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Apr 21, 2017, 07:39 AM
AKA jflyper
Thread OP
Mini-HowTo

OMNIBUS/AIRBOT F4 family: ESC_5V Isolation


Introduction

This is a note for users of OMNIBUS and AIRBOT F4 family those who wishes to isolate ESC_5V rail from rest of the FC so that it could be used to power servos on multi-servo vehicle such as a four control surface airplane.

Background

OMNIBUS and AIRBOT F4 families all share a same design regarding 5V power supply, which is shown below.

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The diode triplet is used to select the highest voltage of the three 5V sources, which are ESC_5V, USB_VBUS, and LDO_5V or SBEC_5V. (These diodes also prevents reverse powering unselected sources.)

To power the ESC_5V with an external 5V source dedicated for servo power, one of the diode triplet, Dx, must be removed.

Modding

In theory, you can use a DMM to determine where the Dx is by testing continuity between a relatively large SMD device (marked Dn with diode icon --i>| in a pin map if you have any) and one of the ESC_5V through-holes/pads. Result of measuring for some of the family members are given below, but builders are advised to verify the continuity before proceeding.

OMNIBUS F3 AIO and F3 Pro
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OMNIBUS F4 AIO
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OMNIBUS F4 V2
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OMNIBUS F4 V3
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OMNIBUS F4 Pro
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AIRBOT F4 (ICM)
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Last edited by teralift; May 04, 2017 at 02:08 PM.
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Apr 23, 2017, 12:58 AM
Registered User
Thanks Teralift. I've attached a wiring diagram with my proposed setup. My concern is creating a ground loop with the UBEC/servo/FC grounds. Does this look right?
Last edited by goog; Apr 23, 2017 at 01:04 AM.
Apr 23, 2017, 06:21 AM
AKA jflyper
Thread OP
Quote:
Originally Posted by goog
Thanks Teralift. I've attached a wiring diagram with my proposed setup. My concern is creating a ground loop with the UBEC/servo/FC grounds. Does this look right?
Look okay to me.

Forget about the ground loop.
Aug 08, 2017, 02:06 PM
Registered User
Thanks for sharing, worked great on my omnibus F3 Pro
Sep 10, 2017, 06:01 AM
Registered User
Awesome ! exactly what fpv plane user (I) needs ! thanks again
Oct 18, 2017, 04:45 AM
Registered User
Thanks terralift. Will be doing this on y newest build. I had heard however that reversing a diode will allow one to run both sources together. Does anyone know anything about this? Is that the diode you are recording we remove?
Oct 18, 2017, 05:26 AM
AKA jflyper
Thread OP
Quote:
Originally Posted by Joesrevolution
Thanks terralift. Will be doing this on y newest build. I had heard however that reversing a diode will allow one to run both sources together. Does anyone know anything about this? Is that the diode you are recording we remove?
Reversing the diode under question will enable onboard 5V sources to drive ESC-mid rail.

At the same time, you will loose an option to power the whole board from the ESC-mid rail.

To summarize

Normal: ESC-mid rail can power the main 5V rail if applied voltage is higher than that of other sources. Other sources does not power the ESC-mid rail.

Removed: ESC-mid rail is isolated from other sources.

Reverse: Other sources will power the ESC-mid rail, as well as rest of the 5V rail.
Oct 25, 2017, 10:08 PM
Registered User
What happens if u don't remove the diode and put 5V ESC on the header?
Oct 25, 2017, 10:40 PM
AKA jflyper
Thread OP
Quote:
Originally Posted by flyandi
What happens if u don't remove the diode and put 5V ESC on the header?
You mean "Apply (or feed) 5V to ESC-5V rail without removing the diode"?

In that case, if the voltage on the ESC-5V rail is higher than other sources, then it will power the entire FC. Otherwise, the diode will prevent reverse voltage/current. These two cases may alternate as voltage on ESC-5V goes up and down when servo moves.

If you are going to power servos, I strongly recommend you isolate the ESC_5V, as back rush from servo propagating to main 5V rail is the worst scenario.
Jan 03, 2018, 05:08 AM
Registered User
What is back rush from servos? Is it where a servo causes voltage spikes that will go through the diode and fry the fc? In that case, won't the other servos and the rx absorb the spikes?

I have a relatively small fixed wing with two 9g servos for ailerons, two 4g for the tail, two 9g for pan and tilt and an rx of course connected to the Omnibus F4 pro v2 (clone). I thought the onboard bec was rated at 3A so do I really need to remove the diode and change esc to one with bec? I think I read the diode is rated at 1A. In what way can that be a critical factor?
Jan 03, 2018, 06:01 AM
AKA jflyper
Thread OP
@RCnator

Servos are inductive load and therefore noisy in nature, if not harmful to other components that shares the same power rail. I have seen a medium 5V servo generating as high as 13V spike on the power rail, and it is a common practice to isolate digital circuit from inductive load.

If you have a working system, that's fine.
Last edited by teralift; Jan 03, 2018 at 06:09 AM.
Jan 03, 2018, 06:12 AM
Registered User
I have not tested it yet but already soldered on an ESC without bec. I could change it though but the other ESC is quite a bit heavier and bulkier. Still, I want a reliable system and not fry anything. I have two other bigger fixed wings though that I use bigger ESCs on from the beginning with inbuilt bec where I will be sure to remive the diode.
Jan 08, 2018, 01:47 PM
...missing the PE Slope Rebels
Flying Beagle's Avatar
Hello
first I want to apologize in advance for being electrictally challenged,
I used wiki to find out that a diode can be viewed as an electronic version of a check valve....

I have this board FLIP 32 - F4 - OMNIBUS V3 PRO
and am planning to run INAV in a Mini Talon with four servos

it appears to be possible to perform this "diode mod"
then just physically plug my ESC and four servos into this row of pins on the FC, just like I do in my sport planes into an Rx
thus avoiding building some sort of external servo rail for + & -

all other board functions ( vtx, beeper, sbus, osd, etc ) would remain functional

am I understanding this correctly ???
if so, what do I need to physically "do" to this diode ???

thx
FB
Jan 08, 2018, 03:27 PM
Registered User
@teralift, 13V sure is a hell of a lot. I actually went ahead and did the mod and exchanged the ESC to one with BEC, partly also because I was told it is better, without further explanation, to use airplane ESCs in airplanes.

@Flying Beagle, I believe you do. I can see you have been a member here since 2004, so just take your soldering iron and desolder the diode.
Jan 08, 2018, 03:32 PM
...missing the PE Slope Rebels
Flying Beagle's Avatar
....you can be in this hobby a lifetime,
design, build and fly dozens of airplanes, and never pick up a solder iron


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