Eachine Tiny QX95 - Page 176 - RC Groups
Shop our Airplanes Products Drone Products Sales
Thread Tools
Mar 14, 2017, 10:11 AM
Registered User
Quote:
Originally Posted by day o
What happened? Too much force pulling the burned one off the board?
Possibly, although I wasnt pulling hard. Or maybe the heat damaged the PCB?
Either way, I soldered it best I could - which isnt very good mind you lol, and its not exactly flat, but it does work - for now. I doubt I'll manage to fix it again if this one fail again though.

@frank4wing, if you are going to try this, you really need a fine soldering iron (assuming you dont have a reflow station) and most of all: good eyes and/or good magnifying lens. If you have that, give it a try, its not like its working now .
Sign up now
to remove ads between posts
Mar 14, 2017, 11:13 AM
Registered User
someone needs to buy a stack of these FET and become an independent distributor, with a realistic shipping charge, I think there is enough customers on this board , and the bat board, and the AIO boards.
Mar 14, 2017, 11:14 AM
Registered User
Scratch "its working now".

I did a brief 10 s hover test after soldering the mosfet, and it flew ok. Then I changed the mounting so that the mosfets get more cooling, I insert a fresh battery and now a different motor spins up full speed and something smells burnt. I cant see damage yet, but Im pretty sure its toast.

Im gonna guess the mostfets burning out are a result of some other underlying problem, not a root cause. No idea what, and thats eachine's job to find out, but in both my cases, I put away a working quad, and the issue began immediately after inserting a freshly charged battery.
Mar 14, 2017, 02:09 PM
ab amicis auxilio parvulo
Aerospacer's Avatar
Quote:
Originally Posted by ZoomNBoom
Possibly, although I wasnt pulling hard. Or maybe the heat damaged the PCB?
Either way, I soldered it best I could - which isnt very good mind you lol, and its not exactly flat, but it does work - for now. I doubt I'll manage to fix it again if this one fail again though.

@frank4wing, if you are going to try this, you really need a fine soldering iron (assuming you dont have a reflow station) and most of all: good eyes and/or good magnifying lens. If you have that, give it a try, its not like its working now .
Did the original failed AO8K look burned before you unsoldered it from the board? Typically, these don't smoke when they fail. I believe the stalled rotor current on most of these 8520 brush motors is under 4 amps so unless there is a direct short, the mosfets shouldn't fail from overcurrent. I think most are failing from the internal failure of their semiconductor junctions, possibly due to momentary inductive high voltage spikes from the motors. But like you, I'm not a semiconductor expert so I speculate.

Your find of the AO3416 looks interesting. It's internal schematic shows ample diode protection around the mosfet internal junctions just like the CPH3448 that I located. In addition to more current capacity yours also has a little lower drain-source impedance which might deliver a few more millivolts to the motors at higher throttle which might result in longer flight times and/or more punch on the throttle.

The other two mosfets found on sinuss.be (TSM2312SX & DMG6968-7) have good looking specs but I don't see an internal schematic that shows the diode protection they have for their junctions. I'm thinking that might be important as the first mosfet I tried (AO3400) didn't have as elegant an internal protection and quickly failed.

Before I located the CPH3448 mosfets, I too was scavenging AO8K mosfets from a spare FC board I had laying around. These transplanted AO8K mosfets even after dozens of flights haven't failed yet. Out of three FC boards that were all AO8K mosfets, I've only replaced 4 mosfets out of the total of 12 so far. So the majority are hanging in there after 40-50 flights. I'm kind of puzzled as to why this is so; most of the failures were with less than a dozen flights on them. Is this what they call high infant mortality rate?

As far as what and how many mosfets to buy so we can keep these Eachine FC based brushed quads flying, I'm not sure what to promote. On the one hand, the nominal 100mm brushless mircos seem to be gaining momentum. On the other hand Eachine seems to be switching all their brushed micros to FC boards using AO8K mosfets (boards sporting plug-in motor connectors.) Eachine initially had FC boards with a square shaped mosfet (direct solder motor connections) that I'm not aware had mosfet failures.

With the soldering difficulties along with finding a source of replacement mosfets, I suspect that few will be motivated to salvage their brushed micro quads. I happen to have 3 quads and four FC boards that use the AO8K mosfets. Right now I've shopped to the tune of about $30 to keep mine going. Based on my four FC boards, I've got about $7.50 per board to keep them from being done in just by mosfet failures. If someone wants to do a mass buy of the right mosfets for everyone to draw on, you could probably get that cost down to $5 per board.
Mar 14, 2017, 06:32 PM
Registered User
Just wondering - how many hours should the brushed stocked motors last?
Mar 14, 2017, 06:37 PM
Registered Fiend
microrcfiend's Avatar
I'm over50 flights on mine and flying fine still.
The only other brushed quads I flew more were the wltoys 939 which I was getting close to 100 on befre changing motors.
The Dark edition MMW I got around 70 before yaw issues started happening but also ran large 750 lipos so they ran very hot.

