# Need help on linear regulator

Discussion in 'The Projects Forum' started by Van748, May 11, 2010.

1. ### Van748 Thread Starter New Member

May 11, 2010
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I was ask to design a power supply unit, using

Input: 230Vac
Output: 3.3Vdc
Use Linear Regulator for your design.

I'm still new to engineering so im a little vague about how should i do it. I tried reading other post about linear regulator but i still don't understand how should i design a circuit. I really need help !

Apr 20, 2004
15,815
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3. ### Van748 Thread Starter New Member

May 11, 2010
4
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thanks man !! im reading it now . but theres one part where i don't understand i hope you clarify my doubts too !

For most power applications, half-wave rectification is insufficient for the task. The harmonic content of the rectifier's output waveform is very large and consequently difficult to filter.

what do they mean by the " filtering " part ??

4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
293
That means placing a capacitor after the rectifier output to act as an energy storage device. The accumulated charge on the capacitor can supply the circuit current during the time of no diode conduction, so the output voltage does not go to zero.

This part of the Ebook and the next discuss making DC linear power supplies - http://www.allaboutcircuits.com/vol_6/chpt_5/6.html

5. ### Van748 Thread Starter New Member

May 11, 2010
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how should i apply it on my circuit design ?

They say to use a capacitor at least twice the RMS AC voltage output of the low-voltage AC power supply. And there is a circuit diagram in that webpage too.

For the RMS AC voltage output of the low voltage AC power supply do they meant by my Output: 3.3Vdc ??

6. ### beenthere Retired Moderator

Apr 20, 2004
15,815
293
Every regulator requires more voltage in that the regulated output voltage. 2 volts is about the minimum difference. There is also an upper limit, where the input voltage will harm the IC regulator. Every regulator has a spec sheet that spells out the limits,such as in his link - http://www.national.com/mpf/LM/LM2937-3.3.html#Overview

Notice that there is also a current limit for the regulator. 3.3 volts is not as common as several higher voltages, so it's a bit harder to find linear IC's in that range.

The rectified voltage from a transformer will tend to charge the filter cap to the peak value, or close to 1.4 times the RMS rating. Ideally, that would mean a 6.3 VAC center tapped transformer for your circuit. Those could be hard to fund, though, as 6.3 volts was common for vacuum tube heaters. That is not quite so much in demand lately.

The problem with higher voltages is in power wasted in the regulator, which may mean a large heat sink to keep it cool enough to operate.

What is the load? How much current at 3.3 volts will you need to regulate?

7. ### SgtWookie Expert

Jul 17, 2007
22,202
1,793
Radio Shack carries several transformers that have center-tapped 12.6v secondaries, for 6.3v-0v-6.3v out.

They also carry LM317T regulators. You could use an LM317 with two resistors to set it's output to 3.3v.
You could use a 220 Ohm resistor from VOUT to ADJ and a 360 Ohm resistor from ADJ to GROUND to create a 3.3v regulator.

You would also need caps on the VIN and VOUT terminals.

The regulator will need a heat sink.

8. ### Van748 Thread Starter New Member

May 11, 2010
4
0
thanks for the help BeenThere and SgtWookie !

9. ### SgtWookie Expert

Jul 17, 2007
22,202
1,793
Ack! - Radio Shack only carries transformers with 120vac primaries. Reading is fundamental....

However, plug supplies aka "wall warts" are cheap and plentiful. Most of them have DC out. The filtering isn't that great, but you can add a larger filter cap, a regulator, and an output filter cap. That would probably be the cheapest way to go.