78 series reg input cap values - Page 1
Forum section: electronics / beginners

author: m3vuv
author_rank: Super Contributor
timestamp: March 21, 2022, 09:28:53 am
content: Hi all,i have a 7812 at the end of some wires about 30cm from the power feed,a .33uf cap is recomended on the input,i only have 1 or 2.2uf,is that ok,i already have a 10nf for the output,TIA.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 21, 2022, 09:50:41 am
content: From the datsheet "Output Capacitor Not Needed for Stability"You will be ok with your values.

author: m3vuv
author_rank: Super Contributor
timestamp: March 21, 2022, 09:53:03 am
content: yes i have read up on it and chosen the 2.2uf input and 10nf output.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2022, 10:00:22 am
content: An electrolytic of about that value, wouldn't be unwelcome.The 30cm run corresponds to about 300nH inductance, which can resonate with device capacitance to make an oscillator (I forget if they're prone to oscillation with a resonant circuit at the input, or output -- you can indeed make e.g. AM transmitters, abusing them in this way), or with high-Q capacitors to oscillate or ring at lower frequencies. In particular, a ceramic of 1uF would resonate at 290kHz (or more due to C(V) effect*) with an impedance of 0.5 ohm, but have much much less than 0.5 ohm of ESR, so would be prone to resonate quite a lot, when so excited. This is also an inrush hazard, if hot-plugging is allowed: the regulator sees a peak of at least twice the applied voltage, as the capacitor charges suddenly to the input voltage then overshoots due to this ringing. (Which is also exacerbated with nonlinear C(V) type ceramics.)*Ceramic capacitance drops with rising terminal voltage, i.e., capacitance is a function of applied voltage, C(V). How much, depends on part type and size; preferably, choose parts with this characteristic in the datasheet, or on MFG's database.Whereas using an electrolytic of -- probably in that value, ESR a few ohms? -- guarantees a dull "thud" startup with no resonance.Tim

author: mariush
author_rank: Super Contributor
timestamp: March 21, 2022, 10:01:53 am
content: The values are some suggestions, they're not obligatory values.You can use higher capacitance, it's just not gonna improve the performance of the component in a significant way so they say a 0.33uF is enough.It's good practice to have a decoupling capacitor right by the input voltage and ground pins, as close as possible to them.. a 0.01uF...0.1uF (10nF..100nF) ceramic capacitor. Ceramic capacitors are best for decoupling, they can handle higher frequencies compared to tantalum, electrolytic, or solid/polymer capacitors.Then, have a higher capacitance capacitor for "bulk energy storage", like they suggest, 0.33uF or more. I would just a common value you have around, like let's say 10uF..47uF 35-50vLong wires can act like an inductor, so a bulk capacitor on the input is good to have to supply energy to the regulator if/when there are sudden spikes in power consumption on the output.On the output, most linear regulators don't need capacitance, or recommend something small, like a 0.1uF ceramic capacitor or a small value electrolytic capacitor. You could just reuse the same capacitor you use on input, like for example a 10uF 35-50v rated capacitor. You don't have to use the minimum voltage rating your circuit needs (ex 16v rated capacitors for 12v output of your regulator, you can use capacitors with higher voltage rating)SOME regulators are picky about the technical specifications of the output capacitor, so pay attention to the details in datasheets.For example, 1117 LDOs are very common and made by lots of companies but some companies make a slightly modified version which is "stable with ceramic capacitors" and others make a version which require a capacitor with ESR value between 0.1 ohm and 1ohm on the output, so you can't just use a ceramic capacitor because those have ESR under 0.1 ohm - but you also have to be careful about what electrolytic capacitor you pick, because some have very low ESR. For example, a 100uF 6.3v rated capacitor may have ESR of around 0.3 ohm, but a 47uF 25v-50v rated capacitor from another series may have an ESR under 0.1ohm .... it depends on the chemical formulas used for electrolyte inside the capacitor.Older datasheets often recommend tantalum capacitors, like let's say 10-22uF tantalum because back then they had better performance compared to electrolytic capacitors, but nowadays electrolytic capacitors are much better and cheaper. So whenever a datasheet suggests a tantalum capacitor, pay attention to the reasons why.For example, back then tantalum capacitors had ESR value between around 0.1 ohm and 1 ohm, but low value capacitors like 10uF 16v electrolytic capacitors had much higher ESR, in the 3-5 ohm range. Nowadays, you can buy 10uF electrolytic capacitors with ESR under 0.5 ohm and solid / polymer capacitors which have ESR under 0.1 ohm.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2022, 10:25:49 am
content: Also, note that you can always increase the resistance, by adding a physical resistor. This works just fine with ceramic. The only cost is the added component itself, and its ESL if that should be important (which can be for low-impedance stuff like chip VCORE supplies, but not here).Also note that, between dry tantalum, tantalum polymer, and aluminum polymer types, a wide range of ESR is available. The latter skew lower (ESRs of 10mohm or less are readily available!), but both have overlapping ranges. So you can shop around quite effectively. The important thing is getting the right combination of values, not what it's made of.Or at least, the ranges had significant overlap when I last checked this some years back. It's probably terrible the last couple years, but that's an unrelated reason, everything's availability sucks these days.Tim

author: David Hess
author_rank: Super Contributor
timestamp: March 21, 2022, 08:23:05 pm
content: Quote from: m3vuv on March 21, 2022, 09:28:53 amThat will be fine. The input capacitor value and type are not critical.It is not strictly required but a 10 to 100 microfarad aluminum electrolytic capacitor or 1 to 10 microfarad solid tantalum capacitor on the output will improve the load transient response. A 0.01 microfarad ceramic on the output is not very useful and could cause problems. Decoupling capacitors should be located at the load and not the source.

author: tszaboo
author_rank: Super Contributor
timestamp: March 21, 2022, 09:35:51 pm
content: Quote from: m3vuv on March 21, 2022, 09:53:03 amNo. This is an old linear regulator, that doesn't work well, if at all, with ceramic capacitors. Either place a electrolytic capacitor, or select a different regulator.The ESR has to be higher than 0.1 Ohm or so.