What does this tantalum cap do? Low pass filter - Page 1
Forum section: electronics / beginners

author: fourierpwn
author_rank: Regular Contributor
timestamp: June 06, 2018, 09:31:07 pm
content: As the title says, I want to understand what the purpose of the 10nF tantalum capacitor is. I have simulated this filter in LTSpice with and without the 10nF capacitor and the response does not change. So why is it there?FYI, this is a 1 Hz low pass filterThanks.

author: Audioguru
author_rank: Super Contributor
timestamp: June 06, 2018, 11:26:16 pm
content: A 1k resistor feeding a 10nF capacitor has a -3dB cutoff at 16kHz. I do not think a tantalum capacitor is made with a value as low as 10nF.A 10k resistor feeding a 100uF capacitor has a cutoff of 1.6Hz and three of these RC filters in series like you have make a very droopy 1Hz filter.But your image shows that the capacitors are actually two 100uF capacitors in series making 50uF and are in parallel with a 10nF capacitor making 50.01uF. Three of these RC filters in series make a very droopy 1Hz filter.The 100uF capacitors filter low frequencies and the 10nF capacitors filter high frequencies.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 07, 2018, 12:42:15 am
content: You did not model the capacitors properly. A capacitor has ideal capacitance, but it also has ESR, ESL and higher order parameters (which may or may not be relevant).Or worse still, LTSpice gave you default values for some of these parameters and the whole thing is nonsense!Needless to say, SPICE, in and of itself, only deals in ideal capacitance and resistance and inductance. It is your responsibility to build a model that is representative of reality. Good models are not simply given to you on a silver platter; hell, manufacturer's libraries are rarely even useful as it is...Tim

author: vk6zgo
author_rank: Super Contributor
timestamp: June 07, 2018, 12:50:08 am
content: Quote from: fourierpwn on June 06, 2018, 09:31:07 pmLTSpice works with ideal components, so the difference between 50uf & 50.01uf is not going to be discernible, so that raises the question:-"Why do they use it?"Real electrolytic capacitors arenotideal components, however, & as well as capacitance, exhibit inductance & resistance, which makes their real characteristics at higher frequencies somewhat unpredictable.The idea of the lower value capacitor of another type in parallel is that, at those higher frequencies, they look more like an ideal capacitor, & effectively bypass the electrolytic at those frequencies.

author: fourierpwn
author_rank: Regular Contributor
timestamp: June 07, 2018, 02:50:44 am
content: Thank you to all for the detailed responses. I was assuming it was for higher frequency spikes as this filter is attached to mains operated equipment which by simulating with an ideal source in SPICE is not going to generate the interference that is present in the real world.

author: tecman
author_rank: Frequent Contributor
timestamp: June 07, 2018, 02:48:38 pm
content: Also if you want to implement this as a 3 pole filter, rule of thumb is to have each stage be 10:1 in value from stage to stage. So first R is 1K, the next R is 10K and the third is 100K, with the C's also cascaded by 10:1. The issue becomes the output impedance.paul