2003.03.13, 09:31 AM
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#1
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Mini-Z Tinkerer
Join Date: Mar 2003
Location: Chambéry, France
Posts: 17
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Are FET Stacks a con?
I really dont get people who stack 14 FET's or someting ridiculous on their controller PCB, especially as one side only controls the servo (i have heard this said but i am not sure, for full foward and revese you need a full H-bridge using two IRF7389s). IRF7389 has a maximum continuous current rating of 7+ amps and an instantaneous curent rating of 30 amps, which is already enough to blow the tracks off the pcb. A normal stock motor draws around 1A full load, a bit more for a complete stall, X- speeds a bit more than that, the tamiya plasma dash motor infamous for killing external 'turbos' around 4.2 amps full load. This is still within the ratings of the IRF7389. At full load a hot motor drawing 5A say has a voltage drop of 5 x 0.029 (Rds of IRF7389 ) = or 0.145 volts which is either 3% of total power. This being the absolute worst case senario.
i can see the sense in possibly adding a second fet to halve the on resistance reducing losses to 1.5% but 7 stack FET's is stupid - that is a continuous current of 50 amps, most Hot 1/10 scale cars dont even need that much and anyway that would destroy the circuit board and the wiring within seconds. for comparison the pieces of wire that connect the switch and motor will burn up at around 4.5 A and have about the same resitance as the FET's (0.06 ohms/metre - 30 cm between switch and motor leads = about 0.02 ohms)
i am just trying to warn people shelling out $$$ for fet stacks where it isn't necessry - a more sensible mod would be a single or double IRF7389 on each with a small heatsink if you have an insane motor (NML and the like)
My car has IRF7389s instead of HAT 4001's (which are truly rubbish) and i have run some very hot motors in it (20T and the like) with no problems at all.
Just my two cents
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2003.03.13, 10:16 AM
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#2
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Mini-Z Novice
Join Date: Feb 2003
Location: Fl
Posts: 16
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I think the concept behind it is the more FETs the lower the on resistance since you're stacking them in parallel. Also you are lowering the current that each FET flows therefore lowering the heat & internal on resistance even further since heat & on resistance are directly proportional.
This can be seen in figure 3 of the Fairchild cross part FDS8958A datasheet. I'm sure the IRF datasheet has this info on it as well I'm just too lazy to look it up...
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2003.03.13, 03:57 PM
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#3
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Z maniac : certifiable
Join Date: Oct 2002
Location: Melbourne
Posts: 447
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Maybe you should try stacking more than 2x2 FETs and you'll be able to see the differences for yourself. Having seen up to 10x2 (which mind you I feel is a touch excessive), I now have 6x2 on my PCB. The results speak for themselves, as friends of mine who have done similar stacks would also attest to.
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2003.03.13, 10:05 PM
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#4
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Registered User
Join Date: Nov 2001
Location: makati, philippines
Posts: 8,702
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i'm not a electronic whiz... but i do have the irf7398 stack in my f-1 (it has a 2x2 stack) and a regular z (2x3 stack)...
although i haven't felt much difference between the two, i have felt the difference between both of my cars compared to a 2x6 stack...
i was just wondering that there must be a limit as to how many fets you can add... like at what number of fets would performance be at optimum, such that adding additonal fets would be irrelevant?
these things can go pretty fast, and i guess there's a limit... and speed without control means nothing right? unless you're after straight line speed that is... but even so, cars trying to beat the fastest land speed record, have unfortunate accidents too...
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2003.03.14, 02:59 AM
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#5
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Mini-Z Tinkerer
Join Date: Mar 2003
Location: Chambéry, France
Posts: 17
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I think that some proper scientifi testing is in order, sadly i am not at home at the moment and dont have the necessary test equipment, and i am not going home until May 2nd but...
What someone needs to do is to take a stock mini-z, a mini z with a FET exchange and a mini z with a huge stack, hook it up to a resistive load so it is drawing say 3A and measure the voltage drop across the ESC unit to find out how much power is really wasted (you'll need two multimeters, a ~1.5ohm resistor, some mini z's and batteries) Hook one multimeter to measure voltage drop across the ~1.5ohm resitor and use I=V/r to find the current flowing, then use the second multimeter to find the voltage drop across the ESC (should be a few tenths of a volt), you can then use R = V/I to find out the overall resitance of the ESC in all three cases and calculate how much power is being lost. Having done that you may wish to measure the voltage drop across one of the motor wires.
As for herman, are you sure that the car with the 6 stack didnt have additional mods like a hotter motor?
