[Resolved] LM7805 outputs 7 to 8 Volt instead of 5V

Thread Starter

ep.hobbyiest

Joined Aug 26, 2014
171
I am using LM7805 in my design for 5 volt. Input i am giving for regulator is 12V. i have connected 0.1uF capacitor across output and ground.

Here is the attached circuit for reference.


1603536745231.png
 

ericgibbs

Joined Jan 29, 2010
11,130
I have connected PIC controller on the output side of the regulator
hi ep,
DON'T connect any equipment load to the output of the 7805 when its outputting 7v!.

Post an image of the 7805 wiring, you may have it incorrectly connected.
E
 

KeithWalker

Joined Jul 10, 2017
1,301
How are you measuring the voltage? It should be 5V between the 7805 output and ground.
You will get 7V if you measure between +12V and 7805 output.
 

Thread Starter

ep.hobbyiest

Joined Aug 26, 2014
171
problem is solved. problem was with regulator connection. i had connected regultor with reverse pin mapping.
After correcting board is working properly.
 

MisterBill2

Joined Jan 23, 2018
7,315
Definitely there must also be a bypass capacitor at the input to that regulator. I had a similar experience and it was oscillating at several megahertz. The fix was adding the input bypass close to the regulator. Just like the app note states. 0.1 mfd within 1/2 inch. consult the application notes for their suggestion of how close.
 

Ian0

Joined Aug 7, 2020
756
Definitely there must also be a bypass capacitor at the input to that regulator. I had a similar experience and it was oscillating at several megahertz. The fix was adding the input bypass close to the regulator. Just like the app note states. 0.1 mfd within 1/2 inch. consult the application notes for their suggestion of how close.
TI's datasheet says 220nF on the input pin (though I also use 100nF), but nothing on the output pin. I prefer a 10uF aluminium electrolytic on the output, as it acts like a zobel network due to its large ESR. I think it was something recommended by Bob Pease.
 

Martin_R

Joined Aug 28, 2019
28
A lesson to learn here - you're lucky that nothing went up in smoke! Mistakes do happen to us all. I always test power supplies first, make sure they output the correct voltage before connecting them to the rest of a project. You had a good outcome!
 

hrs

Joined Jun 13, 2014
256
These regulators need a load of at least 5mA to stabilise properly. Try loading it with 1K and measure the voltage.
Hi Martin, do you have a source for that? I made a lab supply with LM7808, LM7908, LM317 and LM337. The LM7908 starts to regulate correctly around ~3mA. I cannot find this requirement in the datasheet or online.
 

Ian0

Joined Aug 7, 2020
756
Hi Martin, do you have a source for that? I made a lab supply with LM7808, LM7908, LM317 and LM337. The LM7908 starts to regulate correctly around ~3mA. I cannot find this requirement in the datasheet or online.
TI does not specify a minimum load.
 

crutschow

Joined Mar 14, 2008
25,669
The LM317 and other floating type variable regulators, with no direct connection to ground, require a minimum current for proper operation.
The LM7808 and other similar fixed regulators that do have a direct ground connection apparently do not require a minimum load.
 

hrs

Joined Jun 13, 2014
256
LM7805 lists the minimum current here:
Wow, I totally missed that.
NO, that is not a minimum load current, that is the load current range for which the device is guaranteed to meet the voltage output specification. Spec sheets can indeed be confusing sometimes.
How is that different? I mean, it would be the minimum load current required for the device to operate properly wouldn't it?
 

ericgibbs

Joined Jan 29, 2010
11,130
hi hrs,
I would agree, that it suggests that a load current less than 5mA, would mean that regulation cannot be guaranteed.
Adding a Resistor and LED across the Vout, is a useful way of adding this light current load.

E
 

MisterBill2

Joined Jan 23, 2018
7,315
Both miss the point. That is the minimum load at which almost all of the regulators will be within 20 millivolts of exactly 5.00 volts."Proper operation" includes no output current.
 
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