How to Choose the Right Capacitor for Your Circuit

Capacitors Are Not Interchangeable

Picking a capacitor by capacitance value alone is how you end up with a circuit that oscillates when it should stabilise, or a power supply that hums when it should be silent. The capacitance in microfarads printed on the side is only one of six specs that matter.

This guide covers the four capacitor types you will actually use on a workbench—ceramic, aluminium electrolytic, tantalum, and film—and when to reach for each one.

The Five Specs That Matter

Capacitance

Measured in farads (F), but you will work in microfarads (µF), nanofarads (nF), and picofarads (pF). The value tells you how much charge the capacitor stores at a given voltage.

Voltage Rating

The maximum DC voltage the capacitor can handle continuously. Rule of thumb: derate to 80% of the rated voltage for reliable operation. A 25 V capacitor on a 24 V rail is working at the edge. A 35 V capacitor on the same rail has headroom.

Temperature Coefficient and Dielectric

Class 1 ceramic capacitors (C0G/NP0) have a tolerance of ±30 ppm/°C—essentially zero drift. Class 2 ceramics (X7R, X5R, Y5V) shift capacitance by 15% or more across their temperature range. For timing or filter circuits you must use C0G or film.

ESR (Equivalent Series Resistance)

Every capacitor has internal resistance. Low ESR is good for power supply filtering because less energy gets turned into heat. Aluminium electrolytics have the highest ESR. Ceramics have the lowest.

Capacitor Types and When to Use Them

Ceramic Capacitors (MLCC)

These are the workhorses of modern electronics. MLCCs cover 1 pF to 100 µF in tiny SMD packages. Very low ESR, handle high frequencies well. Use for decoupling every IC power pin and high-frequency filtering. A 10 µF X7R capacitor at 5 V DC bias actually measures closer to 6 µF—that DC bias effect catches everyone at least once.

Aluminium Electrolytic Capacitors

High capacitance (1 µF to 2,700,000 µF) in a reasonable size. Polarised—install them backwards and they vent electrolyte gas. Highest ESR, shortest lifespan (2,000–10,000 hours at rated temperature). Use for bulk power supply smoothing. Avoid for high-frequency circuits.

Tantalum Capacitors

Tantalum capacitors pack more capacitance per volume than aluminium electrolytics and have much better stability over time. They are polarised and fail spectacularly when reverse-biased—they catch fire rather than just leaking.

Film Capacitors

Non-polarised, very stable, excellent low-loss characteristics. Polypropylene film capacitors have dissipation factors as low as 0.0005 at 1 kHz. Use for audio crossover networks, snubber circuits across switching transistors, and high-voltage applications up to 2,000 V.

Decision Flowchart

Under 1 µF → ceramic (C0G for accuracy, X7R for general). 1–100 µF decoupling → ceramic X7R. 1–100 µF audio → film. Over 100 µF power smoothing → aluminium electrolytic or tantalum. High ripple current → check the ripple current rating. Reverse voltage possible → non-polarised only.

Common Mistakes to Avoid

Using an X7R ceramic in a timing circuit—the capacitance shifts 15% over temperature. Ignoring DC bias derating on ceramic capacitors—a 10 µF 0805 X7R measures about 4 µF at 12 V bias. Installing polarised capacitors backwards—aluminium electrolytics bulge and leak, tantalums burn. Running right at the voltage rating without derating—always leave 20% headroom.

Sourcing Capacitors in South Africa

TRX Electronics in Pretoria stocks a wide range of ceramic MLCCs (C0G, X7R, X5R), aluminium electrolytics, tantalum capacitors, and film capacitors from Murata, TDK, Panasonic, and KEMET. Contact our engineering support at +27 (0)12 997-0504 or browse the catalogue at trxe.com.

Frequently Asked Questions

Can I replace an electrolytic with a ceramic of the same value?

Sometimes, but check the voltage rating and DC bias derating. A ceramic 10 µF at 5 V bias may only give 6 µF. Low ESR can also cause stability problems in some linear regulator circuits.

What does 105 mean on a capacitor?

105 = 10 × 10⁵ pF = 1,000,000 pF = 1 µF. Same three-digit code used on SMD resistors.

How long do electrolytic capacitors last?

2,000 to 10,000 hours at rated temperature. Life doubles for every 10°C below the rated temperature. A 105°C capacitor running at 85°C can last 40,000+ hours.

Can I mix capacitor types in a power supply?

Yes, and you often should. Use a large aluminium electrolytic for bulk storage (100–1000 µF), a ceramic for mid-frequency decoupling (10–47 µF), and a small ceramic for high-frequency noise suppression (0.1 µF). Each type handles a different part of the noise spectrum.