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@charter.net
>> To: tentec@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@hotmail.com>
>> To: "TenTec reflector"<tentec@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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