<p>Your description of the dramatic voltage drop (<em>from 5V to 0.5V</em>) upon the connection between the flight controller (<em>FC</em>) and 4-in-1 ESC makes me think that the issue you're seeing is most likely due to a short or low resistance path to ground. There are other possible failure modes that could cause this, but a ground fault sounds most probable here.</p>
<h3>Why?</h3>
<p>When a ground fault is created in one of the components attached to the 5V power rail generated by the ESC's <a href="https://drones.stackexchange.com/q/101/97">BEC (<em>Battery Eliminator Circuit</em>)</a>, a substantial current greater than the BEC's rating for continuous power delivery flows. This is due to Ohm's law, <span class="math-container">$I = \frac{V}{R}$</span> which states that a large current (<em>I</em>) will flow if the circuit resistance (<em>R</em>) is tiny.</p>
<p>The usual mode of operation for power supplies when they experience surges in power demand is to drop their output voltage in an attempt to prevent catastrophic failures of themselves or downstream components by decreasing the power output. (<em>Power <span class="math-container">$P = \frac{V}{I}$</span></em>) This is a near-ubiquitous safety feature.</p>
<h3>Confirming a Diagnosis of a Ground Fault</h3>
<p>Since ground faults manifest themselves as low-resistance paths between the power rails, the way to test for the existence of one is to (<em>use a multimeter</em>) measure the resistance between the <code>+5V</code> and <code>GND</code> lines on the FC and the 4-in-1 ESC independently.</p>
<p>It would be best if you could fully disconnect all peripherals from the FC before measuring for a ground fault, as it is possible that the issue is with one of them and not the FC, but this can also be done once a problem with the ESC is eliminated to possibly avoid desoldering and resoldering the components back together.</p>
<p><em><strong>NOTE:</strong> remember to take resistance measurements with no power attached to the device being measured!</em></p>
<p>If the measured resistance for the FC isn't high (<em>the exact threshold differs between circuits, but the difference between a ground fault and a safe circuit is significant and should be noticeable</em>), then there is a ground fault somewhere inside the FC. Likewise, if the measured resistance for the ESC isn't high, then there is a ground fault somewhere inside the FC. If both components have low resistance paths between the power rails, then they both have ground faults.</p>
<h3>Resolution</h3>
<p>You could either attempt to correct the ground fault in the affected component(s) or replace them. Sometimes the cause of the ground fault is some SMD (<em>Surface Mount Device</em>) component that is too small for you to comfortably work on or broken solder mask in the PCB, in which case fixing it may be more trouble than it's worth.</p>
<p>However, the only cost of trying to fix and not buy new is your time. And hey, you never know when a troubleshooting venture like this will teach you something new about electronics!</p>

CC BY-SA 4.0
2020-06-06T20:13:38.990Z
After disconnecting everything from the FC, the resistance increased. So after measuring all peripherals I found that the buzzer AND receiver were shorted. With these disconnected everything works again
Hannes Hultergård
CC BY-SA 4.0
2020-06-06T20:20:02.740Z
Huh, nice catch, @HannesHultergård !
ifconfig
CC BY-SA 4.0
2020-06-06T20:21:17.620Z
Thanks! I wonder how that could have happened at the same time....
Hannes Hultergård
CC BY-SA 4.0
2020-06-06T20:23:18.433Z
Now that's the real question! Could be anything from a particularly hard crash (*physical shock loads*) to out of spec power surges (*electrical shock loads*)...
ifconfig
CC BY-SA 4.0
2020-06-06T20:25:03.337Z
Yeah. I'll just change them and hope it won't happen again
Hannes Hultergård
CC BY-SA 4.0
2020-06-06T19:42:11.800Z
2020-09-25T22:43:06.587Z
2020-09-25T22:43:06.587Z
ifconfig
<p>After crashing my drone recently, my ESC and flight controller won't turn on (<s>the power led lights up on the FC, but nothing else works</s> either it doesn't anymore, or I was wrong before) any more when they are connected together. When I disconnect the ribbon cable between the FC and the ESC, the ESC turns on as it should (Beeps when the battery is connected), and the FC turns on when I connect it to USB, and it seems to function correctly.</p>

