LM7805 Output Voltage Too High

[Peema]

lm7805

Just built my first power supply around the LM7805. Wall Wart input was 9 volts volts but now it is around 12, output volts = 12.4. I have a 100 uf cap on the input side and 10 uf cap on the output side. The LM7805 gets warm to the touch. What's happenning! Thanks!

 

[ctownsend]

power dissipation of the 7805 ic

Do you have the 7805 hooked up backwards? Input to output?
It is normal for the 7805 to get warm, not really hot!

 

[Peema]

if output voltage is too high

I don't think it's backwards, won't have a chance to check until later tonight. I'm going to take it out of the circuit and check it by itself. Thanks!

 

[shoaibali]

lm7805 12v dissipator

Do confirm that you are using the right pins.

 

[crazyduck]

lm7805 iout

Hi,
Sure you've invert input and output. Be careful, 78L05 and 7805 have the ouput and the input invert.


front view
--------
| o |
|------|
| |
|------|
| | |
| | |


IN GND OUT

 

[palesha]

lm78l05 10uf

If the pins r ok then the IC is defective.

 

[ultrabrains]

how to change the voltage of an lm7805

Without any load 7805 ic should be stone cold. If its get warm then there is a little problem which you have to fix, so do the following

1. remove the 10uf capacitor at the output and check it again

2. you can use a digital meter to check if your 7805 is ok, both input and ouput should read like a diode relative to the common, if it does otherwise then you have a defect on your 7805. so change it

 

[cherukukeshav]

7805 output voltage high

if load is connected to it.check with the output current from regualtor to the load.


may be the heat is because of the power dissipation across the regulator.

if input voltage is 9 volts the power dissipation is

(9-5)X current to load

when input vtg is 12V the power dissipation is
(12-5)X current to load ---which is high when compare to the previoius case.

 

[ultrabrains]

checking input and output for 7805

cherukukeshav

when there is no current to load and 7805 is supplied with 12v, power dissipation according to your formular would be

(12-5)X 0 = 0

The point is that when power dissipation is 0 7805 should be stone cold. That was why i suggest the 10uf capacitor at the output be removed, it might just be defective and posing to be a load (or short circuit) at the output of 7805

cheers

 

[cherukukeshav]

Ultrabrains,

good math. but i was telling about when there is a load current and was trying to relate the excess of heat because of the input vtg change from 9V to 12V.

Can u explain the thing with respect to the capacitor please.........

--Keshav