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2003.03.14, 04:17 AM
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#6
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I am a Z-aholic ;)
Join Date: Jul 2002
Location: Melbourne, Australia
Posts: 706
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Now this is exactly what I have been waiting for...someone who appears to really know their stuff...I had a 10x2 with a hot motoron my Z, and recently reduced it to 6x2 with the same motor to see if there was any performance drop..I have perceived none as yet...this is also to determine the minimum FET stacks before either performance drop, or frying occurs.....Hmmm...me tinks I may remove another 2 FETs so it is 5x2, and see what happens.....I am prepared to fry FETs, but going on the specs, I doubt that will happen! It is possible that the there is minimal or zero performance gains above a certain stack amount..BUT, whilst installing FETs, I did notice a difference in the response as each layer was added, insinuating that for each layer added, the gate charge decreases...but this is simply logic and I may be wrong.....if so....who cares
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NOTHING is forever.....ESPECIALLY Mini Z bodies
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2003.03.16, 08:41 AM
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#7
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Registered User
Join Date: Dec 2002
Location: Hong Kong
Posts: 37
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Quote:
Originally posted by Psyklops
I did notice a difference in the response as each layer was added, insinuating that for each layer added, the gate charge decreases...
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For FETs in parallel, by Ohm's law, resistance does decrease. What about Gate Charge ?
Does Gate Charge simply adds up by each additional FET ? If so, that means stack FETs increase the Gate Charge, which is no good at all.
What do U think ?
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2003.03.16, 01:52 PM
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#8
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Mini-Z Tinkerer
Join Date: Mar 2003
Location: Chambéry, France
Posts: 17
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well it could potentially be a problem for large fet stacks, each additional fet adds gate capacitance and it could be that the drive signal no longer provides sufficant current for a fast switch and therfore leading to higher switching losses and overall loss of performance. In order to see if this a problem someone needs to scope up (i.e get an oscilloscope trace of) the gate drive signal. Personally i suspect that it is not a problem, the IRF has a gate capacitance of around 700pF, so a 10 stack (say) has an input capacitance of 7000pF (7nF), at 1000hz (guesstimate) this corresponds to a current of 6.8mA , which i would have thought the drive circuit could probably supply, but i don't know, it could easily be that the advantage of reducing on resistance by adding many FET's could be nullified by the higher switching losses.
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2003.03.16, 08:00 PM
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#9
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LIFTER
Join Date: Feb 2003
Location: florida
Posts: 1,243
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Do FET stacks cause a delay? I know the turbos do.
__________________
Toyota LC #100, Toyota LC #239, White Pajero #201, Red Pajero #200, Black CLK racer, Red Porsche GT3 (Project Pre Runner)
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2003.03.17, 05:31 PM
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#10
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Zen_gineer
Join Date: Dec 2001
Location: The Border, SC
Posts: 1,363
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Quote:
hook it up to a resistive load so it is drawing say 3A and measure the voltage drop across the ESC
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I do believe I smell a smoked ESC in the future............keep us posted.
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Racing is a state of mind....and I've lost it
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2003.03.18, 01:57 AM
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#11
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Mini-Z Tinkerer
Join Date: Mar 2003
Location: Chambéry, France
Posts: 17
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Sadly i can't do anything until i get back home to good'ol blighty, at the moment i am an expat in france as part of my gap year - learning french is cool, not having acess to all my nice test equipment or my mini-Z is NOT. I will be home on may 3rd so expect results sometime shortly after that, unless some charitable soul feels like doing it sooner (hint hint !)
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2003.03.18, 02:50 AM
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#12
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Registered User
Join Date: Nov 2001
Location: makati, philippines
Posts: 8,702
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Quote:
Originally posted by Electra
As for herman, are you sure that the car with the 6 stack didnt have additional mods like a hotter motor?
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actually we all run hot motors... and if i can remember it right, acceleration of the 2x6 stack was a lot more than the 2x3 stack... but i figure that the top speed would be the same...
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2003.03.18, 01:07 PM
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#13
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Registered User
Join Date: Mar 2003
Posts: 74
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Okay, for all of you that have soldered your own FET stacks and into the car...how difficult is it? I am fairly new to mini-z and have never soldered anything....but it seems like an upgrade worth performing.
________
new developments in Wong Amat
Last edited by black13; 2011.09.16 at 12:50 PM.
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2003.03.18, 03:52 PM
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#14
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Z maniac : certifiable
Join Date: Oct 2002
Location: Melbourne
Posts: 447
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Do a search on soldering & FETs as there's heaps of info on that topic. It takes practise,time & money,but is probably worth it in the long run.
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2003.03.18, 08:21 PM
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#15
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LIFTER
Join Date: Feb 2003
Location: florida
Posts: 1,243
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I found it easyest to put a little bit of solder on the iron and rub it on the wanted spot.
__________________
Toyota LC #100, Toyota LC #239, White Pajero #201, Red Pajero #200, Black CLK racer, Red Porsche GT3 (Project Pre Runner)
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