voltage regulator circuit

Thread Starter

jboulasii

Joined Aug 16, 2009
11
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?
 

bertus

Joined Apr 5, 2008
19,939
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
 

Thread Starter

jboulasii

Joined Aug 16, 2009
11
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.
 

bertus

Joined Apr 5, 2008
19,939
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
 

SgtWookie

Joined Jul 17, 2007
22,201
You MUST use a transformer to provide isolation from the mains power.

They are far less expensive than a funeral.
 

Audioguru

Joined Dec 20, 2007
11,251
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.
 

eblc1388

Joined Nov 28, 2008
1,543
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.
Average 54V? What about the peak voltage? Horrible maths. :(
 
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