I am making a small elevator for my daughter and the motor I am going to use for it is a 1.5-3V motor. I am trying to figure out how i can use an old cell phone charger that puts out 5V and converting that down to 3V for the motor, that way I will not be replacing batteries all of the time. I dont know anything when it comes to electronics, so please help me with this and use lamens terms. Thanks

 

Assuming this is a small motor (minimal current draw) and it runs only in one direction you could simply place 3 diodes in series. Should drop the voltage down slightly below 3V.

 

Use 1N4001's. The motor may be happy if you only use 2 diodes.

 

Two diodes = 1.1+1.1 = 2.2
5-2.2 = 2.8v

You can also use a single zener but it's much more expensive or a fixed 3v or three terminal adjustable regulator which even more expensive.

 

Curious... Where does the 1.7V come from?

Nominal forward voltage on a silicon diode is between .6V and .7 V. Nominal forward voltage on a germanium diode is closer to .2V.

 

Yeah, I agree with EEboy - except I think the germanium is closer to 0.3V

Maybe you two were thinking about the drop across a fullwave bridge rectifier from end to end?

Tznuk, another way wind up with a more efficient circuit would be to use a 555 timer to construct a PWM circuit with a base frequency around 1KHz with 60% on time, 40% off time, or 3/5 duty cycle.

Of course, that will require more work than just using a few diodes in series to drop the voltage down - but it'll also be easy to fine-tune the speed of the motor once that's done.

 

Nominal forward voltage on a silicon diode is between .6V and .7 V

Depends on the current. The 1N4001 at 1A has a nominal Vf of 1.1V.

 

Depends on the current. The 1N4001 at 1A has a nominal Vf of 1.1V.

[nitpick]According to the Fairchild datasheet, 1.1V is the max at 1A. Typical is about 0.93V.[/nitpick]

 

you can use zener diod which thresholding voltage eaqual(3V)
parallel with the motor.

 

Why not using an LM317 for safe regulation?

The voltage drop across a diode is quite unstable. It may range from 0.6V to 0.9V. Zener diodes are not a good option either, as they tend to waste energy and the drop voltage also varies with current (except when we are dealing with very small currents).

 

Why not using an LM317 for safe regulation?

The voltage drop across a diode is quite unstable. It may range from 0.6V to 0.9V. Zener diodes are not a good option either, as they tend to waste energy and the drop voltage also varies with current (except when we are dealing with very small currents).

Two volts is pretty iffy for headroom for an LM317, especially since we don't know the load current. There are lots of low dropout regulators that will work, though.

 

Two volts is pretty iffy for headroom for an LM317, especially since we don't know the load current. There are lots of low dropout regulators that will work, though.

For the LM317, the minimum is 1.5V from the input to the output. For this objective I think it will serve. Looking at the datasheet it is possible to see that for a load current of 500mA and a temperature of 25ºC, the dropout voltage will be of 1.9V. It will be the worse case.

See the datasheet here:
http://www.fairchildsemi.com/ds/LM/LM317.pdf

 

For the LM317, the minimum is 1.5V from the input to the output. For this objective I think it will serve. Looking at the datasheet it is possible to see that for a load current of 500mA and a temperature of 25ºC, the dropout voltage will be of 1.9V. It will be the worse case.

See the datasheet here:
http://www.fairchildsemi.com/ds/LM/LM317.pdf

You're probably right. It will probably work as a one-off. However, that graph you read is not worst case, it is typical. If you look at the Electrical Characteristics table, the part is only specified for (Vin-Vout)>3V.

 

Personally, I think SgtWookie has the right idea. A simple 555 PWM circuit like this will allow speed control. I would change the MOSFET to one specified for 5V logic level drive. Fairchild has dozens of them that will work, and I'm sure other mfrs do also.