[TenTec] Airpax Breaker Part and Source

Sun Jan 2 10:05:30 PST 2011

Well, what prompted this discussion was the fact that I zapped the 
finals in my Century 22. It was a series of errors. I had left the drive 
set to full power; band was set to 10 MHz, but autotuner was set to 3.5 
MHz. I accidentally hit the keyer paddle and let loose 3 or 4 dits, 
which was enough to kill the finals. The Ten-Tec 979 power supply did 
not trip, probably because a previous owner had used it with a different 
radio and had readjusted the over-current trip point. I hadn't bothered 
to check it.

While waiting for new finals ( a matched pair from RF Parts Co.) I 
readjusted  the  power supply.  I would like to operate the rig on 
battery, so I'm looking for a circuit breaker to but in the 13V line, 
hence the inquiry to Poco Sales.  I have the Airpax spec sheets.  The 
"instant trip" will trip at 150% of rated load in 100 ms. The fast trip 
will trip at 135% of rated load anywhere between 200 ms and about 8 
seconds.  The slow trip can take as long as a minute to trip.

Older Ten-Tec equipment did not have ALC protection, so they depended on 
the power supply or circuit breaker to protect the finals. Ten-Tec 
therefore chose the instant trip breakers, which are pretty scarce these 
days.

The question is, how fast does a fuse or circuit breaker have to be to 
protect the final transistors?  It may be better to build an outboard 
circuit that duplicates the over-current circuit inside the Ten-Tec 
power supplies.  I've tripped the power supply for my Corsair many times 
without damage.

I will be more careful with the Century 22, as follows:  Keep the drive 
at zero, unless planning to transmit. Switch from an auto tuner to a 
manual one where I can record and physically see the presets for a given 
band.  Use a fast acting circuit breaker when not using the Ten-Tec 
power supply.

                         73,

                         Bob WB2VUF

On 12/14/2010 11:18 AM, art davis wrote:
> Yes they do, but you have to look past the marketing and look at the data sheet. According to the trip curve for their "instant" breakers, the typical operating time for current of 2X the breaker rating is just over 100 milliseconds, where the "non-instant breakers can hang in for over a second at the same current. Definitely faster, but far from "instant". In fact the fastest time they are guaranteed to operate is 10 milliseconds and that's at currents at least 8 X the breaker rating (i.e. 160A for a 20A breaker).
>
>> From: Mike_N4NT at charter.net
>> To: tentec at contesting.com
>> Date: Tue, 14 Dec 2010 10:49:16 -0500
>> Subject: Re: [TenTec] Airpax Breaker Part and Source
>>
>> Airpax calls some models "instant," don't they?
>>
>> ----- Original Message -----
>> From: "art davis"<n4uc at hotmail.com>
>> To: "TenTec reflector"<tentec at contesting.com>
>> Sent: Tuesday, December 14, 2010 1:24 AM
>> Subject: Re: [TenTec] Airpax Breaker Part and Source
>>> 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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