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wrong voltage from 7905 and 7912

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Rohith_elec

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My power supply using the following board using 7912 and 7905 gives -14V and -9V.


**broken link removed**

but the 7812 and 7805 in the circuit is giving exact 12V and 5V.


what can be wrong ?
 

You have to provide more information.
 

It's possible that they require a minimum load in order to regulate properly. Try loading each with 1-10ma.
 

nandhu015 said:
Try to post the full circuit
Click to expand...

they didnt gave the schematic

mtwieg said:
It's possible that they require a minimum load in order to regulate properly. Try loading each with 1-10ma.
Click to expand...

how can i attain that ?

---------- Post added at 04:03 ---------- Previous post was at 03:36 ----------

The heat sinks of the 7xxx s were touching, now i insulated the heatsinks now the 7812 and 7912 are almost 12(11.95) and -12(12.35)

but no change in 7805 and 7905
 

Rohith_elec said:
they didnt gave the schematic



how can i attain that ?

---------- Post added at 04:03 ---------- Previous post was at 03:36 ----------

The heat sinks of the 7xxx s were touching, now i insulated the heatsinks now the 7812 and 7912 are almost 12(11.95) and -12(12.35)

but no change in 7805 and 7905
Click to expand...

possible heatsink 78xx with 79xx short .. be careful heatsink 79xx with 78xx different
 

Li_ereunpisan said:
possible heatsink 78xx with 79xx short .. be careful heatsink 79xx with 78xx different
Click to expand...

yes,
i removed the heatsink of all,
now 7912 and 7812 are almost correct

12(11.95) and -12(12.35)


but not with 7905 and 7805 counterpart
 

Rohith_elec said:
yes,
i removed the heatsink of all,
now 7912 and 7812 are almost correct

12(11.95) and -12(12.35)

but not with 7905 and 7805 counterpart
Click to expand...

are you sure ... with the correct 78xx placement?
or whether IC regulator you are in good condition?
I've had reversed its placement 7905, its output so-9V .. maybe it could be a my experience for maybe it could be as my experience for your analysis of materials
 
I placed as per the silkscreen writings in the board,

ok,

i will check is there any wrong with the boards routings

Anyway I am building the same circuit on a strip board

thank you
 

The pinouts (and power tab connection) differ, and maybe
you got a bad footprint or something if it was made by copy?
Start at the bottom.
 

dick_freebird said:
The pinouts (and power tab connection) differ, and maybe
you got a bad footprint or something if it was made by copy?
Start at the bottom.
Click to expand...
the board was not made by me,
I bought it from a supplier,

ok,
I will check all
 

Check if there are diodes drawn at pin #2 of both 79xx devices.
If circuit is working properly, sounds as a willfull voltage shift.


+++
 

I noticed at some designs the insertion of a direct polarized diode across GND pin of fixed regulator.
This is required if an absolute voltage greater than specified by device is required.

see that :
**broken link removed**


+++
 
I added a zener from the output to get the 5V,
 

andre_teprom said:
I noticed at some designs the insertion of a direct polarized diode across GND pin of fixed regulator.
This is required if an absolute voltage greater than specified by device is required.

see that :
**broken link removed**

I think if add a diode at the input, the output current will be reduced and not in the datasheet applications, except for adding a resistor 5-10 ohms at the input to prevent more heat in the regulator


+++
Click to expand...

Rohith_elec said:
I added a zener from the output to get the 5V,
Click to expand...

I think if add a zener on its outputs is inefficient because it is only natural output does not fit 7805 or 78xx 5V which is between 4.7 V - 4.9 V, unless you need the right output 5V, I suggest you use a variable regulator ,such as LM340, LM 317 and other
 

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