If they are bashed about, especially downwards on the shaft that can shorten the life drastically too.
Mar 14, 2017, 06:42 PM
Registered User
I started having yaw issues with mine last weekend, and figured I have a motor failing. I have around 80 flights on it, so it sounds like that's about right.
Mar 15, 2017, 02:05 AM
Registered User
Quote:
Originally Posted by Aerospacer
Did the original failed AO8K look burned before you unsoldered it from the board? Typically, these don't smoke when they fail.
I didnt look carefully. That mosfet stood out and looked burnt because it was pressed against some sticky foam which melted. Possibly the mosfet itself would look ok after cleaning. Certainly the next mosfet that failed looks fine (as it wasnt pressed against any foam)

Quote:
I believe the stalled rotor current on most of these 8520 brush motors is under 4 amps so unless there is a direct short, the mosfets shouldn't fail from overcurrent. I think most are failing from the internal failure of their semiconductor junctions, possibly due to momentary inductive high voltage spikes from the motors. But like you, I'm not a semiconductor expert so I speculate.
If you read my latest post, another mosfet failed on that same board, and it happened again when connecting a fresh battery. Does that rhyme with your speculation?

Either way, this failure rate is just not acceptable. The original xq95 board flew fine for >100 battery charges until I plunged it in to.... well, some polluted water. This one seems to burn through a mosfet every other flight. Admitedly, this is just 2 datapoints, but they are the exact same mosfets AFAICT, so I think the root cause is somewhere else.

Just a thought; are you guys powering your VTX off the 5V on the FC ? Maybe there is a correlation there? I dont know what Im talking about, but If I get a replacement FC, i will try powering the VTX straight from the battery, like with the qx95 board, perhaps that reduces the load on the internal 5V regulator?
Mar 15, 2017, 02:11 AM
Registered User
Quesion; is there supposed to be an SMD component on the pads I marked in the attached picture, right next to the fets? Thinking a resistor or something? Its so small, I just cant see. But I probably desoldered one in my latest repair attempt.
Mar 15, 2017, 02:25 AM
Germany, NDS, H
frank4wing's Avatar
Quote:
Originally Posted by ZoomNBoom
@frank4wing, if you are going to try this, you really need a fine soldering iron (assuming you dont have a reflow station) and most of all: good eyes and/or good magnifying lens.
Got bad eyes, that's why I have various magnifying lenses and there's a nice fine soldering iron too. But...

Quote:
Originally Posted by ZoomNBoom
Quesion; is there supposed to be an SMD component on the pads I marked in the attached picture, right next to the fets? Thinking a resistor or something? Its so small, I just cant see.
Yes indeed there are resistors or something. They are actually the reason why I say I might not be able to properly replace the transistors even with the equipment mentioned above....
Mar 15, 2017, 02:45 AM
Registered User
If its a resistor, I can just harvest one from my other FC. Thats assuming I manage to desolder and solder it without losing it, but at least its worth trying. My worry is that its a diode or something that needs correct orientation.

edit: there comes a point where one just has to say: the weather will be good, you can fly your big quads or planes outside while you wait for replacement parts for the micro. This is just too small for me to solder, even if I knew in what orientation. On my finger tip you (barely) see the SMD component I desoldered from the old FC. Its hard enough to even position it, everytime my soldering iron comes near, it just sticks to it.
Last edited by ZoomNBoom; Mar 15, 2017 at 03:35 AM.
Mar 15, 2017, 02:52 AM
Registered User
BTW, I just saw this little frsky receiver on bg:
http://www.banggood.com/Mini-FrSky-8...ampaign=fd1000

Full range "1Km", telemetry, ULF antenna connector, and yet only 1.5g. Almost sounds too good to be true? I ordered one, along with the old style flight controller with integrated nothing. I'll lose the OSD, but at least I will be able to replace flight controllers without throwing away a perfectly fine receiver each time, I'll get a full range receiver and the square mosfets aerospacer says never fail.
Mar 15, 2017, 04:36 AM
Registered User
jvdmeulen's Avatar
What's the difference between the black and red racestar 8520?
Mar 15, 2017, 05:13 AM
Registered User
None AFAIK, besides the color.

And hey, my RS conversion FLIES AGAIN.
I just remembered my brother stored this little gem in a cupboard in my hobby room:


A reflow station. I never used it, but Ive seen it used, and if ever there was a time to try it . Desoldering components is a doddle with this. Then I cleaned off all the solder of the board with some copper wick, applied some solder paste that has been in my refrigerator for over year to the mosfets and that whatever it is, resistor ? Put it in place, hit it with the gun, and voila..

well, of course in my first attempt I managed to accidentally desolder the mosfet for the first failed esc I had already replaced. Then it turned out I chose the one broken mosfet on my old board to replace the second broken one on my new board. But on the third attempt, she flew. For how long, we'll see, but to be safe, I already wired the VTX to the battery.

edit: connecting the VTX +5V directly to the battery does give a little bit of line interference. And now the two front motors spin up whenever I plug in a battery. Those are the ones that behaved normally until now, and the only 2 mosfets I have not yet replaced.

The board looks like a mess since I damaged the solder mask , but soldering itself looks good to me.
Last edited by ZoomNBoom; Mar 15, 2017 at 05:49 AM.
Mar 15, 2017, 05:30 AM
Registered User
jvdmeulen's Avatar
thanks,

ah nice, a reflow station.
Maybe clean the board with a bit of alcohol?


Thread Tools