Bulk capacitor math that fits your drawer

Pick three values and stop

Most home-lab power supplies don't need twenty different bulk capacitor bins. Three aluminum electrolytics—say 100 µF, 470 µF, and 1000 µF at your common working voltage—will handle the ripple filtering and hold-up time for almost every linear regulator, DC-DC converter input, or motor driver rail you build on a weekend.

The trick is knowing when to parallel them instead of hunting for the exact value some online calculator spat out. Two 470 µF caps in parallel give you 940 µF, close enough to 1000 µF that the difference won't matter in anything but the most marginal designs. If your design is that marginal, you have bigger problems than capacitor inventory.

Ripple current and ESR matter more than raw capacitance

A single 1000 µF low-ESR cap often outperforms two older 1000 µF caps in parallel because equivalent series resistance eats your filtering budget. Check the datasheet ripple current rating—if your switcher or bridge rectifier dumps 500 mA RMS into the cap, and the part is only rated for 400 mA, you'll cook it even if the capacitance calculation says you're fine.

When you buy bulk caps for stock, spend the extra dollar per part on low-ESR types rated for high ripple current. They'll survive more abuse, and you won't second-guess every power supply six months later.

Hold-up time is where you actually do the math

If your circuit must survive a brief input sag—say 20 ms for a motor controller riding through a compressor startup—then capacitance directly sets your hold-up time. The formula is simple: t = C × (V_start - V_min) / I_load. Plug in your minimum acceptable voltage, your load current, and solve for C.

But here's the reality: most home projects don't need calculated hold-up. If you're running a 5 V microcontroller off a bench supply, the supply itself has enough bulk storage. If you're driving a motor, the inrush matters more than the hold-up, and that's a fuse and inrush-limiting problem, not a capacitor problem.

Save the hold-up math for battery-backed systems or anything that absolutely cannot brown out. For everything else, a 1000 µF cap on the input rail is cheap insurance you'll never regret.

Voltage rating overhead is not optional

If your rail is 12 V, use a 25 V or 35 V cap, not a 16 V cap. Electrolytic capacitors age faster near their rated voltage, and transient spikes—especially on motor rails or anything with inductive loads—will push you over the edge.

The size penalty for higher voltage ratings is small at these capacitances, and you'll thank yourself when a flyback diode glitches and your bulk cap doesn't bulge.

Parallel ceramics for high-frequency cleanup

Bulk electrolytics handle low-frequency ripple and energy storage, but their ESR and lead inductance make them useless above a few hundred kilohertz. Stick a 100 nF ceramic right next to every IC's power pin, as close as your soldering patience allows, and let the bulk cap sit farther away near the power input.

This isn't controversial—every datasheet will tell you the same thing—but it's easy to forget when you're focused on the big caps. The ceramic does the high-frequency work; the bulk cap does the energy work. Both matter, and neither replaces the other.

When to calculate, when to guess

If you're building a one-off motor controller or a custom bench supply, run the ripple and hold-up numbers once and pick a cap 20% larger than the math says. If you're prototyping something that might turn into a small production run, document the calculation so you remember why you chose that value six months from now.

For everything else—microcontroller projects, sensor boards, anything under an amp—just use a 470 µF or 1000 µF cap on the input and move on. Your time is worth more than the penny you'd save by optimizing capacitance to three significant figures.

Keep three values in your drawer, buy low-ESR parts, and you'll spend less time hunting for the perfect cap and more time building circuits that work.