Help in Power Supply

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ahmelsayed

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I'm using LM317 IC with a transformer 24V at 2A the IC is connected to 5Kohm POT to use it to vary output voltage between 1.2V - 30V
this is the circuit:
**broken link removed**

and i add a diode in parallel with R2 as i read in IC datasheet for protection .
the problem :
when i test it , the 5Kohm exploded and i get 33.5V DC output so i turn the power off the output supposed to get0 output . no it count down from 33.5 to 1 V in 45 sec . i don't know where is the problem .
so please some one put me on the right way.
 
Check the pin out of the LM317. In,out,adj
Is D1 going the right direction?
Getting 33 volts out and P1 exploded makes me think then IN, OUT & ADJ are all shorted together, This could happen if D1 way backwards or LM317 is bad (wrong).
 
Hi,

Yes it could be that you connected D2 in the wrong direction. With the pot turned down low, it could draw a lot of current that way and burn up.
Of course you could also have D1 in the wrong direction too.
Now you have to check the LM317 to make sure it is not blown too.

Remember that the diode package cathode has the band not the anode, and the cathode of D2 connects to the output of the LM317, and the cathode of D1 connects to the input of the LM317.
 
ahmelsayed said:
thanks very much
i'm really proud to join this great site
i'll try what u say.
thanks again
Click to expand...

Hi there,

Well you're welcome, and it's nice to have you here. If you have any more problems just come back and yell a little more
 
The datasheet for the LM317 says that the protection diode in parallel with R2 protects against the high discharge current of a bypass capacitor across the pot when the output is shorted. But you don't have the capacitor so you don't need the protection diode.
The datasheet has a schematic showing the polarity of the protection diodes.

An LM317 is supposed to use 120 ohms or less for R2. Then the value of the pot should be reduced to match it.
A more expensive LM117 can use a value up to 240 ohms for R2.

The circuit will provide an output current of up to 1.5A only if the output voltage is 20V and higher and if the heatsink is big enough.
When the output voltage is 5V then the max current could be as low as 400mA.
 
hey
i recheck the connection and the compenents . everything is ok and nothing exploded except POT 5K .
and this is the PCB, please check it for me . may be my brain wasn't with me.
**broken link removed**
**broken link removed**
**broken link removed**

i think the wrong thing i've done is to add D2 in parallel with R2 (in the same hole on PCB)
so please help some one in trouble.
 
You don't need the diode across R2 because you did not add the capacitor to the Adj. terminal. But if you did need it the cathode (end with the line on it) goes to the output pin. My guess is you had it backwards but it doesn't show in your picture.
 
Hi,

I have couple questions too...

1. It's hard to tell the pinout on the LM317 from your drawings. Could you indicated what terminals are IN, ADJ, and OUT please?
2. That diode D1 looks a little strange. What kind of diode is that?

Just an idea: Check the circuit out first without any diodes, see how it goes.
 
D1 is a red LED as shown on the schematic.

D2 is a protection diode that is from the input to the output. It is NOT across R2. It is needed when the output capacitor value is 25uF or more but here it is only 1uF so D2 is not needed.

Please read the datasheet for the LM317 because all the details are there.
 

Hi,

You might want to look at the pinout for the LM317 here on the datasheet.
https://www.electro-tech-online.com/custompdfs/2010/08/LM317.pdf

Note that with the tab down pin #1 (left) is the adjust, pin #2 (center) is the output and pin #3 (right) is the input. With that pinout and if the silkscreen on your PBC indicates the way you placed the regulator on the board, then the adj, pin #1, is going to the bridge instead of the input, the output, pin #2, is tied to the adjust circuit and the input, pin #3, is going to the output.
 
here is a detailed view
**broken link removed**
i think i replace between the adj pin and out pin (hope some one tell me i'm wrong, but pcb wizard cheat me and i was sleepy to do this mistake confused)
and D1------------> LED
D2------------> between input and output.(in right way)
D3------------> in parallel with R2 (in right way)( ithink i'll remove it as no need)

now after all this i think i'll build nuclear reactor.
 
Now you have the pins of the LM317 labelled wrong. Why don't you copy the pins shown in the datasheet?
You show an ADJ pin is connected to the output connector.
You show an OUT pin is connected to VR1.

The datasheet says that your D3 is not needed because the capacitor that it protects against is not there.
The datasheet also says that your D2 is not needed because the value of the output capacitor is low.
 


Hi there,


In your detailed view above, are we looking at the foil side (copper trace side) or the component side?
If we are seeing the foil side then the LM317 looks ok, but if we are seeing the component side then it is not connected properly. I assume the tab is to the left as drawn.

Here's the drawing with a little more contrast
 

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Hi again,


Yes, here is a drawing that shows some changes that would change where one diode is located and the other diode turned around...

There may be other changes required too, so check this over...
He's going to have to go over the entire board and check it over carefully.
Since "D3" was backwards that would blow out the pot.

Note this drawing has color coding, gray for ground, yellow for output, blue for input.
 

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thanks for all who helped me .
specially MrAl , audioguru and MRCecil
now i'll back every thing to its position offcource NOT anew PCB . only i'll connect IC to it right pins using some wires not solder it to pcb .
i think it will work as it supposed to do (hope that)
thanks again.
 
Hi again,


Be sure to check out that new drawing i had posted as that shows a few important changes. Note also that that drawing is looking at the board from the FOIL SIDE.
 
hey!
in the new drawing i think yellow is for adjustment , blue for input and red for output .
as PCB Wizard i used to draw this PCB don't have LM317 Symbol only Voltage regulator general symbol with the reference (adj) in the middle not at pin#1 so that is the mistake ( i depend on a failure prog) and didn't take into account this point so i'll turn to EAGLE (try to learn it).
so let's hope it will work.
 

To be brief, and to encapsulate the intent of my previous post to you without getting into the minutia of the diodes, which is NOT THE ROOT CAUSE OF FAILURE, as others have tried to explain, your selection of the component to place on the board you displayed does not match the pinout of the LM317 as I tried to point out followed on by MrAl's highlighted post, without being abrupt. Hopefully, you will look closely at the posts on this page and reconsider what I and MrAl have tried to convey.
 
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