Why Is My 470uF Electrolytic Cap More Like 20uF?

Why Is My 470uF Electrolytic Cap More Like 20uF? post thumbnail image
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The straightforward capacitor equal circuit taught at school

Inductors are extra like a resistor in sequence with a perfect inductor, resistors may be inductors as effectively, and effectively, capacitors aren’t simply merely a capacitance in a package deal. Little with electronics is as plain and easy in actuality as fundamental concept would have you ever imagine. [Tahmid Mahbub] was measuring an electrolytic capacitor with an LCR and seen it measuring 19 uF despite the device being rated at 470 uF. This was as a result of such components are often specified at low frequencies, and at a mere 100 kHz, it was measuring approach out of the specification they have been anticipating. [Tahmid] goes right into a good bit of element concerning the best way to mannequin the equal circuit of a typical electrolytic capacitor and the best way to decide with a bit extra accuracy what to anticipate.

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An aluminium electrolytic capacitor is extra like this

The fundamental equal circuit for a capacitor has a sequence resistance and inductance, which covers the connecting leads and any inside tabs on the plates. A big-valued parallel resistor fashions the leakage via the dielectric in sequence with the best capacitance, which is answerable for the capacitor’s self-discharge property. Nevertheless, this mannequin continues to be too easy for some use instances. A extra fascinating mannequin, proven to the left, contains a ladder of distributed capacitances and related resistances that end in a progressively longer time-constant part as you progress from C1 to C5. This resembles extra carefully the linear construction of the capacitor, with its rolled-up development. This mannequin is tough to make use of in any sensible sense because of the want to find out values for the parts from a bodily half. Nonetheless, it’s helpful to know why such capacitors carry out far worse than you’ll count on from only a easy equal mannequin that appears on the connecting leads and little else.

To get actual solutions with a sensible utility, if key knowledge should not printed, it’s essential to characterise your components, which is precisely what [Tahmid] did subsequent. Utilizing the wonderful Digilent Analog discovery device, a measurement setup was constructed to find out the reactance versus frequency attribute and make an estimate for the paralleled capacitance and inductance parts. Straight off the display screen, the reactance can dip sharply at round 40 kHz, equivalent to the self-resonance frequency. That is the frequency it is advisable steer effectively away from if you wish to get anyplace near the desired capacitance in your utility. Since [Tahmid] was designing a switched-mode buck converter, which operated round 80-200 kHz, utilizing this half within the enter circuit can be dangerous information – the specified 23 mVpp voltage ripple can be extra like 463 mVpp, and that’s simply not going to work nice. Time to softly place the electrolytic components again within the components bin to await a distinct utility.

The not-so-humble capacitor is a way more sophisticated half than it seems at first look. Total books have been written about them, however for a fast historical past tour, check this out first. Wish to make your personal electrolytics? No Problem. What about when they go bad?

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