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The fan has a 4 position switch, for Off, and speeds 1, 2, and 3. I wanted the same 3 speeds for the 12v conversion. There are several ways to control a DC motors speed, and I decided to use one of the simplest, series resistors. I used a couple of car ballast resistors. Each resistor has a resistance of 1.5 ohms and can handle 50 watts. The circuit below shows how I connected them up. The result was spot on, the slowest speed was a gentle breeze, next up was about half full speed, and full speed was pretty strong!
Here you can see where I mounted the resistors. 12v is a safe voltage so I don't need to cover the connecters, but I will add a cover in the future to protect the resistors againsts bumps and knocks.
The finished fan up and running. I'm very happy with this project, it was a easy build with good results. I can also use the fan as a dump load.
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Efficiency.
While series resistors is a easy way to control a DC motor in this application, its not the most efficient. Some power is lost as heat in the resistors. We can use ohms law and V*I=W to work out how much is wasted as heat. Using a amp meter I took the following readings.
| Speed | Fan Current |
| Min Speed | 1.9A |
| Medium Speed | 2.7A |
| Max Speed | 4.2A |
We can use I2R to get watts dissipated from each resistor. Total power drawn from the battery is worked out with Watts=Volts X Amps.
| Speed | Current | R1 1.5r | R2 1.5r | Power wasted as heat |
Power drawn from battery |
Power used to drive fan |
Efficiency % |
| Min Speed | 1.9A | 5.4w | 5.4w | 10.8w | 22.8w | 12w | 52% |
| Medium Speed | 2.7A | Not Used | 10.9w | 10.9w | 32.4w | 21.5w | 66% |
| Max Speed | 4.2A | Not Used | Not Used | None | 50.4w | 50.4w | 100% |
You can see on minimum speed, almost half the power is wasted as heat in the resistor. The total draw from the battery is 22.8w, 10.8w is wasted as heat and the remaining 12w is used by the fan. On medium speed, about 1/3 of the power is wasted as heat. At full speed, all power is sent to the motor, none wasted.
We are still using less battery power than using the fan at a higher speed, but if you need to save watts, you might want to consider a more efficient way to control the motor power.
PWM ( Pulse Width Modulation ) controllers have a very high efficiency compared to a series resistor controller. No power is wasted in a series resistor, instead, the motor is switched on and off very quickly. This is a picture of the 24 volt motor controller that came with the scooter motor. These controllers have a low battery cut out, so if the scooters battery voltage drops below approx 22 volts, the controller will switch off. As my fan was to run from a 12v supply, this controller wouldn't work. If I had a 24 volt supply, then it would be OK to use, but I haven't tested this. |
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In Australia Jaycar Electronics sell a 24v 10 amps PWM controller for about $25.00. It has a soft start and can be upgraded to 20 amps with an additional MOSFET. Its a good design, I've use one in the past to control 2kw motor ( using 6 mosfets ) |
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If your handy with a soldering iron you can build your own simple PWM controller based on a NE555 chip. Searching Google will find many examples of home made controllers, like this one..... |
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