I want to simply control the speed of a 12v dc motor with the aid of some high wattage resistors.

My voltage supply is 12v, the total current draw with resistors in series is 2.8amp.

Now according to ohms law P = VI = 12 x 2.8 = 33.6watts.

According to this R = V/I = 12/2.8 = 4.2ohm

Total circuit resistance is 26.5ohm 24ohm + motor resistance.

So if P also = V X V/R = 12 x 12/ 26.5 = 5.43watts

if P = I x I x R = 2.8 x 2.8 x 26.5 = 207.76

I'm confused.

I need to know what wattage resistor to use?

 

Your formulas are OK (although they're not all ohms law). Your numbers are wrong.

Total circuit resistance = 26.5Ω

Current at 12V - using ohms law = 12/26.5 = 0.45A

Power dissipated - using someone else's law = 12 x 12 / 26.5 = 5.43W
In the voltage dropper resistor P = I x I x R = 0.45 x 0.45 x 24 = 4.86W

Actually, putting a dropper resistor is not a very good way of controlling the speed of a DC motor. Best to regulate the supply voltage.

 

Actually the best way is PWM.

Pulse Width Modulation

LEDs, 555s, Flashers, and Light Chasers, Chapter 5 - The 555 and PWM

The 555 Projects, 555 PWM Oscillator

 

OK I see what you are saying with I = V 12V / R 26.5ohm = 0.45A

Why I am confused is that my meter on the 10A range reads 2.8A

I am using a 24V 100Watt scooter motor.

The motor drives a spinner which distributes seeds on a small seeder we have.
Some seeds crack if the speed is too high. I am just using a KISS principle.
If a resistor burns out I can quickly replace it. If I go to PWM troubleshooting takes time.

If my current is 2.8A and I wanted say 1/2 speed, would R = 6v/2.8A = say 2ohm. And perhaps P = VxI 6 x 2.8 20 or 25Watt 2ohm resistor.

Thanks

 

You say its a 12 motor in your first post and then that its a 24 volt motor in the next post.

??

How about you try a 6 volt battery...that should slow it down.

 

Sorry the supply is 12v but the circuit I am testing is using a 24volt 100watt scooter motor but I have only supplied 12v

 

How about you try a 6 volt battery...that should slow it down.

Well? How about it? Would be a simple thing to try, would it not?

 

Your resistor will get incredibly hot. This heat is pure waste, the batteries have to provide the energy for that heat. The heat itself can be a problem, I've seen resistors glow, and become a real fire hazard if they get hot enough.

PWM doesn't have this problem, there isn't the heat associated with it, and the batteries will last longer, if the problem is big enough much longer.

 

The OP may be a farmer in difficult circumstances. Sophisticated electronics might not be an option for him.

Perhaps he could make up a dropper by using 12V vehicle light bulbs? He would of course still need to be careful about the heat, but at least the parts might be easily available, for instance from scrapped cars.

 

How about using an LM350 adjustable regulator?

http://www.national.com/ds/LM/LM150.pdf

Might lose 3v at the top end, adjustable down to 1.2v