7809 Voltage Regulator Output Mismatch

Thread Starter

jobayer

Joined Nov 4, 2020
6
Hello, I am learning electronics. Today I tried 7805 & 7809 voltage regulators with a 12V battery.

*Though my battery is of 12V, currently it has 8.76V in it.

In my first attempt, I connected 7805 with the battery and I got ~ 8.70V in the input terminal and in the output terminal I got ~ 4.98V which is pretty obvious.

But the unexpected things happened when I connected the battery with 7809 voltage regulator.

Here in the output terminal, I got ~ 8.69V. But in the input terminal I got ~ 7.91V.

Now this is pretty unusual to me because I am using a 9V voltage regulator and if I assume that due to the internal resistance of the wire, I am loosing around ~ 0.2V then for 7809 the output reading should be ~ 8.67V!!

My question is where is the (8.67-7.91=) unsolved 0.76V? And why this isn't the case with 7805?

Thanks in advance.
 
Last edited:

Ian0

Joined Aug 7, 2020
3,463
Here in the output terminal, I got ~ 8.69V. But in the input terminal I got ~ 7.91V.
Did you measure them both at the same time? If you 12V battery is in such poor condition, the terminal voltage will drop rapidly when any load is connected.
I suggest you get a bettery battery and try again.
 

Thread Starter

jobayer

Joined Nov 4, 2020
6
Did you measure them both at the same time? If you 12V battery is in such poor condition, the terminal voltage will drop rapidly when any load is connected.
I suggest you get a bettery battery and try again.
Ah, ok I think I got the answer. May be the problem is not with the battery.
From the datasheet, I came to know that the minimum input voltage is 11V and that's why 7809 is not regulating the voltage.
 
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