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Repair heli controller

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olly_k

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Hi folks ok I will start by saying I am a bit rusty with fets and so need some help deciding on a possible substitute for a SO 8 dual N-channel mosfet i.c.

Basically the chip is a APM9926 ( APM9926 pdf, APM9926 description, APM9926 datasheets, APM9926 view ::: ALLDATASHEET ::: for data sheet) and contains two enhanced N channel mosfets with shunted diode. This is is out of a receiver from my RC helicopter and controls the tail rotor.

I have already spent a load on spares and can't justify replacing the whole lot ATM. I cannot find this ic over here in the UK so hoping to replace for a couple of individual SMT mosfets & diodes if possible...?

Incidentally reason it blew was a result of my stupidity - I replaced the wires in tail boom with enamled copper to try to reduce weight / minimise volt drop and stupidly drove a screw through them creating a short Arrrrhhh! I wouldn't mind but there was a little voice inside my head sort of telling me to do continuity but I was too eager to fly after a few hours modding various bits and so the price I paid was seeing smoke when I tried to lift off!

Any help appreciated!
 
This is an RC heli ESC? What size heli, and what kind of controller is it (or just voltage/current rating).

Basic guidelines...get a NMOS with the same...
-package (duh!)
-at least the same voltage rating
-equal or less RDson at equal or less gate voltage (pay no attention to the current rating given...the way they got it can't be trusted)
-equal or less "Total Gate CHarge" at similar voltage conditions to those given in the datasheet for that parameter

You want to balance the 3rd and 4rth points because one determines how much the transistor heats up from conducting and the other determines how much it heats up from switching on/off. SInce you don't have the design of the ESC, you don't know why or how they balanced the two parameters, so you have to stick with what they used. If you knew, then you could choose a different balance between the two parameters...but you don't

IRF7402 comes close.
 
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This is an RC heli ESC? What size heli, and what kind of controller is it (or just voltage/current rating).

Basic guidelines...get a NMOS with the same...
-package (duh!)
-at least the same voltage rating
-equal or less RDson at equal or less gate voltage (pay no attention to the current rating given...the way they got it can't be trusted)
-equal or less "Total Gate CHarge" at similar voltage conditions to those given in the datasheet for that parameter

You want to balance the 3rd and 4rth points because one determines how much the transistor heats up from conducting and the other determines how much it heats up from switching on/off. SInce you don't have the design of the ESC, you don't know why or how they balanced the two parameters, so you have to stick with what they used. If you knew, then you could choose a different balance between the two parameters...but you don't

IRF7402 comes close.

I guess it is the ESC part of the controller the whole heli controller is one unit including gyro. It is a Walkera 4#3b heli btw. The main motor appears to be driven with standard surface mount FETS but tail motor with this package for some reason. I initially thought the transistors were ganged together but not so sure now...

For your enlightenment there is a piccy here..

... the 'dot' denotes pin 1 btw :D

**broken link removed**

I am not bothered about the same package as I could work round that I just wanted to be certain I could potentially pull it off.

I will work on your info and check that art you suggested many thanks!

olly.
 
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the most important point is to troubleshoot the couse of this failure
becouse if you replaced the faulty NMOS without knowing the couse of the trouble, the new NMOS will again burn
 
Suggestion for APM9926 substitute

Olly_K: if you are still looking for a part, try this: SI4910DY-T1-E3CT from Vishay Semiconductor. In the US, I get them from DigiKey. I am repairing a Walkera Lama mini helicopter -- this dual MOSFET chip has the same specs and pinout, with a slightly higher dissipation and continuous current. In my mini, the APM9926 controls the two main motors. And in this case, the failure was the result of the heli landing sideways and stalling one of the motors even though the remote was at zero throttle. Instant smoke! The IRF7402 unit mentioned in a previous reply is a single MOSFET so it isn't a direct package replacement.
 
Thanks I sorted out some time ago now with an IRF 9910.

I also decided to mount on the outside of the 4in1 just in case it happens again...

**broken link removed**

Thanks for the help erickphd!
 
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Nice job!

Olly_K:

Good idea! My experience is that there isn't much margin of error on these things, but I'm sure that is part of a marketing plan!
 
Definitely marketing. The one thing I think Walkera got wrong with this great little heli though is motor life... 35 flights on average before it dies really is unacceptable in my opinion.
 
I am tring to fix the board from a Novus CX with the same issue! I have already changed out the dual channle mosfet with a replacment from digikey. However the motors do not run. I notice there are two components on the back of the board that look blown also! I am unable to tell what they are though! I am unable to upload a pic at this moment as I am on my phone, but if anyone has one of these helis and know of the two components I am talking about any help would be great! They are the two on the back of the board that connect to the positve output terminals. Thanks for your help!!!
 
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