# Can a capacitor provide enough current to be used as a fail safe for a servo?

Here's my situation. I'm planning to use a servo (like the ones used in RC models) to rotate a chunk of lead between two positions: 0º and 180º. Since I'll probably be powering the servo from a wall outlet (and a voltage regulator), it's important for this application that I have a way of automatically returning it to 0º position should the power fail.

Apparently the most common way to do this is with a spring, but there's the issue that the servo will be using power to fight the spring even when not moving. I found out that capacitors can be used for this: the capacitor charges when everything is working, and if the power fails the capacitor can power the servo long enough for it to return to 0º.

This is very strange. The servo pulls around $1A$ of current and takes around one second to do the half-turn, which implies that the capacitor would have to store around $1C$ of charge. This would mean either extremely high capacity or extremely high voltage.

And yet it seems that this is a thing that exists and is used for even bigger motors than mine. Can capacitors really provide that much current for that much time? Or am I not understanding something?

• There are high capacity caps, and there are also supercapacitors. But have in mind, that you will need provide a specific discharge rate for you application. How would you do that? Commented May 19, 2015 at 20:19
• 1A @5V for a second is a lot of energy (5J). Consider a battery backup. If you can extract half the energy you'd need a farad or so to use a capacitor. Many supercaps (not all) have too high internal resistance to be used directly. Commented May 19, 2015 at 20:55
• Piece of lead?! For absorbing radiation? The spring sounds like a nice reliable solution. Have you read about the THERAC-25? Commented May 19, 2015 at 21:56
• @SpehroPefhany: maxwells ultracaps have some thousands of farads with milli or even microohm resistance, nice for all kinds of things. Commented May 19, 2015 at 22:09
• @PlasmaHH I heard they can be used to boost batteries in order to start trucks in the frozen Arctic. Commented May 19, 2015 at 22:11

The boost regulator and supercap are available from Digi-Key for $3.50 and$7.54 respectively.