I am trying to make simple voltage regulator circuit from my 9V battery to 5V using 7805c voltage regulator. The schematic looks like this:


simulate this circuit – Schematic created using CircuitLab

My question is - why the output voltage is so low? Did I fried somehow IC or I messed something with values.

One more extra question: Checking a lot of different schematics to build my circuit I found a lot of different values for each of C. So how they should be chosen? Is there any formula for that, or something?

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    \$\begingroup\$ What do you measure for the battery voltage? \$\endgroup\$ – Spehro Pefhany Nov 17 '14 at 21:25
  • \$\begingroup\$ I measure around 7.5V \$\endgroup\$ – sebap123 Nov 17 '14 at 21:30
  • \$\begingroup\$ Your battery is nearly flat. Even still, you should be getting closer to 5V at the output with no load. \$\endgroup\$ – Dave Tweed Nov 17 '14 at 21:55
  • \$\begingroup\$ I know that and this is why I am asking this question. According to documentation even for 5V there should be around 5V on output. \$\endgroup\$ – sebap123 Nov 17 '14 at 22:08
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    \$\begingroup\$ It is a 7805 and not a 78M05, right? I have a distant memory of seeing different pinouts for the 78M05 even in the same package... \$\endgroup\$ – Brian Drummond Nov 17 '14 at 22:19

If you have not mixed up input and output, the chip is toast. You don't need to connect to the tab, but IN/GND/OUT should be as shown below.

enter image description here

It's fairly hard to kill these things with a 9V battery though, so I'd suspect a wiring error first.

  • \$\begingroup\$ What should be the value when I'd connect battery without any capacitor? Just direct connection to chip. I guess that is the last possibility to check it. \$\endgroup\$ – sebap123 Nov 18 '14 at 8:25
  • \$\begingroup\$ It should give you 5V with no capacitors if you have more than about 7V in - just voltmeter and 7805. Unlike most more modern parts the 7805 won't do strange things when you omit the caps. \$\endgroup\$ – Spehro Pefhany Nov 18 '14 at 12:51

The regulator takes power to run. Many of them are switching, and reply on the output capacitor for stability. A 9V battery that is exhausted won't deliver enough of anything to run it.

Use a new battery.

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    \$\begingroup\$ 78xx and 79xx family are not switching but liner 3 terminal regulators. \$\endgroup\$ – GR Tech Nov 18 '14 at 6:33
  • \$\begingroup\$ Sorry, I was thinking of the LDO regulators that will start to switch if there is no capacitor on the output (feedback instability). Like the NS LM2937. I have seen 78xx from ON behave that way too. I stand corrected. \$\endgroup\$ – SpacemanScott Nov 19 '14 at 16:36

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