Why Capacitors Are Important For FPV Drones

FPV Drones

FPV Drones

Capacitors are a crucial component for FPV drones as they help to reduce voltage spikes and electrical noise in the power system. These voltage spikes and electrical noise can cause harm to electronic components, especially the flight controller and ESCs. Adding capacitors to an FPV drone can protect these components and enhance the overall performance of the drone.

What is a Capacitor?

 

A capacitor is one of the most common electronic components.

Capacitors store electric charge in an electric field between two conductive plates and can absorb and discharge electrical energy quickly just like a tiny battery. The capacitor absorbs voltage spikes, and releases the stored energy when there’s a voltage drop to smooth out the voltage fluctuations and prevent damage to other components.

The Benefits of Extra Capacitors in FPV Drones

The FET’s in our ESC are effectively switches. They turn on and off rapidly tens of thousands of times per second to drive the motors, which can generate a lot of electrical spikes and noise. As technology advances allowing for more powerful motors and ESC’s, and higher battery voltage, the problem becomes even more pronounced. The noise issue is absolutely crucial, it can mean the difference between a fantastic FPV drone and something that is completely unflyable.

Capacitors can help to reduce electrical noise and voltage spikes that can occur when the motors spin up or when the drone is performing high-speed manoeuvres. Additionally, capacitors can help reduce the amount of interference that the power system generates, which can improve the quality of the analog video feed from the drone’s camera.

ESC and FC have surface mount capacitors for filtering, but due to the lack of physical space, they tend to be insufficient. As pointed out in my FPV drone build guide, soldering additional capacitors to our FPV drone is almost always mandatory to ensure optimal performance.

To sum it up, adding more capacitors to our FPV drone can provide the following benefits:

  • Filtering electrical noise generated by ESC and motors, reducing interference to ESC/radio/video signals
  • Improving flight performance by “cleaning up” the noise in gyro signal
  • Protecting electronics from damaging voltage spikes. Capacitors act as a buffer, absorbing any excess energy before it reaches the delicate components
  • If you fly analog, having adequate capacitors at the input power can help reduce video noise in your FPV feed

If you don’t use capacitors in your FPV drone, you may experience issues such as:

  • Video noise and lines in your video feed, which can make it difficult to fly
  • Suboptimal flight performance and you can’t push PID and filter as high as you could have otherwise
  • Components are more exposed to voltage spikes damage when you power them directly from battery

Do You Need To Add Capacitor?

You don’t have to use caps, but we recommend it. As mum says:

“Put a cap on, it’s noisy out there!”

If you have a noisy quad (lots of oscillations and it’s hard to tune PID and filters) or noisy video feed, adding capacitors would probably be the first thing you should try.

Here is how you can decide whether your quad is too noisy and need a capacitor.

  • Listen to the motors and see if there are oscillation that you can’t tune out with PID
  • Check for hot motors
  • Noise in your video feed is another sign of electrical noise in the power
  • In extreme cases, you can get ESC/motor desync and “roll of death” due to ESC signals being corrupted by noise

Even if you don’t have a noisy build, it’s still good practice to add caps. Better safe than sorry right? 🙂 Bent and misshapen props can introduce noise in your power, adding caps can actually help with that too.

What Type of Capacitor Should You Use?

There are three types of capacitors commonly used in FPV drones:

  • Ceramic capacitors
  • Electrolytic capacitors
  • Tantalum capacitors

Ceramic capacitors are the most popular type used for surface mount because of their small size, low cost, and high capacitance values. Electrolytic capacitors are larger and more expensive but can provide higher capacitance values. Tantalum capacitors are similar to electrolytic capacitors but are more reliable and have a longer lifespan.

The most common type of cap we install on FPV drone by ourselves would be electrolytic capacitor. It’s a small cylinder with metal casing (usually aluminium) and two legs (one positive one negative).

 

When selecting capacitors for your FPV drone, you should pay attention to the following capacitor ratings:

  • Capacitance: it indicates how much charge a capacitor can store. For an FPV drone, you typically want capacitors with a capacitance between 100uF and 1000uF. Higher capacitance values can provide better filtering, but they are physically larger and heavier
  • Voltage Rating: this is the maximum voltage the capacitor can withstand before failing, it should be higher than the voltage spikes that the drone’s electronics can produce. For example, if the drone is powered by a 4S (14.8V) battery, you should choose capacitors with a voltage rating of at least 25V, and for 6S, that should be 35V or higher. There’s no harm in choosing a higher voltage rated capacitor, but they tend to be larger in size.

