finally venturing into mosfets.....probably

tony_g

Senior Member
ok so after a couple of years and basic circuits with the picaxe the time has come for me to brush aside transistors and look at playing with mosfets.

so firstly the mosfet i was looking over is a dual channel n and p soic8 package,
datasheet: View attachment FDS8958A.pdf

now the project is to replace my old and finally dead winch controller for an rc trailtruck/rock crawler which will also allow me to include an 08m2 soic with my customized operating modes for the winch and setup.

the plan,assuming these are acceptable is to use 2 of them for an h bridge circuit along with other components the ideal end result is a much better controller which can stay within the small size of the old pcb.

now the 08m2 will get its power from the rc reciever aux channel i use for winch control, the speed controller bec is supplying 6v to the reciever and this will be dropped down to just over 5v for the 08m2 with a couple of diodes, i know this works as i made a 6 transistor h bridge circuit for the winch but its not up to the job of coping with winching in under almost stall conditions so thats why i need to look at a beefier circuit.

the winch motor itself is being run from a 3 cell lipo pack(11.1v) which gives more grunt available but also increases the stall current to approx 1.1A though not without a good fight to keep going before stall but for the most part the current varies winching in from 400-580 ma.

due to the difference between the 08m2 power needs and the winch running higher i know that the highside of the h bridge will need to be switched by an npn to keep the high v+ of the winch supply away from the 08m2's pins as was done with the transistor circuit but never playing with mosfets before im still a bit confused after much forum and internet searching about the mosfets gate in respect to its connection to the picaxe pin.

i know that we use resistors to base on transistors to limit the current to protect the picaxe and i assume also the base of the tranny but do i still need to use a resistor on the gate of the mosfet to protect one or both?

also im still unsure completely about the highside switching, can i use an npn transistor to pull the highside gate low when i need to turn it on and if so do i need to worry about the transistors max current rating being exceeded if i switch a mosfet that can potentially draw way more current?

sorry for the ramblings but after trawling so many different sites and reading im definately still confused about a few things, so far to date i have only ever let the magic smoke out of a resistor and want to keep that good streak going, especially with my small supply of picaxe variants lol.

any insights/tips/ideas are greatly appreciated and welcomed.


tony
 

bluejets

Senior Member
For a start, I think you will find using logic level mosfets will save you a lot of lost time wondering why things won't work the way they should.
 

tony_g

Senior Member
no doubt, im still scouring around to find some small enough, i have come across them in sot 23 package but that would end up with a heck of a lot of vias on the tiny(25mmx34mm) pcb just like my transistor version, if possible im hoping to keep the individual parts count down to a minimum so i will have to keep scouring to see if i can find logic level in an n and p package like above
 

tony_g

Senior Member

manuka

Senior Member
Metric multiplier basics - aren't you in SI friendly Canada ? "m" & "M" differ by a 1000 million! Would you prefer a m$ or a M$ ?!

Lower case m= milli = times a thousandeth (10 ^-3)
Capital M = Mega = times a million (10^6)
 

tony_g

Senior Member
thanks stan,

the only time i saw a resistance value with an m was xx"M"ohm and being a high resistance value, the sort that i have in my assortment kit but will no doubt never use lol, but thanks for the note on abbreviation differences, something new learned today.
 

hippy

Technical Support
Staff member
the plan,assuming these are acceptable is to use 2 of them for an h bridge circuit along with other components the ideal end result is a much better controller which can stay within the small size of the old pcb.
Given there were six transistors on the old board then would four discrete logic-level MOSFET's not fit in the same space ?

It would probably be worth experimenting with discrete components to get to understand their characteristics before committing to any final design.
 

John West

Senior Member
ok so after a couple of years and basic circuits with the picaxe the time has come for me to brush aside transistors and look at playing with mosfets.
FYI, Tony, MOSFET's are transistors, they just have significantly different characteristics than Bipolar Junction Transistors, (the historically more common type,) as you're learning.

I actually find them both easier to use and to understand. Just think of the Gate as a capacitor that you charge in order to decrease the resistance (get conduction) through the Drain/Source leads, or discharge in order to increase the resistance between the Drain/Source leads, (switch the device off,) and you're more than halfway to grasping MOSFET's.

