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I'm just a hobbyist and I'm planning to build this circuit. It recommends to use a 33pF 63V capacitor. I'm in trouble to find a capacitor with this specific voltage. I only found a 33pF 50V one.

Since the power supply will be 20V, a lower voltage capacitor should not be a problem, right? What kind of problem I will have if I use a 33pF 50V instead of a 63V?

Searching the forum, I found this question, but it has two answers and each one points to one direction...

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You are correct. Generally speaking, capacitors must not be subjected to voltages higher than what they are specified for. In practice, one always chooses a capacitor with voltage rating somewhat in excess of the highest voltage the capacitor might be exposed to. For example, I would choose a 63V capacitor for a circuit running at 45V.

Your circuit is running at plus minus 20V. A capacitor voltage rating of 25V is marginal, so I would choose a capacitor voltage rating at 35V or more. Your 50V capacitor will be just fine.

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    \$\begingroup\$ Also check the manufacturers derating figures as the voltage withstand decreases with temperature \$\endgroup\$ – JonRB Feb 7 '15 at 20:34

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