi remember right wattage is a measure of energy expenditure. my bad. but thanks for all your help. i had an epiphany last night in which i realized i made a HUGE rookie mistake. resistance in series adds to the sum of the value of resistances. i accidentally hooked up one bank parralell and the other in series so i had biased my MOSFET ioncorrectly. i needed 500 ohms and instead got greater current handling but something like 100 ohms. no bueno. ANd mister GURU your idea with the op amps to make square waves and triangle waves to feed the signal to the transistor is good but i was trying to be as light weight as possible and save room as well ands of course cost efficient cause i crash my planes almost every time i go out to fly and it sucks losing my on board system everytime i cant find my plane.
 

audioguru

Well-Known Member
Most Helpful Member
Hi Jarad,
I have crashed my model airplane a few times too. It was flying perfectly then the ground came up and hit it!

The PWM motor speed control circuit I use is small and lightweight because a dual opamp has only 8 pins and a quad opamp has only 14 pins. Mine is regular size but a surface mount IC is much smaller and lighter.

Have you ever seen a model jet airplane with a real miniature jet turbine engine (very expensive)? It goes so fast that it flys by you and you hear it but don't see it. Like a bullet.
 

jared dunn

New Member
yea i get on youtube and check them out. pretty sweet. i tried to do it a few years agowith some stainless steel tube and sheet aluminum but i had the principle wrong and could get it to light but no thrust. very tedious procedure with me holding a butane lighter in one hand and the starter motor in the other.
 

audioguru

Well-Known Member
Most Helpful Member
A guy made a jet turbine from the turbo for a truck and mounted it on a small cart. It had plenty of thrust and was very noisy. When it ran out of fuel (propane) it produced a big fire ball that nearly set a tree on fire.
 
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