voltage regulator circuit

Discussion in 'The Projects Forum' started by jboulasii, Sep 12, 2009.

Aug 16, 2009
11
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I need some help. I have a 1/2 wave rectifier feeding a LM317T voltage regulator. The input to the rectifier is 120VAC so I'm getting about 54 volt on the other side of the diode. I have this voltage feeding the Vin of the LM317T. According to my calculation if I have a 2200 ohm resistor from the adjustment terminal to ground and 200 ohms from adjustment to Vout I should get about 15volts on Vout. I'm getting 34volts. I also have a .1uf cap from Vin to ground. Any idea on what i'm doing wrong?

Apr 5, 2008
15,799
2,385
Hello,

It is not good to feed more then about 40 Volts at the input.
When the output gets shorted it is the maximum allowed voltage.
You can use a transformer 15 - 18 Volts for the input.
Use a bridge rectifier and a nice capacitor to remove the ripple.
The resistor between output and adjust is normaly 120 Ohms.
The resistor from adjust to ground will be round 1300 Ohms.

Greetings,
Bertus

Aug 16, 2009
11
0
Bertus,

I thought about the transfomer but I'm trying to keep the packaging as small as possible. What about a 20 volt zener diode on the input Vin.

Apr 5, 2008
15,799
2,385
Hello,

You will never get it real safe when feeding the regulator from the mains.
If some component fails the change will be there the mains will be at the output.

Greetings,
Bertus

5. SgtWookie Expert

Jul 17, 2007
22,183
1,728
You MUST use a transformer to provide isolation from the mains power.

They are far less expensive than a funeral.

6. Audioguru New Member

Dec 20, 2007
9,411
896
The resistor from the output to the ADJ pin must be 120 ohms or less for an LM317 or the output voltage will rise when there is no load. The more expensive LM117 can use 240 ohms.

7. eblc1388 Senior Member

Nov 28, 2008
1,542
102
Average 54V? What about the peak voltage? Horrible maths.