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View Full Version : Questions about fet mod for mr-03 board


Velik
2011.01.15, 08:11 PM
greetings,

I just got a MR-03 as a gift and I'm trying to find out what I need to do to safely run an Atomic chili motor. I noticed that the mr-03 already has 4 total fets, 2 on top and 2 on bottom of the board. The stock fets are 3001.

1. Can a stock mr-03 board run an Atomic Chili safely? (even if it doesn't match the motors power potential)

1a. If a stock board is risky, then can I replace the 4(total) stock 3001 fets with 4(total) 4562 and run the chili safely.

Advice is greatly appreciated!
Velik

color01
2011.01.15, 08:26 PM
No.

Yes. :) A 2x2 of any of the popular FETs (4562, 8858, AN0113, etc.) will handle a Chili.

Velik
2011.01.15, 08:35 PM
thanks color01! time to break out the soldering iron then!

SaiTam
2011.01.16, 10:51 AM
I had run Chili on Stock Fets for a while without blowing up the FETs. Low gearing and good drivetrain are a must. Keep an eye on Room temperature/Motor temperature.

If you can solder FETs why don't you give it a try first. You are discarding the 3010 FETs anyway.

EMU
2011.01.16, 12:18 PM
I have used almost every mod motor with the stock fets, the only one that fried the fets was the old Anima II which is discontinued. A fet upgrade is recommended, but as Sai said... proper gearing/gear mesh and room/motor temp are very important. If the motor gets hot, it draws more amps, increasing the load on the fet. I currently use the Z2, PN 39t and PN 33t on different cars which dont have a fet upgrade. If you gear too high, your going to risk damaging the fets.

Velik
2011.01.16, 03:48 PM
well, i opened the fets from shop.tinyrc.com which are vis-si4562dy . I took a look at them and im having a hard time ciphering if the dot in the lower left corner is the orientation dot or a logo? can anyone confirm. it sounds like a dumb question when you see the image, but i was hoping someone could answer who has ordered from tinyrc.com, usually this sort of thing is etched into the casing...

Thanks again!

Update:
Ok, thought I'd have to wait for a reply from tinyrc, but I found my multimeter so I'll pair that with the schematics to figure it out.

Action B
2011.01.16, 07:43 PM
From experience I do not recommend pushing stock FETs unnecessarily. They aren't that expensive to change out and when they go they can cause pretty serious damage to the board traces. Its cheap insurance when you consider the price of the board and the work we put into these cars to make sure they are right.

This should help with orientation:

http://farm6.static.flickr.com/5166/5362238892_ed726acc08.jpg

Velik
2011.01.16, 07:53 PM
YIPPPPEEEEEE!!!!! I didnt blow up my car! :) yep, its not a logo, its a dot for sure. Some fairly tight spaces to solder in there to.

btw, I found a technique where I would insert a strand of wire under the FET legs and pull it through a few inches and twist it together to the other end to lasso the 4 legs, made it easy for me to get good leverage and pry one side up, after that I just broke off the other side and use the iron to remove the metal bits from the stock fet legs. Im sure there are better ways to handle it, but I had limited tools so thats how i went about it.

Cant wait to get the atomic in next week

Thanks for the help everyone

Velik

EMU
2011.01.16, 07:57 PM
I feel that the 3010 fets have the best feel and throttle control, which is why I only have one car that has upgraded fets, which I use primarily as a 70t setup :rolleyes: The faster the motor, the more important it is for me to have a good feel for the throttle, which I feel that I lose with upgraded fets. Even with a 39t, or 35t, I dont feel that I push the fets unneccesarily. The motor runs cool, with a 9/53, and I get good runtime. A Chili with anything more than a 9/54 64p pinion is too fast. I usually use a 7 or 8/44 with the chili. I wish that there were lower gear ratios with 64p, but I dont think that they can do anything smaller than a 9t, and a 55t would just be too large for most rear wheel/tire combos, but would be more than welcome on F1 and LM :)

Action B
2011.01.16, 09:08 PM
Quite frankly, I can't tell the difference in throttle control. Maybe I just don't have the magic touch.