<p>I have measured the connections going between the two, and all of them appears to be okay, except that 5 V from the ESC drops to about 0.5 V when the flight controller is connected. If I connect both the ribbon cable and a USB cable, the FC turns on and the video feed goes from showing black (no OSD) to show the actual video. The ESC still won't turn on though.</p>

<p>I noticed that the capacitor had come loose from one terminal, but it seems unlikely that that caused these problems. Unless it caused voltage spikes that damaged something?</p>

<p>My FC is a HobbyWing FC F4 G2 and the ESC is a HobbyWing XRotor Micro 60A 4in1.</p>

<p>Any help is appreciated, I can't even figure out which board causes the problems.</p>

CC BY-SA 4.0
2020-06-06T18:47:22.347Z
Hrm, quite an interesting debug indeed! This sounds like a short to ground issue to me...
ifconfig
CC BY-SA 4.0
2020-06-06T18:48:22.793Z
What is the resistance between the +5V and GND lines before and after connecting the FC to the 4-in-1 ESC? (*make sure to take these measurements with no power attached!*)
ifconfig
CC BY-SA 4.0
2020-06-06T19:16:48.210Z
Thanks, didn't think of that. < 1 Ω connected, and to high to measure when disconnected. So definitely a short in the FC then
Hannes Hultergård
CC BY-SA 4.0
2020-06-06T19:18:06.613Z
Hrm, yeah. Have you disconnected all perhipherals to the FC to make sure that the issue is with the FC and not the filtering capacitor?
ifconfig
CC BY-SA 4.0
2020-06-06T19:49:10.793Z
Maybe I understood your question wrong at first. The resistance between 5 V and ground on the FC when disconnected is < 1 ohm, and on the ESC it's unmeasurable. So the problem is probably not on anything connected to the ESC (including the capacitor). But I can try to disconnect everything from the FC.
Hannes Hultergård
CC BY-SA 4.0
2020-06-06T19:52:00.370Z
It appears I misread your description of the capacitor: if it's attached to the ESC and you've identified the issue to be on the FC, then you should be looking into the FC now, not the ESC's capacitor.

Your description of the ESC's resistance is confusing me... what makes it unmeasurable?
ifconfig
CC BY-SA 4.0
2020-06-06T19:53:12.290Z
Sorry, not unmeasurable, but too high for my multimeter
Hannes Hultergård
CC BY-SA 4.0
2020-06-06T19:54:10.113Z
Ah, understood! That's a good sign, as too high for the multimeter means that a ground fault within the ESC isn't probable.
ifconfig
<flight-controller><esc>
4 in 1 ESC and FC won't turn on when connected together
Hannes Hultergård
# 4 in 1 ESC and FC won't turn on when connected together

(Asked by: username_0 on 2020-06-06)

After crashing my drone recently, my ESC and flight controller won't turn on (~~the power led lights up on the FC, but nothing else works~~ either it doesn't anymore, or I was wrong before) any more when they are connected together. When I disconnect the ribbon cable between the FC and the ESC, the ESC turns on as it should (Beeps when the battery is connected), and the FC turns on when I connect it to USB, and it seems to function correctly.

I have measured the connections going between the two, and all of them appears to be okay, except that 5 V from the ESC drops to about 0.5 V when the flight controller is connected. If I connect both the ribbon cable and a USB cable, the FC turns on and the video feed goes from showing black (no OSD) to show the actual video. The ESC still won't turn on though.

I noticed that the capacitor had come loose from one terminal, but it seems unlikely that that caused these problems. Unless it caused voltage spikes that damaged something?

My FC is a HobbyWing FC F4 G2 and the ESC is a HobbyWing XRotor Micro 60A 4in1.

Any help is appreciated, I can't even figure out which board causes the problems.

