# Will a large capacitor damage a circuit

I have 32 servos running from a 7.5V battery rated at 2500mAh and I'm sure that it has enough power to handle the servos, but it doesn't look like it can handle all of the servos pushing at once.

I was thinking I could add a capacitor to the circuit to solve this issue, so bearing in mind that I'm very new with electronics and have only just found out that amperes are a direct result to the requirement of the circuit rather than what the battery puts out, please could you answer my questions below?

1. Could I put a capacitor just before the servo controller to deal with the intermittent load requirements?
2. Would a large capacitor damage the circuit?
3. Does a capacitor work just like a battery and only output the required power rather than just discharging everything it has into the circuit?
4. Does a capacitor have to be mounted across the positive and ground or does the positive have to go through it?
• A capacitor holds very little energy compared to a battery, in general. It will smooth out short spikes in current, but will not help with longer bumps or continuous demand. Commented Apr 7, 2014 at 14:42
• That's a lot of servo's. Does the datasheet say how much current they draw?
– RJR
Commented Apr 7, 2014 at 23:33
• @RJR yeah the robot has a lot of degrees of freedom, especially for a biped. I'm sure it does but the servos I've used are cheap due to how many I need, and so they look unnamed. Commented Apr 8, 2014 at 8:16

1. Yes
2. It shouldn't. It should smooth out voltage droops.
3. Yes, it acts more like a constant voltage source. An infinite sized capacitor would be an ideal voltage source.
4. It should be mounted between positive and ground. Basically, it should look like it's a parallel mounted battery.

You can think of a capacitor kind of like the tank on most toilets. It stores extra water for the times that you need extra water (current) to flush. The capacitor in this analogy would be like a pressurized toilet tank. With pressure being voltage and the volume of water being current. When your servo turns on, you now have more temporary volume of water at line pressure to satisfy your needs if you install a capacitor. In short, more current at the same voltage.

• Thank you for your quick answer, though it has sparked a new but very related question. Does a capacitor match the voltage of the battery or do I have to get a capacitor rated for the voltage of the battery? Commented Apr 7, 2014 at 14:44
• If you put the cap in parallel with the battery, then yes, it will match the voltage of the battery. Keep in mind when choosing a capacitor that you will want it rated about 25% higher than the highest voltage you expect it to see (engineering rule of thumb). If the capacitor is placed at some other point in the circuit, say after a regulator, it will match the voltage of the regulator. You still need to have the cap rated for at least 25% above this voltage though. Commented Apr 7, 2014 at 14:49
• Thank you for helping out with this, I like the toilet analogy. Very easy to understand, thank you. Commented Apr 7, 2014 at 14:59
• @horta Out of interest, does the 25% rule vary dependant on the kind of cap? A lecturer I had seemed to hint that electrolitics were far more sensitive to voltage than other sorts and some other kinds cared far less about voltage. Commented Apr 7, 2014 at 19:45
• At least 25% over-sizing is just general engineering guideline for any component. Electrolytics have definitely have a bad rap due to capacitor plague: en.wikipedia.org/wiki/Capacitor_plague Since electrolytic capacitor plague was an aging issue, over-sizing them even more would have mitigated the ageing issues seen. In short though, if you trust a manufacturers spec, it shouldn't matter which kind of cap you use. Commented Apr 7, 2014 at 19:58

In this situation, you can think of a capacitor as a fast charge and discharge battery in parallel to your normal battery.

1 - maybe (see below)

2 - A large capacitor will demand a large amount of current to charge and be capable of sourcing a large amount of current for a short time. This could damage the circuit (i.e. battery), but it depends on the circuit. It would most likely take much larger capacitance than you are going to use to cause issues, with one exception. When connecting a battery to a discharged capacitor, the current drawn can be extremely high as the cap charged up.

3 - In DC realm, both a battery and a capacitor will discharge what is required to satisfy Ohms law. This means that as your load power goes up (i.e. load resistance goes down), more current will be supplied until the voltage drops.

4 - You would mount it in parallel with the battery.