help required on regulated psu

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you can imagine what went wrong!

Yeah, although it hurts. But why should the other one work, then.
Did he insulate the ICs and get the voltage divider correct on the first board, just to make both wrong on the second? I couldn't imagine...

Good Luck, Garbage. ;)
Just don't bother with that LM338. Replace it with a new one and check the old one later. It might still work, but you don't want to determine that the hard way. The part is too cheap to spend more than an evening just to rescue a probably damaged one.

Sebastian.
 
sek said:

You shouldn't leave the chip unconnected. At least use a replaced voltage divider network, no ADJ bypass cap and no OUT bypass cap for a short measurement. With an appropriate input voltage of >15V, the regulator should really show some sensible behaviour now. Otherwise it might be damaged...
ok, will try this.

sek said:

Just to make shure I understand you right: You use a network of 120R and 2.2k according to Pedja's circuit?

Check with the datasheet: On page 4 it shows the equation to calculate the resistor network for a required output voltage. Using 120R as the upper and 2k2 as the lower resistor would calculate to Pedja's required value of around 25V.

For 15V and 120R as the upper resistor, the equation leads to a value for the lower resistor of ca. 1.3k ... :smash:

If you really used the wrong values here, that would explain your problem. It would also explain the too low reference voltage between ADJ and OUT, because for Vin < (Vout+Vdrop) the regulator is fighting against itself, showing ill behaviour at the OUT pin. That makes me wonder why the other board works... :xeye:

Sebastian. ;)
hmm, but i had a 2k2 variable resistor and i tried turning them to one extreme... perhaps i can try the other extreme.

the other psu worked by having output of some low voltage up to about 18 or 19vdc, where further turns on the variable resistor will not yield any increase as that is the max after the rectifiers.

thanks.


carlosfm said:


They were just not isolated from each other.
As this PSU uses two positive regulators to make a +, G and -, you can imagine what went wrong!:hot: :hot: :hot:

i did not terminate the ends of the output to form a common ground.
so the circuits are effectively 2 +15vdc output. just that i reversed the other one as -15vdc for my buffer.
 
mystery solved...

hi all

played around with my psu, and found out that the R1 resistors that i bought in a hurry are not 120R but 120k!!

also one of the psu's variable resistor's pin 2 to 3 is not working, so i had to use pin 1 to 2 instead.

here is one section of the psu running at 15vdc output. (notice the orientation of the variable resistor is switched as one of them is using pin 1 and 2 instead)
An externally hosted image should be here but it was not working when we last tested it.


and here are the 2 psu powering my line drive (opa627/buf634)
An externally hosted image should be here but it was not working when we last tested it.


played john neptune's asian roots cd, track 3 - jegoging on it.
separation and depth are simply great. i did not want to switch to another cd. got to go and listen more.

cya, and thanks for all the suggestions. :)
 
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