Circuit Protection (EE TIP #116)

Circuit protection is necessary to ensure that a circuit will work reliably for 10 years and beyond. Input power supplies are susceptible to spikes from various sources including lightning, high-power machinery (e.g., generators and motors), or interference from outside sources (e.g., microwaves). The figure below shows one way to provide circuit protection. If a voltage spike is applied at VIN, the metal oxide varistor (MOV) will act at 18 V and the positive temperature coefficient (PTC) fuse will limit the current drawn. A transorb (transient voltage suppressor), which can be thought of as an ultrafast silicon zener diode, can be used in place of the MOV. Also, a capacitor in parallel with the MOV will soak up fast transient spikes—an electrolytic capacitor for low-frequency transient voltages and a small-value ceramic capacitor for high frequency transient voltages.

Figure 1Editor’s Note: This EE Tip was written by Fergus Dixon of Sydney, Australia. Dixon, who has written two articles and an essay for Circuit Cellar, runs Electronic System Design, a website set up to promote easy to use and inexpensive development kits. Click here to read his essay “The Future of Open-Source Hardware for Medical Devices.”