I am a novice to practical electronics but here is my problem:
I have a mini transformer 240V AC 50 Hz input - 9.5V DC 355mA output.
I need to lower the output voltage to 3V DC maintaining the 355mA, how
can I achieve this?

 

An LM317 voltage regulator can accomplish that.

 

You only need 2.1vac for 3vdc, you could also look at taking a few secondary turns off, if practical.
Max.

 

Do you intend to hack into the power supply, or do something external?

Is it an electronic switch-mode power supply, or is it a real iron transformer? (You can tell by weight).

 

Do you intend to hack into the power supply, or do something external?

Is it an electronic switch-mode power supply, or is it a real iron transformer? (You can tell by weight).

Hi Mike, it's a real iron transformer; and I intend to do something external.
Regards John (thur10)

 

Using a linear IC chip regulator like a LM317 will do it, but the regulator will produce E*I = (9.5-3)*0.355 = 2.3W of heat. You will have to put the regulator on a small heat sink. Can you do that?

Go to TI.com, download the data sheet for the LM317 in the TO220 package. The circuit is on the data sheet.

Two resistors will program the '317 to output 3.00V.

 

Hi Mike, I checked TI.com and printed out the application information sheet.
Although very explicit and easy to understand I am wondering if this is the path I should pursue, as all I am trying to do is to drive a 3V DC motor intermittently (approx. once every 60 seconds for a duration of about 10 seconds with ridiculously minimal mechanical load). Would a simple resistor not do the job as there is no electronic circuitry involved?
Regards John (thur10)

 

With a very light mechanical load a resistor could do that job fine I think.

How are you switching the motor on and off every 60 seconds?

Maybe that system could also incorporate the feature to reduce motor voltage and speed.

 

With a very light mechanical load a resistor could do that job fine I think.

I agree. The LM317 regulator I mentioned can be thought of as a "smart" resistor. It's able to adjust the voltage even if the load is changing. If the load is nearly constant, and your's sounds like it might be, the LM317 acts basically like a fixed resistor and could be replaced by one. Heat dissipation is still an issue, as mentioned already.

 

With a very light mechanical load a resistor could do that job fine I think.

How are you switching the motor on and off every 60 seconds?

Maybe that system could also incorporate the feature to reduce motor voltage and speed.

The switching is achieved by the means of a magnet acting on a miniature reed switch, the lot is incorporated on a disc and mounted on the second axis of a 1.5V clock movement.

 

The switching is achieved by the means of a magnet acting on a miniature reed switch, the lot is incorporated on a disc and mounted on the second axis of a 1.5V clock movement.

 

He could use zener diode to drop some load?

 

The miniature reed switch can't switch 355mA. You need to add a transistor switch, if that is what you are attempting.

 

He could use zener diode to drop some load?

But the zener would have to dissipate 2.3W so it would need to be a 5W zener on a heat-sink.