Second question: AC power to small DC motor

On the same project as my previous question, I also have a small, 12V DC motor that I am feeding with a rectifier bridge since the project is 120V AC powered. I know I need a resistor in the circuit, but I have had no luck determining what rating. Anyone have a suggestion? How could I calculate the needed resistor? The motor is wired up like a hairdryer motor. Thanks.

If it's a DC motor and you don't need to control it, there is no need for a resistor unless you want to monitor current flowing into the motor to estimate torque. As far as I undertand you only need a capacitor to filter the rectifier output, then the motor and that's it.

Maybe I didn't get your question...let me know.

It is a simple on - off, no other control.

I thought that just the rectifier bridge would work, but I tried that and fried the test motor and popped the circuit breaker for my workbench, so I figure I'm missing something.

Eugene:

In order to work with you, I need to know the plane of the circuit, of the motor characteristics, the type of rectifier if half wave, full wave, etc ... give you more information in good faith because anyone can give you a solution but maybe not the most appropriate

The motor is a 6V - 12V DC, puts out about 6 watts at 5.5 volts, using around 1.2 amps under load. I'm not using a rectifier, just simple diode bridge. It will drive off 120V AC.

Sorry, my rust is showing. Not sure what you mean by the plane of the circuit.

Thank you for giving me a hand here.

If you have 120VAC and you are trying to drive a 6 to 12vdc motor, then you have a number of options: (a) you can get your hands on a small AC-to-DC "adaptor" like those used for everything from laptop computors to small water pumps, to printers, etc. Just look for one where the label says it puts out a DC voltage in the range you can use. Be sure to test to "+" & "-" connections at the output. (b) you could also insert a film capacitor in series with your rectifier bridge to act as an AC impedence. The value of the cap will depend on how much current you are trying to put through the motor. Start with a 1.0 uf / 250+ Volt rated cap. The connection would be thus:the film cap in series with the AC input in series with the AC bridge which is in shunt (parallel) with the DC motor. If you do not have a cap or an ac/dc adaptor, then you can also try inserting an inductor in series with the AC as well. However, you will need a large inductor in order to have the same effect (impedence). You can try using stator sections from old inductance motors like those from old AC fans.

I am not satisfied the previous answer, sorry, the response is very empirical

Using just a resistor and diode in series with the motor is possible, but the resistor would be very high power and get very hot !

You say the motor is wired like a hairdryer motor. Does this mean it is in series with the rest of the circuit ? If so, it must take the same current as the rest of the circuit. It would be helpful if you provide a circuit diagram of your project.

Circuit attached for capacitive AC dropping and rectification. Start experimenting with low value capacitors, I guess about 0.05 microFarad, and work up in steps, checking the voltage you are getting across the motor.
(Sorry, my theory is rusty, please can someone calculate the values needed ?)

Also, keep your fingers of the 120 VAC wires when the circuit is on !

Use two capacitors, one in each supply line, to isolate the high voltage from anything you might touch.

Input capacitors:
Must be of sufficient voltage rating for the supply.
NOT electrolytic (unless bipolar) or tantalum.

Smoothing capacitor on output is optional.

Sorry, image didn't load - trying again.

Still not working - can anyone please offer advice on loading images with a reply ?

Until I find out how to load images: Feed the rectifier bridge from AC supply through a pair of capacitors to drop the extra volts.

(I think Angel might mean PLAN of circuit.)