I can't really think of a good reason why upgrade FETs would reduce throttle control if not stacked. Anyone with a solid answer on this phenomenon?

color01
2011.01.16, 10:53 PM
Every FET has a different profile for its gate-opening characteristics, i.e. the amount of current it flows to your motor for the amount of voltage you apply to the gate. The control electronics can apply a set range of voltages to the FET gate, something like 0-3.3V IIRC, so if the profile is very linear (0% at 0V, going to 100% at 3.3V) then you'll get very linear control as you raise the voltage, whereas if the profile has a sharp jump (say, 0 to 100% in the first 0.5V) then it'll feel very punchy but you lose some fine control.

There are further affects brought about by PWM, too -- if you hold partial throttle the PWM is on-off-on-off, but the voltage rise and drop takes a small amount of time. A sharp-profile FET will spike lots of current-zero current-lots-zero-lots-zero, etc. to the motor, if the voltage is in that threshold where the gate output suddenly shoots to 100%. On the other hand, if the gate voltage is much above or below that threshold (more or less throttle) then the FET will either send lots-lots-lots-lots or zero-minimal-zero-minimal current. This inconsistency is the source of "poor throttle control" with some FETs.

If you have a linear-profile FET, the FET doesn't have a particular range where the gate output varies too much, and the gate output due to any PWM signal from the control circuitry becomes more-less-more-less no matter what range of the throttle you are at (low-mid-high). This smooths out the motor's output, giving you better throttle control at all ranges of throttle.

That's my understanding of how this whole kit works, if there are any experts here I too would like some further education. :)

Action B
2011.01.16, 10:59 PM
Is this something I can look at on a spec sheet?

I find it hard to believe that the difference is that huge if I can't even feel it on my own car. I mean I have TRIED too. I put my car on the speed tester and slowly pulled throttle and stabilized the speed of the car at each KM/H from 1-24 for at least 5 seconds with the same car but multiple boards installed so that there was no other variables. This indicates that there are at least 24 and in my opinion likely more but with only whole numbers on the speed checker I cant say for sure.

That being said I can say for certain I have tested in the above manner the following setups.

PN FETs in 2x3
8858 in 2x2 and 2x3
Stock FETs


I did find that certain boards no matter what the stack or FET type had less throttle resolution.

In my opinion, if you need more than 24 steps (probably much more) on a stock car you are either 1. imagining it or 2. VERY SENSITIVE. My honest guess is that its highly over-exaggerated at the least.

color01
2011.01.17, 12:34 AM
Yes, the spec sheets for FETs should have a graph of some sort showing the gate opening profile. I don't know the actual name for it though, like I said I'm not claiming to be an expert.

I haven't driven enough cars to actually feel a difference between different aftermarket FETs, but I will say that my car got noticeably punchier and faster top end when swapped to 3x2 AN0113 FETs (I had stock 3010). Arguably that's just lower resistance and better current handling in general. IMO the batteries make a much larger difference, just changing the charge rates for your batteries noticeably affects the smoothness and punch of a run.

Action B
2011.01.17, 09:36 AM
Yes, the spec sheets for FETs should have a graph of some sort showing the gate opening profile. I don't know the actual name for it though, like I said I'm not claiming to be an expert.

I haven't driven enough cars to actually feel a difference between different aftermarket FETs, but I will say that my car got noticeably punchier and faster top end when swapped to 3x2 AN0113 FETs (I had stock 3010). Arguably that's just lower resistance and better current handling in general. IMO the batteries make a much larger difference IMO, just changing the charge rates for your batteries noticeably affects the smoothness and punch of a run.

I think you hit the nail on the head here with this comment. I 100% agree with your opinion and your suggestion of the feeling of lower resistance/current handling of upgrade FETs.