Choose voltage rating based on LiPo cell count, voltage spikes can be higher than the battery voltage, so it’s safer to pick a cap with higher rated voltage than the LiPo.

  • 3S: 16V or higher
  • 4S: 25V or higher
  • 5S: 35V or higher
  • 6S: 35V or higher (e.g. 50V if you want to play safe)
  • 8S: 63V or higher
  • 12S: 80V or higher

Where to Install Capacitor in FPV Drone?

FPV drones typically require capacitors to be added to the power system, either across the power leads or directly onto the ESCs’ power pads. The general idea is to connect the capacitor as close as electrically possible to the MOSFET’s in the ESC can maximize its effectiveness. For 4in1 ESC, that usually means the power solder pads.

 

 

If you don’t have space, you can shorten the legs of the cap, then solder some flexible silicone wires for extension and place the cap somewhere else inside the frame. But make sure to keep the wires as short as possible to minimize resistance, and use larger wires (e.g. for 6S 5″ 20AWG or even 18AWG is a good choice).

 

 

It’s extremely important to understand the polarity of a capacitor, the longer leg is positive (+) and the shorter leg is negative (-). The marking on the body (white label) also indicates which terminal is negative. If you make a mistake, the capacitor won’t work, it might even explode, please be careful!

 

Solder the capacitor to either the ESC power or XT60 solder pads. Negative to negative, positive to positive.

Cleaning Up Gyro Power

If your mini quad has electrical noise issue, such as oscillations and hot motors, by simply adding a large capacitor to the XT60 or ESC power doesn’t solve it, then you might want to consider adding a small capacitor directly to the Gyro’s power supply. This can target the noise present in the gyro’s power more effectively.

I only recommend this hack to experienced users. It’s not easy finding where to solder the capacitor to, and it also requires decent soldering skills. You don’t have to worry about this if you don’t have issues. If you do, try other easier methods first and treat this as the last resort.

Adding Cap to 3.3V

The Gyro on our flight controllers is powered by a 3.3V LDO regulator, so you just need to solder a capacitor to the 3.3V pin of the Gyro, or the 3.3V output of the LDO. The other end of the cap solder to ground.

Popular options are 4V 220uF, 330uF or 470uF tantalum capacitor: https://amzn.to/3XN4Hhn

The 3V rail should be filtered as close to gyro as possible for the best result.

Very few FC’s share the same 3.3V rail with the MCU and spektrum receivers, so you might be able to access the 3.3V rail on an external solder pad. IMO this is not a good FC design by not having a dedicated low noise regulator just for the gyro, but it does make it easier for you to solder the capacitor to if required.

Adding Cap to 5V

The 3.3V LDO for the Gyro gets power from the 5V rail, so you can also filter the 5V rail which will also clean up noise for the gyro. It might not be as effective as filtering in on the 3.3V rail but it’s worth a try if it’s more accessible.

Popular options: 6V 220uF, 330uF or 470uF tantalum capacitor: https://amzn.to/41blcqv

Capacitor Gets Hot After Flight

Potential causes of capacitor gets hot after flight (or it exploded):

  • Capacitance is too small. For example if you are using a 680uF, you should try a 1000uF
  • It could be a sign of the voltage spikes are too high, you should get a cap with higher voltage rating. For example if you are using a 35V cap on 6S, you might want to get a 50V one.

Is Adding Too Many Capacitors Bad?

Short answer – No.

If you are filtering signal of certain frequency range, then you should control the capacitance and inductance precisely in your circuit. But what we are dealing with here is just DC voltage and a low pass filter, you won’t “over-filter” DC voltage, so you can use as much capacitance as you want and it won’t be a problem. In fact, the more capacitors you use in your drone, the stronger this low pass filtering works.

So, use as many capacitors as you wish, as long as you have space to mount them.

Do Capacitors Wear Out?

Short answer – Yes.

Electrolytic capacitors used in FPV drones have a limited lifespan and can wear out over time due to factors such as high temperature and high voltage stress. When an electrolytic capacitor wears out, it can leak or bulge, and its capacitance may decrease. This can result in instability and noise in the power supply, which can affect the performance of the electronics. It’s a good idea to periodically inspect and replace capacitors in your drone, even when they look “ok”, it’s might be worth to replace them regularly.

LC Filter vs Capacitor

LC filters are often used in FPV systems to combat video noise as well, but it only protects the system after the noise is induced. Adding capacitor at the source of the power on the other hand, protects the entire system by preventing the noise from being induced in the first place.

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