# How to prevent a 5V supply from sagging to 4.6V when the FPV camera is on?

I have issues with my FPV camera. My camera is a RunCam split 2S (concept is similar to Caddx turtle).

This camera turns off almost instantly after I have plugged in my battery. When I probe the voltage between 5V and ground, I can see the voltage drop from 4.9V to 4.6V as soon as the camera starts recording (which it does automatically when powering it). Then, when the camera shuts completely off, the voltage goes up again. Also, I can press a button and the camera will turn on but also back off again

Hooking up a phone charger to the camera gets the voltage up again (and makes the camera run stably), but I obviously don't want to fly around with a power bank attached to my drone.

Unfortunately, the VTX is also wired to the flight controller's 5V converter instead of its own.

TL;DR: The issue is not that the voltage has too much ripple. The issue is that the voltage converter on my FC is not strong enough.

Would a capacitor at the input (XT60 connector) make any sense? If yes, which size should it be? I have 220µF and 1000µF electrolytic caps.

Links to the mentioned parts: Flight controller, Camera, VTX

EDIT

short answer for future people: additional, external voltage converter/BEC; use all available voltage converters (VTX, FC, (PDB)); do not make them parallel!

• Welcome to Drones and Model Aircraft! May 7 '20 at 12:29
• thank you @Pᴀᴜʟsᴛᴇʀ2 :) May 7 '20 at 12:30

I've pulled a few specs together from the parts you linked to:

• Flight controller:

• Input: 2S to 6S LiPo (7V to 20V)
• Output: 5V 3A
• VTX:

• Input: 7V to 24V
• Output: 5V 0.3A
• Camera:

• Input: 5V 0.65A --OR-- 12V 0.27A

You don't mention what size battery you are using, but I will assume it is in the FCs range of 2S to 6S.

• The camera needs too much current (0.65A) to get it's 5V from the VTX (max 0.3A)
• The VTX should be connected to the battery
• The FC should be connected to the battery
• The camera should be connected to the FC 5V supply

This wiring should work, if you have no other large 5V loads connected to the FC.

    ╔══════════════════╗  ╭───────╮
║ LiPo Battery     ╟──┤ 7-20V ├────┬──────────┐
╚══════════════════╝  ╰───────╯╔═══╧═══╗  ╔═══╧═══╗  ╭────╮
║  VTX  ║  ║  FC   ╟──┤ 5V ├────┐
╚═══════╝  ╚═══════╝  ╰────╯╔═══╧═══╗
║  Cam  ║
╚═══════╝


Alternatively you will need a separate DC-DC converter or regulator to supply the 5V. A capacitor is great for smoothing out surges in demand, but won't hold up the 5V rail if the load is constantly too large.

• Thank you very much! My suggestion with the input capacitor was based on my experience with this buck converter. Using this thing taught me, that the input capacitor was very important when there's a constant high load applied, whereas an output capacitor is necessary for short bursts. But you're right, my drone is 4S May 7 '20 at 13:07
• A buck converter works (in summary) by turning on and off quickly so that the average output voltage is correct. Capacitors and inductors are used to filter this output, so that you actually get 5V and not a rapid 12V and 0V. With larger loads, the internal capacitor may not be big enough to hold this 5V level, so you are correct about the capacitor helping in this case - just the pulses are from the supply, not the load. May 7 '20 at 13:18
• Actually, the buck converter I linked to doesn't have anything internal but stray capacitance so it is meant to be used with an output cap (and inductor) anyways. But what I meant was that without an input capacitor, this buck converter was unable to even supply single-digit mA. As soon as I hooked up an input capacitor though, it was able to supply a significant amount of power. Such that it was almost possible to drive a load without an output cap at all. In a nutshell, there was a direct correlation between the sustained output current and the input capacitance. May 7 '20 at 13:30
• I know, that might (or might not) sound strange to you. At least it sounds strange to me. More input cap => more output current. And that was, what I thought, might happen on my quad - which is why I suggested this in my original question. May 7 '20 at 13:32
• Thanks @Kralc. It works flawlessly now! Thankfully, the little tiny rest of wire (I cut it off cause I previously thought I'd not need it) on the 7-24V input of the VTX was enough to solder to :). As you suggested, I did not connect both voltage converters to each other. May 7 '20 at 21:33

I am not an electronics expert, but it sounds like it is a problem with the voltage regulator and as such, adding a capacitor wouldn’t solve the problem.

I would suggest you try running the camera off of another BEC, especially if you can power it from a VTX output as this will also reduce noise.

If you have no other BEC available, you can add a separate one, like this.

• Fortunately, my VTX also has a BEC onboard. Unfortunately though, I blindly followed one of UAVFutures (very few) bad advice: I already clipped off the 7-24V input wires of the VTX, not knowing that the VTX has its own, even better (because low noise), voltage regulator. I will try if I can somehow get a new cable on there but I've cut it very short so I doesn't dangle around mid-air (that's what I thought) May 7 '20 at 12:37