Thursday, November 30, 2006

oops ... pop :( learning about MOSFETs

In books, the descriptions of MOSFETs are so simple; look at a MOSFET data sheet and it isn't quite so simple. The most big question in my mind is the drain current min - why is there a min. Does that mean that the MOSFET can support at least that specific current?

The IRF531 looked nice, so I bought a couple to learn with. I'm have a pretty basic multimeter and a limited set of resistors, so I bread-boarded some circuits with the intent of getting a most basic understanding. Here are my notes in preparation for bread-boarding and testing:

Specific Constraints
- 0-25 mA and 0-250 mA test ranges on multimeter
- 330 ohm and 470 ohm resistors are only small resistors available
- both types of small resistors are 1/2 watt resistors

Playing with MOSFET
- assuming no resistance at all on gate-source
- circuit: +5 ... 235ohm ... gate-source ... Gnd
- 21 mA used
- if resistance on gate-source, then current used will be smaller
- assuming negligable resistance (< 1ohm) on drain-source
- circuit: +12 ... 660ohm ... drain-source ... Gnd
- 18 mA used
- if 5V @ 20 mA isn't enough to turn MOSFET on, then current will be smaller

You know what's coming - after careful consideration of my constraints and building my circuit, I blew the fuse in my multimeter! I have no spares, so I'll have to finish testing tomorrow :( My mistake, besides doing this in real life rather than with Crocodile Clips, was that I didn't plan my volt versus amp metering locations. Live and learn - at least I lived...

3D representation of the MOSFET test using Croc Clips

Now that is the smart way to learn! The 3D view is pretty and neato, but what I'm not showing is the schematic with ammeter and voltmeters installed. Good stuff!

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