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View Full Version : Tolerances in the Capacitors for Voltage Regulator Circuits



Martin_H
09-03-2011, 07:16 PM
I'm a hobbyist and often need voltage regulator circuits which require a capacitor before and after the regulator. I am wondering how much leeway I have with capacitor specifications for regulator circuits between 3.3 to 12 volts. The reason I ask is that Radio Shack often has regulators, but not the capacitor with the required specifications. While you can mail order, the shipping charges or minimum order size can be prohibitive for one or two parts.

Can I substitute a capacitor with the same capacitance, but a higher voltage rating for one with a lower rating?

Can I substitute a capacitor with a higher capacitance for a lower one (e.g. 100 uF for 47 uF)?

Phil Pilgrim (PhiPi)
09-03-2011, 07:23 PM
Can I substitute a capacitor with the same capacitance, but a higher voltage rating for one with a lower rating?
Usually, yes, unless the ESR value gets too high for the regulator's specs.


Can I substitute a capacitor with a higher capacitance for a lower one (e.g. 100 uF for 47 uF)?
Sometimes, with the same caveats as above. You really have to look at the regulator's datasheet to see what the capacitor requirements are and then follow them to a "T". Also be aware of the capacitance tolerances, sometimes as bad as -20% to +80%, and design for the worst case.

-Phil

Martin_H
09-04-2011, 02:42 AM
OK Thanks Phil. I read through the datasheet and if I read it right the schematic I was following was already using capacitors with a higher capacitance than suggested. That's probably because of the bad tolerances you mentioned. In any event it looks like 100 uF is out of the question, but a lower value around 20 uF will probably be fine.

Loopy Byteloose
09-04-2011, 06:19 AM
Generalizations can be hazardous as different voltage regulators have different means of filtering ripple AND some are quite sensitive to the discharge of a large capacitor on the output side. Specifically, the 78xx series can in certain situations suffer damage from voltage collapse back flowing through the regulator if the capacitor is above 10uf or so and no protective diode is used. On the other hand, there are low dropout regulators that need capacitors of above certain size in the right positions to properly function.

So it seems that it is always best to read the particular components PDF and not just think that a larger capacitor with improve performance. Also note where and why protective diodes are mentioned.

Personally, if I am going to use a linear regulator, I try to find one that has all the neat features - outputs 1 amp or more, thermal shutdown, shortage protection, reverse polarity input protection, and low drop out voltage. The LM2940 regulator series seems to be a good choice -- but read the PDF for capacitance requirements and compare it to the 78xx series, very different.