Decoupling cap selection

Frequently asked questions on capacitor types, ratings, brands, use and abuse.

Decoupling cap selection

Postby boblob » 04 Aug 2014, 11:28

Hello.

Just finished a Vorg Warp Sound (op-amp based eq. pedal) build and it made me think of this...

The Vorg has two 10k resistors forming as voltage divider between +9V and ground to provide a reference voltage, and a 33uF electro for decoupling at the junction. I didn't have any 33uFs on hand, and so subbed with a 100uF.

This got me thinking - what design criteria do you consider when selecting decoupling caps in general? I'm guessing the sub of a 33 for 100uF will work fine, but most designs have a smaller cap there. Why is that?
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Re: Decoupling cap selection

Postby Nocentelli » 04 Aug 2014, 12:54

From a design point of view, decoupling caps need to be big enough capacitance to sink AC hum down to very low frequencies (e.g. 60Hz mains hum), so a large electro cap of 100uF should be plenty to remove ripple from the +9v, but not be so physically big it's a problem finding space for it on the pcb. I assume most designers reason that since vref is usually around half the voltage of supply, half the capacitance is needed to remove ripple, so 100uF across supply and 47uF across vref is common. The cap also has a role in acting as a reservoir of charge to stabilise the circuit as the demand for power fluctuates, lower values down to 10uF are common for vref.
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Re: Decoupling cap selection

Postby boblob » 04 Aug 2014, 15:15

Yes, I get that the 10k and cap form a low pass filter, but plugging in the numbers gives a corner freq of < 1Hz - not sure if that is going to help much with mains hum @ 50-60Hz.

Doing a bit more reading and playing with the circuit, it certainly removes some high frequency hiss, which suggests it's doing more of the stabilisation thing. I see a lot of digital guys use a smallish value electro (2.2uF) and another 100nF ceramic/film in parallel to deal with the higher freq content, but not a lot on why those values are chosen.
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Re: Decoupling cap selection

Postby Nocentelli » 04 Aug 2014, 15:37

boblob wrote:Yes, I get that the 10k and cap form a low pass filter, but plugging in the numbers gives a corner freq of < 1Hz - not sure if that is going to help much with mains hum @ 50-60Hz


It's a lowpass filter so it only allows AC of lower than 1Hz to remain, 50-60Hz is above the cut-off so gets filtered out. Think of it as a maxed-out treble control that rolls off any AC signal in the vref right down to sub-audible frequencies.

I think smaller value ceramic or box caps are supposed to be better than electros at killing potential high frequency oscillations (even though the electro on it's own *should* filter them out), so the two are sometimes used in combination.
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Re: Decoupling cap selection

Postby boblob » 04 Aug 2014, 16:18

Duh! Yes, of course. Being dumb.

Apparently the 100nF is useful in logic circuits - stops switching transients interfering with the power supply, potentially adding noise to other parts of the circuit that share the power supply. Probably not massively useful in analog pedals, but sounds like a good idea to have built into a power supply. I have a cheap power supply with non-isolated grounds that does a marvellous job at transmitting an unpleasant digital wash from my Boss TU-3. Wonder if adding a 100nF would kill that. I know the Beavis Dehuminator has just that.

Anyhow, I think this pretty much answers my original question - the 100uF will be fine to sub for the 33uF.
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Re: Decoupling cap selection

Postby DrNomis » 04 Aug 2014, 18:12

boblob wrote:Duh! Yes, of course. Being dumb.

Apparently the 100nF is useful in logic circuits - stops switching transients interfering with the power supply, potentially adding noise to other parts of the circuit that share the power supply. Probably not massively useful in analog pedals, but sounds like a good idea to have built into a power supply. I have a cheap power supply with non-isolated grounds that does a marvellous job at transmitting an unpleasant digital wash from my Boss TU-3. Wonder if adding a 100nF would kill that. I know the Beavis Dehuminator has just that.

Anyhow, I think this pretty much answers my original question - the 100uF will be fine to sub for the 33uF.



Providing that you have room for it on the circuit board substituting a 100uF cap for a 33uF cap will be perfectly fine, the rule of thumb to remember is that the bigger the value of cap, the better the filtering, the only caveat is that the working voltage of the cap needs to be equal or greater than the expected supply voltage..... :thumbsup


Just thought I'd add my two cents worth...... :thumbsup
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