Why use this Capacitor

[gjoo]

I don't understand why there is a 22uf capacitor on the input of lm294t. Did the designer add it because he already was using a 22 uf capacitor on the output?

 

[ZipZapOuch]

It's typically not needed but the designer was possibly expecting some significant surges in load or supply - or trying to bring output swings to a lower level.

 

[gjoo]

I was just curious about why the designer might use the same size capacitor on the input and output, could it be just convenience. Because, he could have used a lot of different values that could have done a similar job.

 

[AnalogKid]

Most switching regulators require a minimum mount of input and output capacitance. It is entirely possible that 22 uF was chosen/calculated for one side, and deemed good enough for the other side. Done that many times in many different kinds of circuits. For example, using the same cap value for both a 555 oscillator cap and its power supply decoupling cap.

ak

 

[Nigel Goodwin]

I don't understand why there is a 22uf capacitor on the input of lm294t. Did the designer add it because he already was using a 22 uf capacitor on the output?

Possibly, but I don't see any reason not to use a 22uF (or higher) on the input, as it provides a lower source impedance for the chip.

 

[gjoo]

Why not use a 10uf or 33uf

 

[AnalogKid]

Have you read the datasheet?

ak

 

[gjoo]

Yes. It says to put a 22uf capacitor on output only.

 

[AnalogKid]

It says to use 22 uF *minimum*. It also says that an input capacitor of a certain minimum value might be required depending on the wiring.

Note that the input cap recommendation also is a minimum value. A larger value is fine, but increases the energy storage to keep running the regulator after power is removed. A large input cap also is an energy store that can damage the circuit if a fault occurs.

My guess is that 22 uF is used because of parts commonality - that value already is used somewhere else. A 10 uF or 33 uF input cap should be fine, as long as it is in parallel with a smaller ceramic or film part with better high-frequency characteristics.

ak

 

[gjoo]

Thanks. That's what I was thinking.

 

[Nigel Goodwin]

Why not use a 10uf or 33uf

You could say that about almost every component on aboard

Designers choice, 33uF would probably be 'better', where 10uF may be 'poorer', but I suspect any of the three would be perfectly fine. As others have suggested, using an existing value means you have less stock to keep.

 

[crutschow]

using an existing value means you have less stock to keep.

I often use the same value resistor or capacitor more than once in the same circuit, if feasible, for that reason.

 

[gjoo]

Thanks for all the helt

 

[gjoo]

Help

 

[drtechno]

I don't understand why there is a 22uf capacitor on the input of lm294t. Did the designer add it because he already was using a 22 uf capacitor on the output?

even though you don't have to, it makes the regular work less, but the other reason they might is because they are splitting the unregulated voltage across different voltage regulators and the cap could be installed to prevent voltage sagging.

 

[ZipZapOuch]

I think they put it there so someone, somewhere, asks, "why is it there?".

 

[aliarifat794]

These are for filtering the input signal.