The caps surrounding the LM7812 regulator?

[Kevin Weddle]

No. Read the datasheet.
A 7805 voltage regulator keeps the output voltage very close to 5V if the input voltage changes from 7.5V to 30V and/or if the load current changes from zero to 1.5A.

That's what I said. It regulates 5v with a 7.5V to 30V input. So this is what it must focus on because ....

The output resistance is spec'd on the datasheet at 0.008 ohms for a 7812 an is less for a 7805.

There is no load that can compete with that low of resistance. That means the load is nothing to the regulator up to 1.5A.

 

[audioguru2]

There is no load that can compete with that low of resistance. That means the load is nothing to the regulator up to 1.5A.

The actual output resistance is about 1 ohm. The high gain and high amount of negative feedback make it function the same as if its output resistance is only 0.008 ohms.

With a load of 1.5A then its output voltage would drop 1.5V across its actual reistance of 1 ohm. The gain and negative feedback cause its output to rise almost 1.5V so the voltage drop is only 12mV.

 

[Kevin Weddle]

Okay. I think we weren't on the same page. About the design, the circuit values are chosen around an ideal circumstance. Do you think it was designed around an open load, or a mid value of about .75A. I wouldn't choose a 5A regulator over a 1.5A regulator for a 1A application.

 

[audioguru2]

The load current doesn't affect the voltage regulator circuit. It works perfectly with zero current up to max current.
It also works perfectly with minimum input voltage up to its max input voltage.

 

[Herman the German]

The datasheet says the input capacitor must be used and when. It also says the output capacitor stops certain problems. They are small and inexpensive so why not use them?


The datasheet says "The input voltage must remain typically 2.0V above the output voltage" but some ICs need 3.0V. At 1A, the dropout voltage is typically 2.0V but then it is not regulating. It is typically a little less at lower currents.

Omit them if you like it hot! Like any oscillation (mechanic or electronic) it produces heat! Of course the poor regulator will some time die because of the tremendous heat inside. You can accelerate this process also omitting a heat sink.

Concerning the input voltage for a stable output three Volts is the best figure. Due to the "typically" built-in error two Volts will sometimes work, sometimes won't work. Electronically spoken this device will be a "perhaps-gate" using two Volts above the desired output voltage.