Filter Cap Tolerance?

Thread Starter

brightnight1

Joined Jan 13, 2018
75
When should filter cap tolerance be small and when does it not matter and you can have say a 20% tolerance for a filter cap? Is there a rule such as in general, the smaller capacitance the filter cap is, the tighter tolerance it needs to be?
 

Delta Prime

Joined Nov 15, 2019
1,026
Hello there :)
Rule of thumb is the higher the frequency, the smaller the filter capacitor or bypass capacitor you need. If you have very high frequency components in your circuit, you might consider a pair of capacitors in parallel. One with a large value, one with a small value. If you have very complex ripple, you may need to add several bypass capacitors. Each cap is targeting a slightly different frequency. You may even need to add a larger electrolytic cap in case the amplitude of the lower frequencys is too great
 

kennybobby

Joined Mar 22, 2019
75
But doesn't adding caps in parallel equate to a single cap with value equal to the sum of the values? then the largest value cap will dominate the filter and appear as a near-short at the high frequencies and negate any significant contribution by the lower value parallel caps? i have seen such parallel cap filters added in circuits and schematics, but it looks to me that they fail to actually perform the assumed filter action. Current takes the path of least impedance.
 

MrChips

Joined Oct 2, 2009
23,565
But doesn't adding caps in parallel equate to a single cap with value equal to the sum of the values? then the largest value cap will dominate the filter and appear as a near-short at the high frequencies and negate any significant contribution by the lower value parallel caps? i have seen such parallel cap filters added in circuits and schematics, but it looks to me that they fail to actually perform the assumed filter action. Current takes the path of least impedance.
In theory, yes. You add capacitors in parallel.
In practice, no. Capacitors have internal resistance and inductance. Smaller value capacitors are better at filtering higher frequencies. Capacitors are placed in parallel to cover different frequencies.

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dl324

Joined Mar 30, 2015
12,873
When should filter cap tolerance be small and when does it not matter and you can have say a 20% tolerance for a filter cap? Is there a rule such as in general, the smaller capacitance the filter cap is, the tighter tolerance it needs to be?
A typical tolerance for electrolytic capacitors is -20%/+80%. If you design assuming values are 20% less than nominal, you won't have problems.

When we design circuits, we typically take component tolerances into consideration, so we don't need to cherry pick parts or specify tight tolerances.
 

MrChips

Joined Oct 2, 2009
23,565
In most designs, ±20% is acceptable for capacitors. New high capacitance electrolytics are commonly +20% or higher.
 

Reloadron

Joined Jan 15, 2015
5,935
Decoupling Capacitors and Bypass Capacitors – Working, Applications and Differences is a good read on the subject. The link will give you what should be a good understanding of how they are selected and what they do. Taken from the link:

Value of the Decoupling Capacitor
Unlike Bypass capacitors there are not much riles to choose the value of a decoupling capacitor. As the decoupling capacitors are used widely there are certain standards for choosing the value.

  • The Low frequency noise decoupling capacitor value should be typically between 1 µF to 100 µF
  • The high frequency noise decoupling capacitor should typically value between 0.01 µF to 0.1 µF.
The exact value of the capacitors to be used is always provided with the ICs data sheet. The decoupling capacitors should always be connected directly to a low impedance ground plane for its effective operation.

Also note post #4 which is why it is not unusual to see decoupling for both high and low frequency noise removal side by side.

Ron
 
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