# ±5V using Voltage regulator L7805 and L7905, from DC input

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.

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?

• The schematic you've drawn will work sort of okay, as long as no net current flows in the ground. Otherwise your neutral point will drift because of the capacitors you're using to clamp it. It's not really a design I'd recommend using. Mar 12, 2019 at 17:19
• Large-value resistors across the neutral-point clamping capacitors will help, though. The smaller the resistors, the more power wasted but also the more current imbalance can be handled without losing regulation. Mar 12, 2019 at 17:21
• @Hearth is there any recommendation on the circuit diagram using DC input? or do I have to figure out how to use AC input? Mar 12, 2019 at 17:21
• If you want to use DC input and have a usable bipolar power supply, you want a switching converter, not linear regulators. Using AC input would be the easiest method for a beginner. That doesn't have to be 110V AC, though; you can get wall-wart transformers that output just 12V AC to your board, for instance. Mar 12, 2019 at 17:23
• Not only does this design smelly fishy, the reality is that building your own power supplies is rarely worth the trouble - the reasons to do it would be if you had a very unique requirement (which you do not) or you were studying power supply design in which case you would be looking at classic architectures, or you were designing the power supply of a product, which this is far from. Buy something suitable (at the very least something with true positive and negative outputs, even if you then need to regulate more) and put your effort elsewhere. Mar 12, 2019 at 17:36