Designing a Voltage Regulator Circuit w/ LM7805

[ws0619]

Hi!

I'm working on designing my own voltage regulator circuit by using LM7805.

From what I found from website the capacitors connected parallel between common leg and input and common leg and output,its values varies from one website to another.

Actually how to determine what capacitor should be used on the circuit?Can someone teach me?What is the function of those capacitors? For capacitor between common leg and input is greater than capacitor between common leg and output,why?Is it due to the voltage on input and output?

Thanks!

 

[berkeman]

Hi!

I'm working on designing my own voltage regulator circuit by using LM7805.

From what I found from website the capacitors connected parallel between common leg and input and common leg and output,its values varies from one website to another.

Actually how to determine what capacitor should be used on the circuit?Can someone teach me?What is the function of those capacitors? For capacitor between common leg and input is greater than capacitor between common leg and output,why?Is it due to the voltage on input and output?

Thanks!

Input and output capacitors on voltage regulators are generally there for stability and to minimize transient variations. The datasheet for each regulator should give you guidelines on the values and any other specs (like equivalent series resistance ESR of the caps).

The output caps on low-dropout linear regulators are often important for stability, and both the input and output capacitors are often important for switching regulators (generally the input caps are important for stability, and the output caps with low ESR are important for low output ripple).

 

[vk6kro]

Neither capacitor is very critical.

The input one adds a little to the filtering of the previous power source, but it must also be a good bypass at high frequencies for stability reasons. So, 0.1 uF non electrolytic would be OK.

The output one also has to be a good bypass at high frequencies. 0.1 non electrolytics would be OK here too. Miniature 0.1 uF monolithics seem to be best if you can get them.

For some reason, negative regulators like the 7905 can oscillate readily if you don't have adequate HF filtering on their input and output. 7805s are not as bad.

It is important not to have too big a capacitor on the output or it will hold the 5 volts when you remove the power supply and this would be a voltage reversal on the regulator chip.
Some chips can fail if you do this.

The regulation of the device can also suffer if you put a big capacitor on the output. The chip can't sense changes as quickly as if it had a small capacitor there. Again, 0.1 uF is usually OK.

 

[MATLABdude]

I think the TI reference design calls for something on the order of a few dozen or few hundred nF. For a 7805, I generally just use 10 or 100 uF electrolytics, which helps keep the input and output fairly smooth (even with not so great harvested wall warts).

EDIT: I should point out that my experience is in fairly non-critical hobbyist-type applications.