4QD Motor Speed Controller
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MOSFET is an acronym - I'll give you the whole phrase, so you know why they're always called MOSFETs.. Metal Oxide Silicon Field Effect Transistor.
MOSFETs are very near to perfect semiconductor switches, capable of switching high currents at high speeds. All modern low voltage controllers that are properly designed use them - and 4QD's controllers are no exception.
MOSFETs are certainly the best devices for operation below 60v but above this voltage they become less perfect (they can handle less current) and somewhere above 100v, other types of technology (such as IGBTs or Thyristors) start to become a sensible alternative.
A MOSFET has a 'channel' through which the current flows and a 'gate' terminal. If there is no voltage on the gate, the channel is closed and no current flows. If there is a sufficient voltage on the gate, then the channel opens and current flows. The channel opens somewhere between 3 and 7 volts. Of course, it takes a little electrical work to open or close the gate, but once open, keeping it open takes no electricity.
MOSFETS, if you know how to use them, are also incredibly robust and difficult to damage. However they are extremely fast devices with extremely well defined operating conditions. Give a MOSFET even a sniff of a voltage (or current) which exceeds its limitations and the SFET becomes and acronym for 'Smoke and Fire Emitting Transistor'. These conditions are often transient so difficult to examine and this sensitivity has caused some people (who don't understand them) to find then unpredictable and unreliable!
We at 4QD love the beasties!
'Chopper' means that the MOSFETs are switching the battery current (i.e. chopping it) rapidly on and off: if the battery is only connected to the motor half of the time, then the motor only 'sees' half the battery voltage. By varying the chopping ratio you can vary the amount of battery the motor sees, so varying the speed of the motor.
If you are a beginner to electronics and want to learn more, Understanding Electricity - an analogy with water may help you.
If you are technical enough to understand circuit diagrams, there is more information in our circuits archive.