To:  qrp-l@lehigh.edu
Subject:  2N7000's: important information
From:  burdick@interval.com (Wayne Burdick)
Date:  Tue, 17 Oct 1995 21:47:01 -0800

This is important reading for designers and kit builders about a component
that has recently become popular in QRP circles.  [Please publish
widely--QRPp, "72", etc.]

The 2N7000 is a DMOS MOSFET transistor that is great for switching uses: it
can be used just like an NPN transistor, but requires no current-limit or
bias resistor at the base lead (its gate lead draws no current).  In fact,
in the TO-92 package, its leads are in the same order as a bipolar
transistor:  S-G-D correspond to E-B-C.

BUT: this part *IS* subject to static damage.  Before you handle one, sit
down and touch the nearest metal surface that is grounded (rig, scope
ground lead, etc.).

Both the Cascade (NorCal) and the KC1 (Wilderness Radio) use 2N7000s.
Wilderness guarantees that their 2N7000 transistors are good when they go
into the anti-static bags, and after that it's up to the builder.  99% of
the time, you won't have a problem even if you handle them improperly (see
story at the end of the article).  It's the 1% I'm trying to eliminate.

I can't speak for NorCal's packing of the Cascade, but I know Doug
Hendricks was careful when he packed them.  (On the other hand, Dave
Meacham confirms that there have been some failures of this part in the
Cascade, and there have been a couple in KC1s as well.  You can bet the
next revisions of both manuals will include more anti-static precautions.)

Anyway, this is a very useful part that won't go away, so here are some
strongly recommended guidelines:

1.  NEVER buy surplus 2N7000s or similar parts unless they are still on or
in  their protective anti-static shipping material.  Loose parts in bins
must be 100% tested before use.

2.  NEVER remove them from the shipping material until you're properly
grounded.  It is then OK to put them into anti-static bags but NOT into
ordinary plastic bags.

3.  If you suspect one is bad, you can tentatively substitute a 2N2222A or
other NPN transistor for it in many cases, BUT you must add a resistor
(typically 10K) in series with the base lead.  This is necessary because in
most circuits, the 2N7000 is driven directly by things like microprocessor
output pins, and you'll fry the NPN transistor without base current
limiting.

Now, just to confuse the issue, here's an amusing counter-example:

When I designed the 2N7000s into the KC1, I attempted to do "destructive
testing" of one of the parts.   I held a 2N7000 loosely in my hand, scuffed
my feet around on carpet, rubbed the part on my cat, shocked myself on the
sliding glass door transistor-first, and other things too horrible to
mention.  Then I put it back in the circuit.  It still worked!  Perhaps
some of the parts are better than others!  After all, they are made by more
than one manufacturer.

Hope this clarifies the use of these devices.  Assume the worst, despite my
good luck with one of them.

73,
Wayne Burdick
N6KR

- posted by Leigh @ 22:22 z