[sdiy] Power supply and FETS

[Tony Clark]

> A few questions for my first post.
> I've built a few modules that require a +12v -9v power supply however the
> rest of my modules run at +12v -12v as does my power supply.I've tried using
> a resistor to drop the -12v to -9v but it reads as +9v when I do this.Is it
> a case of using different value resistors from the -9v rail?If so how do you
> calculate the correct values to use?Is there something I'm missing?

   Hi Rich,

   It sounds like you just have the leads of your voltmeter reversed!  
You can certainly drop the voltage this way, but depending on how much 
current you draw, it may not regulate at -9V very well.
   As long as the current isn't too great, I would recommend using an 
op-amp buffer to create your -9V supply.  Just divide the -12V down with 
two resistors with a 1 to 3 ratio to ground.  Connect the midpoint (now 
-9V) to the positive input of an op-amp, with the negative input pin 
tied to the output.  This way the op-amp will regulate the output up to 
its maximum current capacity.

> How do you measure the Vp of a J fet?

   According to my text book, the knee of the Vgs = 0V curve occurs when 
Vp = -Vds.  So ground the gate and source pins and monitor the drain for 
voltage and current.  Increase the voltage to the drain until the current 
no longer increases.  That voltage would be Vds or -(Vp).
   Hopefully that would do it.  I've never had to make that sort of 
measurement, so if I'm wrong, someone please correct me!

   Cheers,

   Tony

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