# LM78XX imput/output capacitors

• posted

Dear All,

Sorry if this is a basic question. I deed search the web for an answer, but could find one.

I'm designing a power supply with various outputs (+24, +15, +10, -15,

-5), using LM78/79XX line regulators. The datasheet states that a .1 uF capacitor should be placed at the output to improve transient response, and a .22 uF at the input, if the regulator is far from the filter capacitor. But when I look at designs published in journal and such, I found all kind of variations, including different values for the input cap., even if the regulator is close to the filter capacitor (in a PCB), and usually an additional electrolytic capacitor at output.

So I guess my question are: 1 - do I need a capacitor at input and what is is value? (and is purpose).

2 - Do I need an additional electrolytic capacitor at output, besides the 0.1uF, and how do I calculate is value.

If anyone knows of a bibliographic reference ware this things are explained, I would appreciate.

Regards

Jos=E9 Mariano

• posted

Hello,

these are minimum values.

the input capacitor doesn't replace the filter capacitor.

for the filter capacitor, take 1000uF per Ampere for 1 volt input variation.

for the output, put big value if you want low impedance...you can add a 220 nF plastic capacitor cause it is better at high frequencies.

Regards

Vincent

"jmariano" a écrit dans le message de news: snipped-for-privacy@r34g2000hsd.googlegroups.com...

Dear All,

Sorry if this is a basic question. I deed search the web for an answer, but could find one.

I'm designing a power supply with various outputs (+24, +15, +10, -15,

-5), using LM78/79XX line regulators. The datasheet states that a .1 uF capacitor should be placed at the output to improve transient response, and a .22 uF at the input, if the regulator is far from the filter capacitor. But when I look at designs published in journal and such, I found all kind of variations, including different values for the input cap., even if the regulator is close to the filter capacitor (in a PCB), and usually an additional electrolytic capacitor at output.

So I guess my question are: 1 - do I need a capacitor at input and what is is value? (and is purpose).

2 - Do I need an additional electrolytic capacitor at output, besides the 0.1uF, and how do I calculate is value.

If anyone knows of a bibliographic reference ware this things are explained, I would appreciate.

Regards

José Mariano

• posted

vincent.thiernesse TOP-POSTED [using Outlook Express]:

For someone who has posted so many times,

you are STILL posting like a rookie.

Hasn't anyone ever mentioned that you should get a better tool

*-use-Micro.oft-programs&strip=1#quila ...or learn to configure and use the one you have properly?

• posted

If the LM78XX regulator can see an inductive input line it is likely to oscillate. You want a low impedance capacitor right at its input terminals. The values in the data sheets are minimums based on several assumptions about how the rest of the system looks. They are usually enough.

Another reason to put a larger capacitor is that logic circuits often draw current in narrow pulses. A capacitor to ground and some impedance in the input wire can stop these pulses from making their way up the power wiring.

One thing that people often overlook is that nearly all capacitors cost about the same by time they are installed. Using a low number of different values can often save you more than the price differences.

I normally put added capacitance at some small distance from the output. The purpose is to provide a low impedance path to ground for the supply. The output impedance of a 78XX rises at high frequencies. I prefer the impedance to fall.

• posted

Yes, you should have the capacitors. Their purpose is to keep the regulator stable and stop it from oscillating.

Not to keep the regulator stable, no. You may need additional capacitors on the output located elsewhere on the board to provide localised decoupling to those parts of the circuit.

Be aware that "low dropout" voltage regulators are a different beast and have fairly critical input and output decoupling requirement, this will be explained in the datasheet for each device. Standard regulators like the 78xx series are much more tolerant to this.

You might see extra caps in some designs because people tend to get overzeleous in use of decoupling caps. Nothing wrong with that apart from extra cost and board space.

This might get you started:

Dave.

• posted

"JeffM" a écrit dans le message de news: snipped-for-privacy@w3g2000hsg.googlegroups.com...

What is this, a rookie ?

*-use-Micro.oft-programs&strip=1#quila

• posted

"JeffM" a écrit dans le message de news: snipped-for-privacy@w3g2000hsg.googlegroups.com...