Capacitor Hold Up Time Guide
What Hold Up Time Means
Hold up time is the time a circuit keeps running after input power drops. A capacitor stores energy before the drop. The load then draws that energy until voltage reaches its safe limit. This matters in controls, relays, sensors, converters, and backup rails.
Why Designers Calculate It
A rough capacitor choice can fail during short brownouts. It can also waste board area and cost. This calculator gives a practical estimate before testing. It includes capacitance derating, voltage limits, load type, efficiency, ESR, leakage, and design margin. These options help compare real parts, not only ideal values.
Key Inputs
Start with the total rated capacitance. Choose the unit and enter parallel capacitor count. Add a usable capacitance percent. This can cover tolerance, aging, temperature, and bias effects. Then enter the charged voltage and the minimum operating voltage. The difference between these voltages drives the useful time. A higher starting voltage stores much more energy. A lower shutdown voltage also increases available energy.
Load Behavior
Constant current loads are simple. The current stays fixed as voltage falls. Constant power loads are different. They draw more current at lower voltage. That makes the final part of discharge harder. Resistive loads draw less current as voltage falls. Their discharge follows a logarithmic curve. Pick the load type that best matches your circuit.
Losses And Margins
Efficiency adjusts load demand when a converter is used. ESR creates voltage sag and heat. Leakage slowly drains the capacitor. A safety factor reduces the reported time. This makes the estimate less optimistic. It is useful when temperature or part aging is uncertain.
Practical Use
Use this tool for early sizing. Then check the result against datasheets and tests. Capacitors may have wide tolerance. Electrolytic parts age over time. Ceramic parts can lose capacitance under DC bias. Supercapacitors need voltage balancing and leakage review. Always verify ripple current, voltage rating, polarity, and surge limits. For critical equipment, test the real board with the real load.
Reading The Result
Read the result as an engineering estimate. If the required time is close to the value, choose a larger capacitor bank. Extra margin protects startup delays, relay chatter, and supply dips.