# 5V to 3V

Discussion in 'General Electronics Chat' started by tznuk, Dec 14, 2007.

1. ### tznuk Thread Starter New Member

Nov 29, 2007
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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

2. ### eeboy Active Member

Sep 27, 2007
90
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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.

3. ### beenthere Retired Moderator

Apr 20, 2004
15,808
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Use 1N4001's. The motor may be happy if you only use 2 diodes.

4. ### mrmeval AAC Fanatic!

Jun 30, 2006
833
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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.

5. ### eeboy Active Member

Sep 27, 2007
90
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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.

6. ### SgtWookie Expert

Jul 17, 2007
22,201
1,807
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.

7. ### nanovate Distinguished Member

May 7, 2007
665
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Depends on the current. The 1N4001 at 1A has a nominal Vf of 1.1V.

8. ### Ron H AAC Fanatic!

Apr 14, 2005
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[nitpick]According to the Fairchild datasheet, 1.1V is the max at 1A. Typical is about 0.93V.[/nitpick]

9. ### h.d Active Member

Oct 22, 2007
150
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you can use zener diod which thresholding voltage eaqual(3V)
parallel with the motor.

10. ### cumesoftware Senior Member

Apr 27, 2007
1,330
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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).

11. ### Ron H AAC Fanatic!

Apr 14, 2005
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681
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.

12. ### cumesoftware Senior Member

Apr 27, 2007
1,330
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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

13. ### Ron H AAC Fanatic!

Apr 14, 2005
7,012
681
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.

14. ### Ron H AAC Fanatic!

Apr 14, 2005
7,012
681
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.