I am trying to get ±5V using voltage regulator L7805 and L7905.
The datasheet says to use AC source as input and transformer, but I am not really familiar with transformers, and I don't know if I can handle 110V AC as a source.
So I came across YouTube clip, creating ±5V with DC input.
Figure 1: Circuit Diagram
Then I also learned that size of capacitors are enough as long as they are more than what datasheet requires, from searching through StackExchange
How do I decide what capacitor to use in a circuit?
Figure 2: Datasheet from L78xx and L79xx
I choose 220uF, 50V. This was the biggest I have right now.
Figure 3: Test on breadboard and DC adaptor
I used DC 12V as an input.
I tested L7805 and L7905 individually, to check whether they are producing voltage +5V and -5V, respectively.
Figure 4: Individual regulator test
From voltmeter reading, I get that individual operation is fine.
Then I tried the whole circuit diagram (Figure 1), and I don't get the voltage properly.
Figure 5: Whole circuit test
As can be observed from Figure 5, L7905 produce voltage of -5, but L7805 doesn't.
I replaced L7805 with other L7805 to see if that L7805 was damaged, but it wasn't.
I searched more on Stack Exchange, and found one,
Incorrect Output From 7805
but it doesn't solve my problem.
Then I found an website, that suggests I need more input voltage.
I changed to DC adapter of 18.75V with 3.15A as an input, however, it doesn't solve my problem either.
Was it had to do with wrong(?) circuit diagram (Figure 1) from the very beginning?
Best Answer
The circuit above has a floating ground and the supply feeds both. This is not the way these dual voltage regulator circuits are built, they usually use two supplies, a positive and negative supply, then regulate it down to the voltage needed.
The problem with regulators is they have dropout and need more voltage on the input terminal than what they regulate to on the output terminal. With a positive regulator, this is easy since most of the voltages we have are positive, you have a 12V source and regulate down to 5V. With the negative regulator, you'd need a -12V (or at least roughly -5.5V).
So either find a dual supply input (like ±12V) OR do this:
These regulators are 7805 drop in compatible and are DC to DC regulators, they have little loss and can generate a negative rail from a positive one.