mosfets and other single points of failure, including wifes

[meepmeepmayer]

20 minutes ago, Patton250 said:

I’d just love to know what the heck a MOSFET is and what it does.

It's a switch that can switch power on and off rapidly.

The motor needs power to run. The power requirement can vary smoothly between "off" and "full power" (whatever that is). There's two ways to do this:

• Build some analog circuit that can smoothly create any desired power/current/voltage curve. That circuit is going to be complicated, physically big, expensive, etc.
• Have power either completely on or completely off with a digital switch (the mosfet) controlling it. If you oscillate between off or on fast enough, and by having the power on x% of the time and off 100-x% of the time, it looks like you're at x% of the full power to the motor. That's how you can create smooth power output with only digital switches, the method is called PWM (pulse width modulation - you modulate the length of the "on" pulses compared to the altogether time).
  You can build mosfets cheaply just from an easy to mass-produce semiconductor chip (the mosfet) inside a cooling casing (the mosfet package, the black blocks you see on the board).
  All the power to the motor goes through the mosfets, which is why they heat up and need to be strong enough and need good cooling (the package and the heatsink, without glue in between) to not explode.

Warning: I'm not knowledgable about this stuff, and probably it's 50% wrong, but you get the idea.

Side note: The typical whine of a wheel is the PWM frequency vibrating the motor coils (or vibrating something). It used to be around 6kHz but they increased it above audible levels so wheels would no longer have the annoying whine.

[Patton250]

6 minutes ago, meepmeepmayer said:

It's a switch that can switch power on and off rapidly.

The motor needs power to run. The power requirement can vary smoothly between "off" and "full power" (whatever that is). There's two ways to do this:

• Build some analog circuit that can smoothly create any desired power/current/voltage curve. That circuit is going to be complicated, physically big, expensive, etc.
• Have power either completely on or completely off with a digital switch (the mosfet) controlling it. If you oscillate between off or on fast enough, and by having the power on x% of the time and off 100-x% of the time, it looks like you're at x% of the full power to the motor. That's how you can create smooth power output with only digital switches, the method is called PWM (pulse width modulation - you modulate the length of the "on" pulses compared to the altogether time).
  You can build mosfets cheaply just from an easy to mass-produce semiconductor chip (the mosfet) inside a cooling casing (the mosfet package, the black blocks you see on the board).
  All the power to the motor goes through the mosfets, which is why they heat up and need to be strong enough and need good cooling (the package and the heatsink, without glue in between) to not explode.

Warning: I'm not knowledgable about this stuff, and probably it's 50% wrong, but you get the idea.

Side note: The typical whine of a wheel is the PWM frequency vibrating the motor coils (or vibrating something). It used to be around 6kHz but they increased it above audible levels so wheels would no longer have the annoying whine.

Thanks man.  I apologize for side tracking the thread. 

[erk1024]

1 hour ago, meepmeepmayer said:

It's a switch that can switch power on and off rapidly.

I know even less about this than @meepmeepmayer, but I'm going to chime in anyway. It's the internet after all.

The control board runs at very low voltage and current (something like 3.5 volts maybe). So it needs a way to control the massive voltage and current going between the batteries and the motor, and those are the mosfets. This is true whether accelerating or regenerative braking (current is going from the motor back to the batteries). You can get spikes in the amount of current both when the wheel is trying to get over an obstacle and needs some extra power, or if somebody rides down stairs and there is a sudden increase in power coming from the regenerative braking. Those spikes can blow mosfets, so they need to be high capacity.

(I won't be upset at all if somebody corrects me)

Edited July 23, 2019 by erk1024

[Rehab1]

9 hours ago, Marty Backe said:

When you've been around EUCs for years you learn that the MOSFETs are what fail in the vast majority of cases. Thus the obsession over MOSFET talk 

 

[Harvey Pooka]

Thanks Meep and Erk. I'm not a techie so my understanding of the science behind my EUC is pretty limited. Your explanations were great. Easy to understand. Way better than the ones I found on the internet. My only remaining question is where is the hamster powering the wheel kept?

[Bob Eisenman]

7 hours ago, meepmeepmayer said:

Warning: I'm not knowledgable about this stuff, and probably it's 50% wrong, but you get the idea.

Does that mean your community reputation score is only 50% accurate?