Re: ATM MOSFET Blues

[Dwight Elvey <elvey@hal.com>]

Michael Lindner <mikel@att.net> wrote:
>  If not, is there a simple
> way to determine it with an ohmmeter? 

Hi Michael
 Yes, an Ohm meter can be used to determine the pins.
In order to do this, you need one of the older analog
Ohm meters. This is because they usually use a higher
voltage on the high Ohms scales. Most digital meters
only apply .2 volts in normal Ohms and then only
2.0 volts in diode mode. To check the MOSFET, you'll
need more than 5 or so volts.
 There are 4 types of MOSFETs that come as descretes.
There are P channel and N channel. The P channel
are more rare but do exist. The nexts is that some
have a zener diode to protect the gate while others
have no gate protection ( Be especially careful
with the unprotected gate ones. Almost any static
discharge to the gate can destroy them. Most of these have
a maximum gate voltage of 60 volts. One can generate
several thousand by just getting out of a seat. ).
 The diferences between the P and N channel are just
that the meter leads will be swapped for all of the test.
 First you need to know that there is a diode from the
source to the drain. This is caused by the substrate
connection. There may also be a diode from the gate to
the source. This will be from the zener protection
diode on the gate if there is one.
 Now, knowing the source as the common of the two diodes,
connect the meter lead to one of the other pins ( usually
the same as the heat sink tab ). Put the meter on the
highest ohms scale. This will select the higher voltage
forcing function. Short the unconnected lead to the source.
There should be infinite ohms. Momentarilly short the lead
to the drain. The meter should now show some Ohms.
It may gradually go up scale as the charge leaks of the gate.
 This method has always worked for me. Like I said, it
only works with the non-active analog meters ( not
digital, VTVM or FET VOM ). You do need to be careful about
static. If you have one of the ones without the zener
on the gate, these can be easily damage, even with an
ungrounded soldering iron, like the cheap ones from
Radio Shack.
Dwight