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ECU Burn-out problem

Old 08-27-2008, 07:36 PM
  #1  
CraigG
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Default ECU Burn-out problem

I've had a recurring problem with my P-120SE ECU's burning out on my MIBO A-10 during engine start. I've gone through 6 in 12 flights. Bob Wilcox has been working with me and we initially focused on shortening the long runs from the ECU's to the engines but the problems still persist. Although no one who has looked at my installation (including Bob and other JetCat reps), can find anything obviousy wrong, there has to be something causing the problem. I would like to briefly describe my setup and entertain any suggestions on what I might change to solve the problem.

I have located my ECU's, UAT's, fuel pumps and starting gas valves up front under the cockpit for accessibility and cg purposes. All the fuel, power and data cables are running together (away from the servo leads) down the center of the airplane. The batteries (JetCat supplied A123's) are in the nose.

The problem occurs during engine start and usually aborts after the kero starts flowing with a "slow accell" message. Afterwards, the ECU will not spin the motor, either manually or for start (although all other functions are normal and I can actually start the motor with a blower and go fly). The problem is intermittant and I may get 1 to 4 normal starts before the ECU fails.

I have been offered suggestions including separating the data cables from the other leads, replacing the carbon fiber ECU, IO and RX boards with ply, and using anti-static additive and fuel lines.

It's a little hard to tell from these pics but the fuel pumps are mounted on the bottom and the ECU's on a board about 2" above them.

I'm ready to try almost thing (within reason) to isolate and eliminate this problem.

Thanks,

Craig



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Old 08-27-2008, 09:46 PM
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Airforce7
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Default RE: ECU Burn-out problem

Not sure exactly what is causing your problems, but I have a few suggestions based on your photos. I wouldn't group the pumps and ECUs together like you have it. I would have tried to isolate the components along the side wall of the fuse going to each turbine. If you have the original 6 cell batteries I would try those. Are you trying to regulate the A123s? For the propane valves, I took those out of my jets a long time ago. I hook mine up when I start the turbine via a 4mm Festo fitting. I like to hold the can and gently shake it during start-up so that the propane feeds the turbine better (just my little technique). You may also need to nudge the electric starter on the turbine to improve the o-ring grab on the compressor nut to get better rpm ramp-up. Your com cables are grey, normally they are black. Did you get those from JetCat? I would dump the I/O boards too, you can just leave the com cables connected to the ECUs and hook them up directly to the GSU. One last thing, I would try a totally different receiver just to see if there's any difference in the start-up. Just my 2 cents.
Old 08-28-2008, 06:24 AM
  #3  
CraigG
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Default RE: ECU Burn-out problem

Thanks Airforce. I had the same problem with the original JetCat 6 cell Nicads and I switched to the A123's (unregulated) on the suggestion that the extra amps might help the situation (but did not). I have also replaced my homemade (gray) data cables in those pictures with some JetCat custom length ones (also no help).

I like your suggestion to eliminate the IO boards. Any electrical component I can get rid of is one less potential source of the problem.

Just curious as to what the advantage is of separating the ECU's and pumps. The only connection between the two systems is the throttle channel which is connected via a "Y" cord (which I have now switched to 2 separate outputs from the rx). I have noticed that the problem usually occurs when the first engine is running and I'm trying to start the second, so maybe there is some sort of interaction.

Whatever the case, something is causing a serious electrical spike in a specific part of the ECU and it seems the electrical guru's could come up with an explanation/fix.

Thanks Again,

Craig

Old 08-28-2008, 08:16 AM
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RCISFUN
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Default RE: ECU Burn-out problem

Hello Craig, we talked at Heart of Ohio.

Your problem could be a couple of things in my opinion;

1) The inductance from the long leads are inducing a back EMF* voltage problem for the MOSFET drive transistor located in your ECU from the highly inductive load (i.e. starter motor and leads). As the magnetic field collapses in the windings of the motor and leads it induces a high reverse voltage that is detrimental to the MOSFET.

2) As I do not know the part number of the MOSFET that is used I can only theorize that the Rds on resistance of the MOSFET may be too high and it is causing a localized over temperature of the MOSFET which in turn could cause it to burn out (i.e. too much power dissipation in the device and not enough heat sink). However this seems unlikely because this would have nothing to do with lead length of your setup and everyone would be complaining of ECU starter failures in normal (i.e. short lead) installations.

To solve for number 1 is easy if you are comfortable using a soldering iron.
You can install a Flywheel Diode to protect the MOSFET; ideally you would want the flywheel diode as close as possible to the device you are protecting; however, I’m sure you don’t want to try to stuff the diode in the ECU enclosure; therefore, I would install it as close as possible to the connector of the ECU.
You want the Cathode of the Shottky Diode connected to the most positive lead going to the starter motor and the anode connected to the most negative lead.

I would use a Shottky Diode that has forward current rating of 20 A (or more) and a reverse voltage rating of at least 15V, one suggestion is as follows:

http://search.digikey.com/scripts/Dk...PS20L15DPBF-ND


* http://en.wikipedia.org/wiki/Back_EMF

Rich
Old 08-28-2008, 09:19 AM
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Default RE: ECU Burn-out problem

Thanks Rich,

I'm not an electronics guy but this sort of explanation sounds plausible to me. To be clear, I would bridge the diode between the positive and negative leads from the ECU to the starter motor? This is a pic of the connector from the ECU to the motor. One positive lead and then a negative for the starter and one for the glow plug (I presume), not sure which is which. Is it OK that the positve is shared by the starter motor and glow plug?

