Introduction: Burnt ESC? How to Revive a Dead ESC?
Hi, fellow RC hobbyist. First of all, I'm not very serious in RC since I play more on robotics, but in real case, RC components are widely used in this field too, especially servos and outrunner. They are great for low budget robot with reasonable build quality, unlike other robotic shop that sell the components at killing price, well, you know.
Anyway, What I'll like to mention in this post is to do something which may be done by experienced electronic guys. ESC (Electronic Speed Controller) is a great stuff, I never see something as small as this can actually suck up so much current from battery. My first ESC is the 25A Turnigy ESC, as advertised by HobbyKing (so do I owned it), my outrunner is rated at 30A but I used that ESC since they said can actually support up to 60A (Really? o.O). So I tried it up. It actually works, but won't last long. After a couple of satisfying moment, it simply "POP", I can see some smoky fireworks around the ESC. That is a bad news, since I don't have any backup ESC left, so my project halted. I have to wait for another 3 weeks to get the new ESC.
You guys may say that I should get another better ESC, but I'm just as stupid as buying the same ESC again, for my second purchase because I just love its programming card, and they are somehow equipped with many programming options as required in my robotic project. So I set it up and run again. As expected, it works, just the ESC last longer for this time, it did break down at last. Then I realized that something is wrong in the advertisement. Well, I actually want to complain such case to HobbyKing, but since I lost both of the ESC, why not just hack one of it and have a look its inside? At least I can have fun to look what's inside, maybe.
Step 1: Inside Look of the ESC
*Well, I'm sorry, I've forgotten to take the real picture, so the pictures shown in this page is the ESC taken over the internet, which is exactly the same as mine one.
As you can see, there are arrays of MOSFET Half H-Bridge with NMOS installed. The NMOS are dead, killed by reverse polarity connection. I didn't use proper socket for power source for last time, so MAKE SURE you use socket to prevent such things happen!
Back to the point, assuming you know what is MOSFET, the transistor is LR7843 N-Channel MOSFET, with 3.3mOhm on resistance (Rds on) as shown in datasheet. Simply speaking, MOSFET is a transistor just like BJTs, but is voltage controlled . Well, considering power loss, if my outrunner draw 30A continuously, the power loss as heat for each MOSFET will be (30*30)*0.0033=2.97Watt, so 6 MOSFET will dissipate 17.82W of heat for continuous 30A draw, which is awful for such a small heat sink in my project. So there are 2 options to overcome such problem, either using a larger heat sink or use MOSFETs with lower turn-on resistance.
Anyway, I don't trust the LR7843 MOSFET as it spoiled twice, maybe the 1st time is my fault, but for the second time it just burnt out without reason, like what have been complained by other people too. So I decided to replace it with the better one. I found a relatively cheap MOSFET, PSMN1R2-25YLC N-Channel MOSFET with 1.3mOhm on resistance, just released in 2011. That was great, the chip should improve the efficiency and performance of the ESC and probably increase the switching current capability, but the package and pinout is totally different from the previous MOSFET. I can use another MOSFET with same package but I simply love the MOSFET's electrical properties too much, and the MOSFET of lower on resistance with same given package is way more expensive. So I decided to replace the LR7843 MOSFET with the PSM*@%$#@ (just kidding).
Step 2: Replacing the Transistors
But the packaging of the new MOSFET simply won't fit the on board layout, so I set out to make a adapter board.
After that, the new MOSFET will be soldered on this board, and the board will be soldered on top of the ESC. I used toner transfer method to make the PCB. About how to make the PCB, you can refer to the other great posts, maybe I'll make one post about that next time.
Maybe you may ask why don't just get the MOSFET with same package to replace. Firstly, I'll like to see whether this small MOSFET can perform as well as the bigger one. If so then I can use the smaller MOSFET for next projects to save my precious money and time, right? Actually they only differ from power ratings, bigger package can drive at higher power with given enough heat dissipation.
Step 3: The Results
Walla, all transistors are replaced with the new one. I don't even consider to place heat sink given that its ultra low power dissipation, so I soldered it unevenly. Even by calculation, the heat dissipation now should not more than 7W at full thrust (Even the oturunner can't handle full thrust for long time too since it can overheat for long run)! Check out the video for the results!
As you can see for the background picture, I'm not a mathematician, I'm currently at university studying engineering. But I'm more interested to practical hands on like this one ^^.
Thanks for your interest.