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Old 2018.09.07, 08:37 AM   #1
LJH
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MR03 RWD FET's?

Hello All,
Newbie to the Mini-Z's and having a ball with it. I have about 15 hours on my car doing hot laps on a hardwood floor. It took a bit to figure out how to get traction but between chassis set up and floor cleaning my Circuit works well and offers pretty decent traction.

OK, with that said last night I cleaned the floor to do some hot laps, set up my timer and put the car down. It went forward for a second and then nothing. Reverse still works and steering works. I checked a few things and saw nothing and then did the sniff test...seems I let the smoke out of something. I opened it up and the FET's looked fine until I looked at them under some 10X glasses and could see a very small blister on the inboard FET. To be honest I am surprised this happened because I am still on the stock motor and running the 7T pinion and have not done anything that would cause the failure. In taking a look at the FET Kyosho uses 4606 FETs.

I am pretty decent at soldering so I want to try to swap out the FETs myself. Looking at how the board is laid out I am not sure stacking the FETs will work because of space reasons. On the RWD board the FETs are on the bottom of the board and there is just not that much room.

I have a couple questions:

1. Has anyone stacked FETs on a MR03 RWD board? Is there space to do that? There are no traces on top of the board to do a over/under setup.
2. What FETs should I go with? If I am doing this I would like a bit more overhead so I could run a hotter motor in the future. To me the 4606 FETs feel fine in response and I understand some FETs are not as linear as others.

Thanks all.

Cheers,
Jim
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Old 2018.09.07, 11:27 AM   #2
EMU
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Welcome to MZR Jim. We have chatted a bit on RCtech.

Here are some data sheets for the fets.

4606 fet
http://dalincom.ru/datasheet/AO4606.pdf

8858 fet
http://www.onsemi.com/pub/Collateral/FDS8858CZ-D.PDF

I have been out of the game for a while, and the 8858 was the best fet to use at the time that I was racing.

Some key notes from the data sheets:
8858 N
Max rDS(on) = 17mΩ at VGS = 10V, ID = 8.6A
Max rDS(on) = 20mΩ at VGS = 4.5V, ID = 7.3A

8858 P
Max rDS(on) = 20.5mΩ at VGS = -10V, ID = -7.3A
Max rDS(on) = 34.5mΩ at VGS = -4.5V, ID = -5.6A

===================================
4606
N P
ID = 6.9A (VGS=10V) -6A (VGS=-10V)
< 42mΩ (VGS=4.5V) < 58mΩ (VGS = -4.5V)

Amperage capability of the 4606 is not really shown at 4.5v, but you can see that the 8858 has a higher amperage rating at 10v, and has a lower resistance rating at 4.5v. The resistance rating will be seen as more punch out of the corner.

I am not an electrical engineer in any shape or form, but these are what I look at when judging a fet. The older 3010 fet has a LOT more resistance than the 4606. The 4606 actually looks quite good, and I am really surprised that it was damaged especially with a stock motor. The long runtimes definitely could be a factor here, but with such low load I wouldnt expect it to have issues. I think the issue could be more that the motor overheated, and with that drew considerably more amperage?
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