Anybody else see a problem with trying this?

Thanks,

Craig
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Old 08-28-2008, 09:20 AM
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Default RE: ECU Burn-out problem

Hi Craig,

Bob and I discussed this last night for almost an hour.

I shared Rich's thoughts about the reverse EMF. The "flywheel" or clamping diode is already in the system, it is located inside the turbine. I thought maybe this part has failed, but JetCat has changed out these boards, and both turbines are affected as well.

There are many people flying almost identical setups. And as best we can tell this is utterly unique to your plane. Bob is speaking with Germany to brainstorm and I think that Bob is going to talk to you this morning, Craig.

Regards,

Old 08-28-2008, 09:57 AM
  #7  
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Default RE: ECU Burn-out problem

Mr. Matt

I think you might have hit the source, while the Flywheel diode is ok being located in the Turbine as it protects the components in the Turbine from the back EMF of the motor however the long leads are exasperating the situation, and protection from the long leads needs to be closer to the device you are protecting, i.e. the MOSFET.
As I stated before I do not know the P/N of the MOSFET being utilized in the Jet Cat ECU therefore I do not know if the device has a built in flywheel diode, many MOSFET’s will have this feature while just as many do not.
It seems like a simple solution to add a protection diode back at the source


Rich
Old 08-28-2008, 10:01 AM
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Default RE: ECU Burn-out problem

Craig,
you want to be as close as possible to the exit of the ECU, as Mr. Matt has stated the Turbine end is already protected, ideally the protection diode would be soldered directly on the MOSFET transistor on the PC board; however you may not want to do this for obvious warranty situations.

Rich
Old 08-28-2008, 10:24 AM
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Default RE: ECU Burn-out problem


ORIGINAL: mr_matt

Hi Craig,

Bob and I discussed this last night for almost an hour.

I shared Rich's thoughts about the reverse EMF. The "flywheel" or clamping diode is already in the system, it is located inside the turbine. I thought maybe this part has failed, but JetCat has changed out these boards, and both turbines are affected as well.

There are many people flying almost identical setups. And as best we can tell this is utterly unique to your plane. Bob is speaking with Germany to brainstorm and I think that Bob is going to talk to you this morning, Craig.

Regards,

Thanks Matt, I really appreciate it. I'm off to work and except for occasionally being able to answer my cell phone (770-314-1289) I will be out of touch at home except before 11:00 am and after 9:00 pm EST the next couple of days.

Craig
Old 09-02-2008, 07:14 PM
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Default RE: ECU Burn-out problem


I had the opportunity to open up a buddy trashed V6 ECU to see what MOSFET was used, it turns out that it does indeed have a flywheel diode integrated into the device for protection against back EMF. The device is rated at 75 Amps, 60 Vdc with an Rds on of 10 mohms. The catch is the gate voltage needs to be driven at 10 V in order to get these ratings.

I don’t know what the gate drive circuit from the processor is; however if the gate is being driven at 5V (typical uP voltage) the RDS on increases substantially which in turn will cause the package power dissipation capability to be exceeded due to the increased source to drain resistance.

"My theory" is you can end up in a case of diminishing returns, the high current required during the start sequence (motor and glow plug) causes a voltage drop of the supply voltage, and the voltage drop lowers the gate drive voltage which in turn lowers the devices capability to switch high current, the device then goes into thermal runaway until ultimate destruction.

This seems to follow what Craig explained as to what was happening with his ECU’s during the start sequence.

I copied the graphs from the MOSFET data sheet, the right hand graph show the relationship of gate voltage vs. drain current.


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Old 09-09-2008, 08:33 AM
  #11  
RCISFUN
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Default RE: ECU Burn-out problem

Craig, any progress to report?
Old 09-10-2008, 10:37 AM
  #12  
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Default RE: ECU Burn-out problem

ORIGINAL: RCISFUN

Craig, any progress to report?
Rich,

Yes, I got word from JetCat that they experienced a similar problem in Europe that was traced to carbon fiber mounts allowing electrical contact between components of the left and right engines. Turns out that any such contact can cause a burn out in the ECU starter circuitry such as I was experiencing. I have removed my carbon fiber trays and relocated my ECU's further back in the fuselage on a plywood mount as well as seperating the fuel pumps. I had several flights last weekend with no ECU problems so this has apparently solved the problem!

I still have some issues with "accel.slo" start aborts but these appear to be related to getting the "U excel" values matched to my new component configuration. I also discovered one of my fuel pumps is weak/bad and this may have been masking/contributing to the ECU problem.

Thanks to Matt Carroll and Bob Wilcox for getting to the bottom of this problem.

Thanks also to you to Rich for your suggestions.

Craig

Old 09-10-2008, 11:20 AM
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Default RE: ECU Burn-out problem

Craig,
Great news on solving your difficulties, I look forward to talking to you in the near future

Rich

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