**Tags:** flight-controller, esc

> **Comments:**
> **username_1**: Hrm, quite an interesting debug indeed! This sounds like a short to ground issue to me...  
> **username_1**: What is the resistance between the +5V and GND lines before and after connecting the FC to the 4-in-1 ESC? (*make sure to take these measurements with no power attached!*)  
> **username_0**: Thanks, didn't think of that. < 1 Ω connected, and to high to measure when disconnected. So definitely a short in the FC then  
> **username_1**: Hrm, yeah. Have you disconnected all perhipherals to the FC to make sure that the issue is with the FC and not the filtering capacitor?  
> **username_0**: Maybe I understood your question wrong at first. The resistance between 5 V and ground on the FC when disconnected is < 1 ohm, and on the ESC it's unmeasurable. So the problem is probably not on anything connected to the ESC (including the capacitor). But I can try to disconnect everything from the FC.  
> **username_1**: It appears I misread your description of the capacitor: if it's attached to the ESC and you've identified the issue to be on the FC, then you should be looking into the FC now, not the ESC's capacitor.  Your description of the ESC's resistance is confusing me... what makes it unmeasurable?  
> **username_0**: Sorry, not unmeasurable, but too high for my multimeter  
> **username_1**: Ah, understood! That's a good sign, as too high for the multimeter means that a ground fault within the ESC isn't probable.  

---

## 1 answers

### Answer by username_1 on 2020-06-06. Score: 4 | Accepted answer

Your description of the dramatic voltage drop (*from 5V to 0.5V*) upon the connection between the flight controller (*FC*) and 4-in-1 ESC makes me think that the issue you're seeing is most likely due to a short or low resistance path to ground. There are other possible failure modes that could cause this, but a ground fault sounds most probable here.

### Why?

When a ground fault is created in one of the components attached to the 5V power rail generated by the ESC's [BEC (*Battery Eliminator Circuit*)](https://drones.stackexchange.com/q/101/97), a substantial current greater than the BEC's rating for continuous power delivery flows. This is due to Ohm's law, $I = \frac{V}{R}$ which states that a large current (*I*) will flow if the circuit resistance (*R*) is tiny.

The usual mode of operation for power supplies when they experience surges in power demand is to drop their output voltage in an attempt to prevent catastrophic failures of themselves or downstream components by decreasing the power output. (*Power $P = \frac{V}{I}$*) This is a near-ubiquitous safety feature.

### Confirming a Diagnosis of a Ground Fault

Since ground faults manifest themselves as low-resistance paths between the power rails, the way to test for the existence of one is to (*use a multimeter*) measure the resistance between the `+5V` and `GND` lines on the FC and the 4-in-1 ESC independently.

It would be best if you could fully disconnect all peripherals from the FC before measuring for a ground fault, as it is possible that the issue is with one of them and not the FC, but this can also be done once a problem with the ESC is eliminated to possibly avoid desoldering and resoldering the components back together.

***NOTE:** remember to take resistance measurements with no power attached to the device being measured!*

If the measured resistance for the FC isn't high (*the exact threshold differs between circuits, but the difference between a ground fault and a safe circuit is significant and should be noticeable*), then there is a ground fault somewhere inside the FC. Likewise, if the measured resistance for the ESC isn't high, then there is a ground fault somewhere inside the FC. If both components have low resistance paths between the power rails, then they both have ground faults.

### Resolution

You could either attempt to correct the ground fault in the affected component(s) or replace them. Sometimes the cause of the ground fault is some SMD (*Surface Mount Device*) component that is too small for you to comfortably work on or broken solder mask in the PCB, in which case fixing it may be more trouble than it's worth.

However, the only cost of trying to fix and not buy new is your time. And hey, you never know when a troubleshooting venture like this will teach you something new about electronics!

> **Comments:**
> **username_0**: After disconnecting everything from the FC, the resistance increased. So after measuring all peripherals I found that the buzzer AND receiver were shorted. With these disconnected everything works again  
> **username_1**: Huh, nice catch, @username_0 !  
> **username_0**: Thanks! I wonder how that could have happened at the same time....  
> **username_1**: Now that's the real question! Could be anything from a particularly hard crash (*physical shock loads*) to out of spec power surges (*electrical shock loads*)...  
> **username_0**: Yeah. I'll just change them and hope it won't happen again  

---

