Opinions (& answers?) needed regarding FET stacks….

[Admin]

Mechaone here. Ive been into xmods and mini scale RCs for over a year, contributed a bit to Xmodworld.com and am a fledgling member of InfernalRC. Though I am fairly knowledgable of Gen1s, I still have yet to wrap my head around the dynamics of FETs and how stacking affects current – resistance, etc. I have posted an inquiry on the above two websites regarding FETs, but havent received any feedback or assistance. That being said, given that I bought the FETs I have from ausmicro on ebay I thought Id try here….

What follows is the text posted on XMW & IRC. Any help or links to relevant resources would be awesome:

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‘Allo fellow RC freex.
I have ten FDS9934 FETS I want to drop on some gen1 xmod ESCs. Im not sure if I should do a 2×2 stack on each of two xmods and replace the stock FETs on a third board, or if I should just 2×2 stack one speed controller and 3×2 stack a second.

The opinions or answers I need are regarding where the two extra FETs would be more effective and beneficial. Would it be worth swapping stock 450 mOhm FETs for the 58 mOhm 9934’s, or are the extra FETs best added to a 2×2 stack to create a 3×2 stack?

What is the general resistance difference between a 2×2 & a 3×2 stack? Is there an algorithm for determining how stacking will affect resistance, and thus current capacity? If only a nominal current capacity increase from 2×2 to 3×2, Ill prolly just replace the stock FETs on the third car.
My last question or posed criteria for thought is: If the stock gen1 FETs could only tolerate scarcely a few amps or roughly a 50-52 wind motor, does anyone know how to rationalize the potential motor wind count tolerance or capabilities when applied to the 1×2 replacement with 58 mOhm FETs?

Suggestions? – mech

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So thats it. Like I said any help would be great. At this point Im willing to accept anything, including sarcasm & snarky comments.

Thanks guys. – mech

[klims]

given 3 resistors, R1 R2 and R3, connected in parallel the total resistance, Rt, follows the equation

1/Rt=1/R1 + 1/R2 + 1/R3

if you have a 2×2 stack of 50 ohm FETs the effective resistance would be
1/Rt = 1/50 + 1/50
1/Rt = 2/50
Rt = 25 Ohm

hope this helps!

[Traveler]

I’ve had similar questions about FETs. Not the technical stuff, just whether I should go with 2×2 or 2×3. I bought some SP8M4s from Aaron for use on Gen1 Xmods and MR02 Mini-Zs. So far I have done a 2×2 and 2×3 stacks on the Xmods, and WOW!

Not sure about the current, etc. but the 2×2 stack made the car fly with a stock motor (Speedy BB05) and the 2×3 stack made my second car REALLY fly with a Speedy BB07.

All I can say is I love these FETs! Do a search on this forums for SP8M4 and you’ll see what others have said.

Can wait for the Spider v4.0!

[Aaron]

My personal feeling is that you’re always better off using better FETs (9934, 4562, SP8M4) in place of the stock FETs.

2×2 is the best combo of performance and “easy”, but even just as a stock replacement 1×1 you get massive improvements in performance, run times and all that.

3×2, 4×2 and 5×2 are all awesome but are progressively harder to build up and can affect throttle response etc…

I sell 10 packs becuase it’s a good number to do a few cars so people can race with matching electronics. (and most people who fiddle with their electronics have more than one car!)

A.

--
Site Owner Guy.

[Admin]

Thanks guys. I really appreciate the feedback.

@klims 226940 wrote:

given 3 resistors, R1 R2 and R3, connected in parallel the total resistance, Rt, follows the equation

1/Rt=1/R1 + 1/R2 + 1/R3

if you have a 2×2 stack of 50 ohm FETs the effective resistance would be
1/Rt = 1/50 + 1/50
1/Rt = 2/50
Rt = 25 Ohm

hope this helps!

Klims, the resistance equation helped greatly.
That brings the approx FET stack resistance to 29 mOhms with a 2×2 stack arrangement. Formidable is what that is, when compared to the 450 mOhms of the stock FETs. That being said I think Im going with 2×2 (x2) and a straight swap on the third car.

Traveler, Ive had the 9934 FETs for a while, but I intend to buy some SP8M4s once Ive made use of them. I had no idea they were out till a few weeks ago.

Aaron, you da man. I had a feeling I should have posted my inquiries here first. Cheers for the awesome ebay deals as well…

So the last questions would be:
What motors or wind count would a 29 mOhm (2×2) stack tolerate @ 7.4V?
PN anima? PN Xeric?

What motors or wind count would a 58 mOhm (1×2) stack tolerate @ 7.4V?
Im guessing I could probably run a Speedy07BB with neos at 7.4V and still be safe unless I stall with the throttle pulled for a while.

What is the relationship between the resistance and current capacity of the FETs? Does the 29 mOhms of the 2×2 stack translate to a figure in terms of current?

Im a pain in the arse, I know. – mech

[klims]

all the characteristics you will ever need are clearly laid out in the manufacturers datasheets of the mosfets.

just keep in mind that because you are running a stack and not an external turbo you run the risk of frying the tracks on the stock board under high currents.

[Admin]

Thanks again Klims.
Ive been thinking about that. Does anyone know if the spider is still commercially available?

[Traveler]

The latest and smallest spider v4.0 is due out any day now. I’ve been waiting anxiously for a couple of months for them, but there has been some delay as a result of a problem in manufacturing. See Aaron’s thread about the new v4.0 under technical. I’ve used the v2.1s and they are awesome, so I have high expectations for the v4.0!

Just keep an eye on the store (I check it daily) or on eBay. Would attach a link here, but don’t know how!

[Aaron]

Just waiting for the boards to arrive…

Should be with me on the other side of Easter and I will have the first units availabile within 24 hours of them arriving.

More details will be posted in the next few days as I’ll start allowing pre-orders.

A.

--
Site Owner Guy.

[Admin]

Gracias mi amigos.

[Author]

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