# New to OHMs Law

Discussion in 'General Electronics Chat' started by MJB, Mar 10, 2011.

1. ### MJB Thread Starter New Member

Mar 10, 2011
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0
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?

2. ### JDT Well-Known Member

Feb 12, 2009
658
85
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.

Mar 24, 2008
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4. ### MJB Thread Starter New Member

Mar 10, 2011
3
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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

5. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,789
945
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.

6. ### MJB Thread Starter New Member

Mar 10, 2011
3
0
Sorry the supply is 12v but the circuit I am testing is using a 24volt 100watt scooter motor but I have only supplied 12v

7. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,789
945
Well? How about it? Would be a simple thing to try, would it not?

8. ### Wendy Moderator

Mar 24, 2008
20,766
2,536
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.