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

Question

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?

Answer 1

Yes, you can do this with supercaps. According to this calculator, 400,000 µF or 0.4F will give an RC time constant of two seconds with 5v and a 5 ohm load (i.e. 1A). Half of that (1 second) will drop the voltage of the supercap to about 60% of its voltage, or 3v.

To make up for the constantly falling voltage during discharge, you can connect the supercap up to a boost converter like the ISL97519A, configured to have a 5v output. You can "or" the output of the boost converter and your regular supply together with two Schottky diodes. The boost converter has an enable lead that you should normally assert low, and then when power is lost, keep it high by tying it to the output of the cap.

Most supercaps in the 1F range have a voltage rating of 5v or less. To get a higher rating, you can put two in series, although this halves the capacitance. The Eaton Bussman PM-5R0H155-R is rated at 1F, 5V with an ESR at 100 mOhm. This ESR is higher than we'd like, especially since we have to double it so the voltage drop will be 200 mV. However using a boost regulator will mitigate that.

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