[R-390] RE- OA2 replacement

[Michael Murphy]

Steve,

If wasting time and effort while learning and good old fashioned danger is
just as important as actually achieving good regulation...

If you have a junkbox full of old bipolar power transistors TO-3 or TO-220,
the actual breakdown voltage of many of these devices can be in the 150 VDC
range. I call this a poor mans Zener.

How? The maximum reverse bias voltage that can be applied to a transistors
p-n junctions is limited by breakdown. Breakdown is characterized by the
rapid increase of DC current under reverse bias. The corresponding applied
voltage is referred to as the breakdown voltage. Two mechanisms can cause
breakdown, namely avalanche or tunneling of carriers through the bandgap.
Neither of the two breakdown mechanisms is destructive. However, heating
caused by the large breakdown current and high breakdown voltage causes the
diode to be destroyed unless sufficient heat sinking is provided. With a
TO-3 or TO-220 package, we can heatsink the diode.

We can look at the VCEO, the Collector to Emitter breakdown or the VCBO, the
Collector to Base breakdown. We do not consider VEBO which is always a
comparatively low voltage. For instance a 2N3055 has collector to base
breakdown voltage of around 12 VDC.  A Hint: with an NPN  transistor, the
tab or case can be directly grounded with no insulation needed which is
handy, but remember - do not ground the base.

Set up a test stand with a variable high voltage supply capable of at least
100 mA and a series resistor of about 1K at 2 Watts. You will need a series
mA meter and a voltmeter across the junction. Use some insulated clips to
make contact with the transistors legs. Slowly increase the voltage until
the current pops up - Bingo - Avalanche. After going through a few hundred
devices you will get the hang of it and eventually find a 150 Volt junction.

As usual, the Reflector is not responsible for crazy ideas like this, nor
the consequences.

Mike Murphy  WU2D


----- Original Message ----- 
From: "Steve Hobensack" <stevehobensack at hotmail.com>
To: <kc0lwn at yahoo.com>
Cc: <r-390 at mailman.qth.net>
Sent: Thursday, October 20, 2005 8:29 PM
Subject: [R-390] RE- OA2 replacement


>      Yes, I have done this on my Super Pro 600, but I replaced the 0D3
with
> a 150 volt stud mounted zener rated at 10 watts. I bolted it directly to
the
> chassis. It makes just as much heat as the VR tube. Since the heat
generated
> is considerable, an axial style diode wired to the pins will soon fail due
> to inadequate heat dissipation.
>
> Date: Wed, 19 Oct 2005 21:58:33 -0700 (PDT)
> From: g kwitka <kc0lwn at yahoo.com>
> Subject: [R-390] OA2 replacement
> To: r-390 at mailman.qth.net
> Message-ID: <20051020045833.16530.qmail at web30606.mail.mud.yahoo.com>
> Content-Type: text/plain; charset=iso-8859-1
>
> Can the OA2 be replaced by simply putting in a 150
> volt 10 watt zenner in the appropriate socket pins?
>
>
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