what is a mosfet?

[stevo]

please gear the explanation for a non-techie 

[Leslie]

Mos was the inventor and FET = field effect transistor

It was 20 years ago but I think I nailed it without google...

[pdonahue]

Mos was the inventor and FET = field effect transistor

It was 20 years ago but I think I nailed it without google...

Close....   MOS = metal oxide semiconductor

It's basically a voltage controlled switch.   Similar to using a relay except that it can only be used to switch the current in one direction.  It has 3 terminals, the Source, Drain, and Gate.  The Source and Drain are hooked up to the two wires you would like to connect/disconnect with the switch.  When the Gate voltage is equal to the Source voltage, the switch is turned off, and when there is a voltage difference between the Gate and Source the switch is turned on. 

If you're doing anything with mosfets you will also need to understand the difference between N-mosfets and P-mosfets (each one conducts the current in a different direction, and the Gate voltage required to turn them on are the negatives of each other)

Hope this helps,

Pete

[Leslie]

Mos was the inventor and FET = field effect transistor

It was 20 years ago but I think I nailed it without google...

Close....   MOS = metal oxide semiconductor

Hope this helps,

Pete


AHH "I" should of googled it after all

thanks

I have heard of this term metal oxide semiconductor and know it well, but I have no idea as where the inventer thing came from.  20 years gone and its all muddleing around my head.


Yes they are a great invention.  I know of them in my audio learnings and use the n and p mosfet for to output the current in both directions to give signal generataion...

Correct me if I am wrong but they need little current at nominal volatges compared to bnc types to operate higher loads due to the "field effect" effect happening.

[stevo]

thanks guys.  So what's their role in an electric bike (conversion kit)?

[Leslie]

Now you want tech...

Right im not the best man for this atm but Im getting to it. without google.

The MOSFET is a three leged creature and amplifier that takes the rail voltage being 48volts to the source leg and uses a smaller variable voltage from a smaller transitor controlled by your throttle and PWM (Pulse Width Modulation) frequency modulator to the gate leg.  The load is the motor and is taken off the drain leg. When you throttle up you increase the PWM amplitude and some increase the frequency to the gate leg, this within the field effect is used to increase the output to the load by allowing electrons to pass from the source to the drain to through the load...

I did a quick edit on a image I found of an amp based on a Jfet



edit:
overvolt conversion.  (not)

[stevo]

ok thanks

[Spacelander 1946]

OK, so the Mosfets are locaterd where?.....in the hub motor or in the BMS unit? & can they burn out with regn braking mode?...Cheers

[Leslie]

The Fets are soldered to the PCB (printed circuit board) of the controller unit.  You will notice then pretty much straight away...


On a 3 phase controller you will see a line of these suckers screwed tight to an aluminium heat sink with a white silcon heat transfer paste.

[shakkan]

ive got 2 cooked g/m controllers(thanx g/m). ive looked inside and nothing is burnt out or smoked.i was thinking it was the mosfets that died- can I check them with a multi-meter or do I need to power up the unit.maybe I can make 1 from 2. I know ziltch about electronics.

[Leslie]

You can test them with a multimeter, well sort of! you can never really tell though unless you really load them.  These PWM circuits are quite complex in there operation.  You have a 3 phase frequency modulated (pwm) Hall sensor controlled device and now I find out it has pic (programable intergrated chip) onboard.  Man this is a little deep for me.  Repairing these are not an easy job.  Sure you can pick up those FETs cheap enough and go forth and tinker away and replace them.

You can make a real mess of thing though if you don't have the right gear.

1: Build/buy yourself a regulated power supply with 12v 10 watt soldering iron for custom sized jobs.
2: Get some decent light source and a desk magnifier glass thingy.

You can fix one thing and that will blow another.  So if you do fix stuff make sure you trace the circuit and check all the components around the said fets.

Im not sure how to tap into the pic and what interface is going to work.  it looks USB to me.  In my day we used serial ports.  Shesh.

I would rather the pic out of the circuit and things done a little more hard core.  Computers are easy and you can replace thing with a new board here and there but pics op amps and comparators you cant replace unless they install them with a ic socket.  Its a bitch just to remove them little only resolder them.

God speed with the repair......

[Lanchon]

> ive got 2 cooked g/m controllers

what type of controller? (voltage? regen?) how did you brick them?

[shakkan]

i blew the standard controller.first one lasted 5 seconds then stopped turning,went backwards and died. the second one lasted about 40 miles then threw its hand in.one of the 3 power wires broke away from the connecter. the motor went slow/juddery for 30yrds and packed in.nothing was burnt inside both of them.

[Lanchon]

thanks. that' strange, an open phase shouldn't be that problematic. maybe it short-circuited with another phase when it broke out loose.

[pdonahue]

i blew the standard controller.first one lasted 5 seconds then stopped turning,went backwards and died. the second one lasted about 40 miles then threw its hand in.one of the 3 power wires broke away from the connecter. the motor went slow/juddery for 30yrds and packed in.nothing was burnt inside both of them.

I hat to be the bearer of bad news, but chances are that if any of the mosfets are blown, then the whole gate drive circuitry will likely be fried as well.  Unless you happen to have some spare parts kicking around then it'll probably be cheaper to just replace the controller than to go blindly replacing parts without knowing a rough idea of what's wrong... 

If, however, you can figure out for sure that a particular part is fried, then it should be fairly easy to replace one or two parts...   If you want to see if the mosfets are completely fried, hook up the controller with the power leads to the motor disconnected (keep the small hall sensor wires connected to the wheel)  Using a voltmeter, measure the voltage between each of the three motors...  You should find that the voltage across one pair is equal to your battery voltage, while the others are roughly zero.  If you slowly rotate the wheel, the pair of wires with the voltage across them should alternate between the three power wires.  Also, look closely at the board to see if the white markings on some of the chips look darker than they should...  The markings tend to turn a little yellow/brown when they're overheated even if it's not hot enough to actual bubble and burn the surface.

Good luck,

Pete