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Re: [TenTec] Airpax Breaker Part and Source

To: tentec@contesting.com
Subject: Re: [TenTec] Airpax Breaker Part and Source
From: "Dr. Gerald N. Johnson" <geraldj@weather.net>
Reply-to: geraldj@weather.net, Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Tue, 14 Dec 2010 02:41:46 -0600
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>
In my experience, no fuse or circuit breaker is as fast as a
semiconductor so the blown fuse or tripped circuit breaker is often an 
indicator that the semiconductor has failed shorted.

That being said, the "instant" trip breaker or fuse is considerably
faster than time delay parts that are intentionally slowed to allow
repeated switching of loads with a turn on surge, such as incandescent
lamps that draw 15 to 16 time their operating current a the instant of
applied power, loads with large filter capacitors and consequently large
charging currents when energized, and motors, particularly AC motors or
shunt wound DC motors that draw up to 6 times full load running current
while starting. In a fuse the time delay comes from parts with thermal
inertia, and that's true of thermal trip circuit breakers. Magnetic trip
circuit breakers depend on mechanical inertia aided in Airpax breakers
by a hydraulic dash pot to add delay. The "instant" trip devices lack
the added delay elements. There still is some delay due to the minimum 
practical thermal or mechanical inertia in the device.

Fortunately for our Tentec radios, the PA transistor are robust and can
take a lot of overcurrent abuse but need mostly thermal protection under
those abuse conditions so the instant trip breaker is fast enough. I
have compared the Airpax delay curve to that of fast fuses and have run
my Corsair II on a large supply that runs my whole shack with fast
automotive fuses, but I cheat, I use 15 amp rated Buss type ATC fuses on
a 20 amp circuit. That speeds up their opening because a fuse is
generally rated to carry 125% of nameplate current indefinitely. I use
only Buss fuses, not any western Pacific rim part. I have blown several
fuses while tuning antennas with full power out, and haven't caused
detectable damage to the radio. And owners of radios run with the 
specified Airpax breakers have tripped the breaker under the same 
conditions. User of current limited Tentec supplies have also found the 
power supply protected their radio PA from damage due to a high current 
overload from a high SWR load that reflected a low impedance to the 
transistor collectors. These protection techniques DO work with Tentec 
transmitters, but probably not many or any other maker's radios.

73, Jerry, K0CQ

On 12/14/2010 12:24 AM, art davis wrote:
> I'm no expert, BUT...
> be very careful with the use of the term "instant trip" when
> referring to a circuit breaker or fuse. All circuit protection
> devices take a finite amount of time to operate, no matter what type
> they happen to be. Look at the response curves (i.e. trip curves) for
> any breaker or fuse and you'll see that it may be very quick indeed
> (less than a millisecond in many cases) if the fault current is high
> enough, but not "instant". And in the world of semiconductors very
> quick might not be fast enough. And look at the curves to see the
> level of current that can flow through the device without it tripping
> at all. You might be surprised.  I may be wrong but I doubt that
> there is any circuit breaker on the market that, when located
> upstream at the power supply, can operate fast enough to protect
> semiconductors from damage due strictly to overcurrent. If the fault
> current is high enough (many times the circuit breaker rating) they
> may offer some limited protection from the heat generated
> by the overcurrent flowing through the device, but the semiconductor
> itself must be capable of surviving the current in the first place.
> The primary reason to specify different response time characteristics
> (trip curves) for circuit breakers (slow blow, fast, etc...) is to
> make sure that the power feeder circuit maintains proper trip
> coordination among the various circuit protection devices in the
> circuit in case of a fault in the wiring upstream of the load. In
> most power distribution systems the fuses and circuit breakers
> upstream of an end user (i.e. radio) are there only to protect the
> wiring between the power source and the load. The load (in this case,
> the radio) must protect itself internally.  My guess is that there
> could be significant damage done inside a transceiver long before a
> 20A breaker all the way back at the power supply knew what was
> happening.
> Of course, the flip side to all this is that it couldn't hurt, right?
> You certainly won't have any protection if you don't use something,
> so put the fastest breaker in there that you can find  and keep your
> fingers crossed! Just don't call it "instant"
> Art, N4UC
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