Problem LM317 output voltage adjustment

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muneeb.khan

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hi
i am making a 1.5-28V 8A power supply using LM317 and 2N3055 as pass transistor, the problem is that i am not getting adjustable voltages at the end rather it remains constant to 24V but i am getting correct adjustable voltages in range of 1.7-27V at the OUT pin of LM317. any help and suggestions will be appreciated. The schematic is attached that i got from **broken link removed**
 

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The schematic is correct. If your output voltage does not go up to 28 V under load, check the rectifier output. Your transformer may need some change, too.
If you need 1.5 A max output, why not use a LM350 only, it can take 3A max without additional 2N3055.

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Also check if P1 is full-scale. You may try to add some resistance there or reduce R2 value a bit.
 

thanks for reply,
the rectifier output is 40V(peak), i am using 36V 10A transformer. I need 6A current for my steppers in full stepping mode.:sad:
 

A few things of concern here. If you are using a 36V transformer , how do you get to 40V peak? Even with 2V drop in the bridge(likely to be less , closer to 1.4-1.5V)
you should still be getting close to 48V on your output caps. There are no values in your schematic , which does not help.
Unless you are using a HV version of the 317 , you are going to blow it. Max input is 37V.
Follow the voltage from the output of the 317 to the base of the transistors. With P2 set to 0 it should be in the region of 1.8 - 2V less than the output of the 317.
We really do need some component values to make more advised calls here!!
Cheers
Neddie
 

It isn't a well thought out power supply, it has no over-current protection and a short across the output could damage one or both power transistors, potentially letting full voltage through. Limiting base current which could be up to an Amp or so with a variable resistor isn't a good idea either.

It would be better to use a conventional PNP based 'wrap around' current booster which would give the same output with over current and thermal protection as well. It has about the same component count as the present circuit.

Brian.
 

The circuit will have terrible voltage regulation because the transistors are simple emitter-followers and the LM317 cannot adjust their output voltage accurately.
The datasheet for the LM317 shows a PNP transistor at the input driving NPN output transistors for excellent voltage regulation.

The original circuit is wrong because it uses 240 ohms for R2. The datasheet for the LM317 shows 240 ohms for the more expensive LM117. The LM317 needs 120 ohms so the output voltage does not rise when there is no load. Then the value of R1 must also be reduced.

The LM350 can pass up to 3A and the LM358 can pass up to 5A only when there is 10V or less from input to output.
 
I agree with betwixt.
This is not a great circuit at all. Rather go with the lm317 + PNP transistor to boost the current.
Current limiting and fuse etc should be added as well. Check your input voltage does not exceed 37V for the 317.
 

I have shared the link of author's page here all the values of components are mentioned..
and its strange why i m getting 40V:???:..

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thanks to all guys, i found 2N3055 faulty and replaced them n its working. Now its working well n i am getting desired adjustment. But there is a problem when i apply load, output fluctuates n after releasing load its again constant. any help
 

A properly designed linear power supply monitors it's output voltage and adjusts it's pass element to maintain a constant level. That power supply does not. The LM317 is used only as a variable voltage source and the actual output comes from the two 2N3055 transistors. Any base current in those transistors, and the drop in the diodes and variable resistor will make the output fall under load.

As neddie and myself mentioned, there is an alternative configuration which gives better regulation and a degree of overload protection and potentially uses fewer components.

Brian.
 

thanks betwixt! is there any circuit you can share that gives better regulation?
 

The circuit of a high current adjustable voltage regulator with excellent voltage regulation is shown on the datasheet of the LM317 as I said before.
Why don't you look at the manufacturer's datasheet??
 
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