Then realize that the more time the device spends between the ON and OFF states, the more time it will spend in a state where it has both a resistance, and a current passing through that resistance, meaning it's dissipating heat. Therefore, when it's used as a switch it's best to charge and discharge the Gate capacitance quickly.
 

tony_g

Senior Member
Given there were six transistors on the old board then would four discrete logic-level MOSFET's not fit in the same space ?

It would probably be worth experimenting with discrete components to get to understand their characteristics before committing to any final design.
yes that could definately be an easy way to transition the board over to the more powerful mosfets, my 6 tranny board i made was a bit bigger than the original board that was supplied and ideally i would like to be able to eventually get its size reduced.

so i can try some sot23 mosfets in place of the sot23 transitors and eventually change over to something along the lines of what i was perusing over on digikey's site, they did have some soic 8 mosfet h bridge chips so effectively replacing 4 of the sot23 and less tracks and via's to run on the board, i did have a go at redesigning my board schematic last night with design spark and was able to get the size and track/via placements significantly reduced so just need to read up some more and ensure i eventually choose the most appropriate one.
 

tony_g

Senior Member
FYI, Tony, MOSFET's are transistors, they just have significantly different characteristics than Bipolar Junction Transistors, (the historically more common type,) as you're learning.

I actually find them both easier to use and to understand. Just think of the Gate as a capacitor that you charge in order to decrease the resistance (get conduction) through the Drain/Source leads, or discharge in order to increase the resistance between the Drain/Source leads, (switch the device off,) and you're more than halfway to grasping MOSFET's.

Then realize that the more time the device spends between the ON and OFF states, the more time it will spend in a state where it has both a resistance, and a current passing through that resistance, meaning it's dissipating heat. Therefore, when it's used as a switch it's best to charge and discharge the Gate capacitance quickly.
for me i think the common problems of selecting the right one for the job and all the specific values that have to be correctly identified as suitable are what have made me delay looking into using them, although i understand their are more advantages to using fets over bjt's.

i think i will do a bit more reading to ensure that i can select something thats completely suitable for me and my needs to ensure i can avoid any mishaps, im sure i read on here a while ago that someone has a mosfet shaped bald patch on their head due to a fet going "pop",now was that hippy or dippy?

but for my circuit i only need to either have the fet on fully in one direction, off, on fully in the other direction or if i can do it like my bjt circuit both n channels pulled high to "brake" the motor so the winch does'nt spool back out if i have to stop winching when the truck is going up a rocky/steep incline.

no need for pwm with the fets so hopefully one hassle their to not have to deal with switching speeds at least.
 

premelec

Senior Member
What size/weight vehicle are you considering? Any transistor has the possibility of failing shorted - so you should consider what would happen if a short occurs and put in some protection... sometimes plain old mechanical relays are the right answer to on/off. [or fancy vacuum relays if switching kilovolts & amperes... :)]
 

tony_g

Senior Member
the truck itself is a 1/10th scale trail truck/light rock crawler and all up weight can normally be around 5 pounds, give or take for add-ons or hop ups/additional scale items.
relays are not going to be practical on this due to size and most of the electronics are normally housed in some sort of waterproof container just incase some light water crossing is involved on the run.

the winch is not solely going to be doing the grunt of pulling up challenging obstacles if needed but can be required to work hard alongside driving up/over some challenging areas,its normally pulling between 400- 600 ma on average but the stall if it gets near that jumps up to an amp or so for a very brief moment before the winch will be turned of from the transmitter if it looks like its going to need some thinking to clear the obstacle.

normally when we trail run these trucks you are only allowed to physically handle the truck once if you get really stuck then after that you can only use whatever functional scale accesories you have in your truck to clear the obstaclewhether its tow straps/lines or scale winches and winch anchors or admit defeat and end your run, its fun :)

so hence my own custom winch controller run by an 08m2 with my code which allows me to use any spare channel on my tx as opposed to the standard only works with one type of channel with various different operating modes dependant on how i use the switch.
 
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