Filtering Capacitor Voltage and Placement

[Max Holdcroft]

Hi All,

This is probably a very easy question, but if I have a 3.7v lipo battery attached to a voltage regulator outputting 3v, and I have two 100nF filtering capacitor to filter out noise before and after the regulator, are these sufficient to filter out the noise. The approach I'm doing is probably completely wrong, but I'll really appreciate any advice.

Thanks,
Max

[CountChocula]

Howdy! A few things to note:


• If you're using a linear regulator and you're getting voltage from a battery, you're unlikely to need much filtering, since these tend to be very quiet devices. If you really want to filter, the capacitors simply connect the input rail to ground, like your C4. For decoupling purposes, it's good practice to place multiple caps of various values to block off different frequencies. You will also need to keep C3, which provides bulk on the output of the regulator.
• In general, you should not place capacitors in series with a DC path like C1 and C2 in your diagram. Capacitors block DC, and so you won't be able to get any current flowing!
• When working with a linear regulator, keep in mind that these devices need “headroom” between input and output voltage to regulate properly. For example, if you read through the TC1262's datasheet, you will see that it needs up to 400mV of headroom to regulate properly when loaded with 500mA. Even with a fully charged battery, you won't be able to get a stable 3.3V in output for very long… if at all. This may or may not be a problem for your circuit—I can't tell—but it's something to think about.

Also, please consider trying to make your diagrams a bit more consistent—ground always pointing down, for example; it makes it easier for others to understand and make suggestion.

Cheers!


—CC

[Vovk_Z]

Your voltage regulator probably has a name and a datasheet? And its load too?
And I guess it's wrong to say that function of 100nF caps is a filtering.

[Max Holdcroft]

Howdy! A few things to note:


• If you're using a linear regulator and you're getting voltage from a battery, you're unlikely to need much filtering, since these tend to be very quiet devices. If you really want to filter, the capacitors simply connect the input rail to ground, like your C4. For decoupling purposes, it's good practice to place multiple caps of various values to block off different frequencies. You will also need to keep C3, which provides bulk on the output of the regulator.
• In general, you should not place capacitors in series with a DC path like C1 and C2 in your diagram. Capacitors block DC, and so you won't be able to get any current flowing!
• When working with a linear regulator, keep in mind that these devices need “headroom” between input and output voltage to regulate properly. For example, if you read through the TC1262's datasheet, you will see that it needs up to 400mV of headroom to regulate properly when loaded with 500mA. Even with a fully charged battery, you won't be able to get a stable 3.3V in output for very long… if at all. This may or may not be a problem for your circuit—I can't tell—but it's something to think about.

Also, please consider trying to make your diagrams a bit more consistent—ground always pointing down, for example; it makes it easier for others to understand and make suggestion.

Cheers!


—CC

Hey,
I really appreciate the detailed response! Thank you for making this forum so welcoming for beginners. I just have a couple clarifying questions for you. If I was to remove C1 and C2, would this schematic be properly filtered? I have heard a lot of conflicting information about this, but apparently the bluetooth module I'm using generated a considerable amount of noise that needs to be filtered out? Also, should I decrease the voltage of the regulator I have in the circuit? The bluetooth module datasheet says that the operating voltage is 1.8. - 3.6, so what is the highest regulator rating I could have to get a consistant voltage into the circuit? Again, thank you so so so much for your answer!
Max

[BrokenYugo]

Thou shalt read datasheets. Linear regulators sometimes need specific caps to be stable.

See page 3 sec 3.1. https://ww1.microchip.com/downloads/en/DeviceDoc/21373C.pdf

[Max Holdcroft]

Thou shalt read datasheets. Linear regulators sometimes need specific caps to be stable.

See page 3 sec 3.1. https://ww1.microchip.com/downloads/en/DeviceDoc/21373C.pdf

Hey, thanks for your response
I had read that and was under the impression that I had correctly implemented these capacitors, but I could be wrong. I've attatched a screenshot of my current schematic, and I'd appreciate it if you could help me figure out where I went wrong.

Thank you!
Max

[CountChocula]

If I was to remove C1 and C2, would this schematic be properly filtered? I have heard a lot of conflicting information about this, but apparently the bluetooth module I'm using generated a considerable amount of noise that needs to be filtered out?

It looks all right to me, though I don't know which module you're using, so I don't know if that requires any kind of additional filtering. The datasheet is your friend in these cases

Also, should I decrease the voltage of the regulator I have in the circuit? The bluetooth module datasheet says that the operating voltage is 1.8. - 3.6, so what is the highest regulator rating I could have to get a consistant voltage into the circuit? Again, thank you so so so much for your answer!
Max

Well, it depends (it always depends!). The question is: what happens when the battery's voltage drops below the minimum required by the LDO to regulate its output? I think the most likely outcome here is that you'll get lower-than-expected voltage on the output, but, given that your device can work all the way down to 2V, you'll probably be okay.

However, you do need to worry about depleting the battery down to the point where it becomes damaged. If your battery already has a protection circuit, then it will automatically cut off before that happens, but if it doesn't you have to either be careful and never let it deplete to that point, or add some protection circuitry of your own.


—CC