0
\$\begingroup\$

I want to know how you can find a decent estimation for the value of a decoupling capacitor without the need of experimentation. I don't want an equation, I'm just wondering if there is a simple rule of thumb for it.

Can you possibly show it in a circuit with the 5V 1.5A Linear Voltage Regulator - 7805 TO-220 from Adafruit as that is what I am going to use it for, but it is okay if you use your own example.

Also if possible can you tell when to use a resistor paired with the capacitor, but this is secondary.

\$\endgroup\$
3

2 Answers 2

2
\$\begingroup\$

I'm just wondering if there is a simple rule of thumb for it.

There is not.

If you have all the information the chip designer has, you can run a simulation to find an acceptable value. That's why we normally look to the datasheets for our chips (which are written by the companies that designed the chips and therefore have the necessary information) to tell us the appropriate decoupling capacitor.

7805 TO-220 from adafruit as that is what I am going to use it for

In the datasheet for the 7805 (TI version) it says this:

enter image description here

So the output capacitor is not required, but it will improve the transient response to add a 0.1 uF ceramic disc capacitor on the output.

Also if possible can you tell when to use a resistor paired with the capacitor

When the datasheet tells you to.

Assuming you're asking about output capacitors on linear regulators, this is sometimes required to maintain stability of the regulator. Newer chips (designed after 2005 or so) will tell you this is required in the datasheet.

For older chips (like the 7805) the datasheet may have been written before capacitors with low enough resistance to cause a problem were available. For those it's better to stick with relatively large capacitors such as were available when the chip was designed rather than use the smallest size SMT capacitor that is available today. So stick with 0603 size (EIA) for 1 uF or 0805 for 10 uF, or even use a THT capacitor as recommended in your 7805 datasheet.

\$\endgroup\$
0
1
\$\begingroup\$

Generally, you don't estimate, you read the datasheet and place what it says. From Adafruit's website

This regulator does not require capacitors for stability, but we recommend at least 10uF electrolytic capacitors on both input and output.

Though that's not really decoupling capacitance as much as bulk capacitance. Let's say you're powering a 741 - that calls for a 0.1uF (100nF) decoupling cap as close to the power supply pins. In the case that a datasheet doesn't specify, then 100nf ceramic is my go-to.

The second part of your question:

Also if possible can you tell when to use a resistor paired with the capacitor,

It's not clear to me what you mean by this. Are you referring to having a resistor in series with a capacitor? This is often done to stabilize a filter, I've not seen it done as part of decoupling.

\$\endgroup\$
2
  • 2
    \$\begingroup\$ LordTeddy - Hi, FYI I know you're quoting Adafruit's website, but that text actually disagrees with the manufacturer's datasheet! The Adafruit webpage says: "This regulator does not require capacitors for stability" but the actual ST datasheet linked on the same webpage gives more details & says that the input capacitor is "Required if regulator is locate{d} an appreciable distance from power supply filter" (Figure 8, footnote 3, page 28). (Same on equivalent TI datasheet). \$\endgroup\$
    – SamGibson
    Commented Nov 22, 2023 at 23:16
  • \$\begingroup\$ I've seen a 7805 regulator turn into oscillator since it did not have capacitors on input which was located after appreciable distance from power supply. So clearly Adafruit web page is wrong. And series resistors are used in series with capacitors to tune the regulator output control loop, or to damp oscillations when you plug in a cable, when using modern capacitors with really low ESR. \$\endgroup\$
    – Justme
    Commented Nov 23, 2023 at 6:46

Not the answer you're looking for? Browse other questions tagged or ask your own question.