Wednesday, June 16, 2010

June 16th - Expensive Lesson:

I received the MOSFETs from Hong Kong today. I bought 50 of them at $1 each + $5 shipping.

I'd ordered these because I saw them recommended on the arduino forum for switching high amp loads with a microcontroller. I did some research and pulled the trigger to get them here.

I pulled that forum post back up tonight and started thinking about building a perfboard to do some load testing. As I'm looking at the schematic, I realized my basic transistor knowledge missed a rather important item. I couldn't really explain the difference between NPN and PNP before - but now it is crystal clear. NPN allow you to switch GROUND, and PNP allows you to switch POWER. I want to switch power, and I have 50 NPN transistors. Oops...

Now I need to identify another transistor and get them ordered. I think I'll read the "Practical Electronics for Inventors" section on transistors a few more times before I spend more money :)

I'm using this post to summarize what I'm learning as I head to picking new MOSFETs

1) P-Channel MOSFET since I need to switch power, not ground
2) Low RDS reduces heat
3) I need 25A capability, but it is typically recommended to buy 2x capacity, so near 50A
4) Since I'm working with microcontroller voltage (0-5v) and switching 12-14v, I'll need to use drivers, since the difference in potential would be too high and the MOSFET would always be on. This means I don't need "Logic Level" MOSFETs since I will be using drivers.

1) STP80PF55 - P-channel 55V - 0.016Ω - 80A - TO-220 - $2.67 @ Digikey
2) IRF4905PBF - P-channel 55V - 0.02Ω - 74A - TO-220 - $2.81 @ Digikey
3) FQP47P06-ND - P-channel 60V - 0.026Ω - 47A - TO-220 - $2.26 @ Digikey (Used in the Arduino RC Lawnmower)

Other random notes:
1) You can MULTISELECT digikey filters. DUH. No wonder I've always struggled to find items. DUH.

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