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LM317T step down voltage Regulator

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mhamzah3

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I am building a Step Down Voltage Regulator from 70V to 20V using 2 LM317T chip. The schematic design is attached with this post.

The problem I am getting is whenever I load with the circuit with 0.3-0.5 amps, the voltage is unstable where the output drops from 19.85V to 10V.

On the datasheet, the maximum current that can be drawn from the LM317T is 1.5A but I am unable to draw that much current. The maximum current draw is less that 200 mA to achieve a stable output voltage.

Please help.

Thanks
 

Please check:

Whether the Vin - Vout < 40 V on each regulator chip?

The 70V supply can provide 1A current?
 

The Vin-Vout of the first chip is 70-42=28V and the second chip is 44-20=24V.

The 70V supply can provide a max of 7A.

I used a decade box to provide the load to the circuit.
 

mhamzah3 said:
The Vin-Vout of the first chip is 70-42=28V and the second chip is 44-20=24V.

The 70V supply can provide a max of 7A.

I used a decade box to provide the load to the circuit.
Click to expand...

The Vin-Vout of the first one is 70-42....The Max output of LM317 is 37V...reduce the output of first chip to 35V not 42V and then use second chip to make 20V....
 

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    mhamzah3

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I guess the datasheet did not specify the output voltage or i missed it out.

Thanks for helping me out. This is my first time posting on a forum. Will try that out.
 

Hey I need some help, I already stepped down the voltage to 37V but now my problem is I can't draw current up to 0.3 A on the Voltage Regulator design that I made. The maximum current that can be drawn on this circuit is 0.14A. If I draw more than 0.14 A which is up to 0.27A, the output voltage will become unstable. which drops significantly to less than 10V.

Is there any design changes that can be made to correct this problem?
 

Use an external pass transistor you are overheating the 317.
 

As was already stated, you are overheating that part. If you read the data sheet, a very quick power dissipation calculation from the data sheet says
PD=((VIN-VOUT)*IL)+(VIN*IG). If you ignore the VIN*IG and just use the first part of the equation and lets say 1A for simplicity PD=(70-37)*1A=33W in the first stage. There is absolutely no way that package is handling 33W without any sort of major heat sinking. You are hitting thermal shutdown in the first stage. That is why you are seeing the voltage drop as you increase the current. You are probably hitting it in the second stage as well. Read the section on the heat sink requirements.
 

if current is motorboating you are definitly overheating, heat sink.
 

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