[TenTec] PA current control

[Bob McGraw - K4TAX]

As I recall, and I don't have a manual in front of me, the Corsair series and the Delta series did not use current sensing the similar way that the Omni and Paragon series of implementation.  Thus the reason some external current limiting is required.  Now current limiting can either be a fast magnetic {not thermal} breaker such as the Airpax or a current limit circuit in the power supply or even a "fast blow" fuse which is typically rated at 125% of rated current for a period of time.  Never use a "slo-blow" fuse in the circuit unless you intend to "let the smoke out of the radio".  I for one have never favored using a power supply that is over rated for the intended load.  Oh a margin of safety is desired but that typically is in the 10% to 20% range max. It just makes no sense and actually causes some element of danger to be introduced.  Plus additional circuit protection is required.

The original use of the Airpax breaker served two purposes; one to turn the radio on an off when used with a source like a vehicle battery and to limit the current to a safe value and two to protect the radio if thermal run-a-way or excessive current flowed for a period of time due to PA load conditions.

The fuse method is really too slow to offer much silicon protection but more to protect the supply and the radio cables, connectors, PC boards and maybe, to a lesser degree,  the PA devices.

Here is the Bussman data for the Fast Acting  AGX series of fuses.  
Rated Current % of Amp Rating Opening Time

1/500 - 30A 110% 4 hours minimum

135% 60 minutes maximum

1/500 - 2A 200% 5 seconds maximum

2.5A - 30A 200% 2 minutes maximum

Note that a 20 amp AGX fuse can handle up to 22 amps {110%} for 4 hours and 27 amps {135%} for 60 minutes max.

Here is the data for the Fast Acting AGC series of fuses.

% of Amp Rating Opening Time

100% None

135% 60 Minutes Maximum

200% 120 Seconds Maximum

The data information above was taken from the Bussman specification page.

The point being, fuses won't protect your PA silicon.


Someone asked "have I ever operated my Paragon into a dead short at 100 watts".  Well the answer is NO.........however I did test my new Omni VII under these conditions just for the purpose of these discussions.  The short was a paper clip folded and bent to fit into the load side of the Bird 43 meter. The results were observed on the Bird meter and the current meter on Astron SS30M that powers the radio.  So you asked, did I risk a $800 or $900  radio,  nope, I used a $2800 radio instead.  Now folks, that takes knowledge, caution, intestinal fortitude and well I won't say it here but it's part of the male anatomy. 

Now some may say that I'm stupid or apply a host of other adjectives.  However, I know what I'm doing and, more importantly, I do these type of tests in a very carefully controlled and monitored environment.  As some might say, "try this yourself and you may find that you are a very underpaid, under educated, inexperienced and inadequately equipped design engineer".  And might I add, don't try this with a non-Tentec radio.  You'll most likely smoke these puppies 100 out of 100 times.  I know, I've repaired many {and make good money at it too} of these that were accidentally dumped into a bad coax or load and allowed to sit there generating heat while the operator was trying to figure out what was going on.  

OH, the results on the Omni VII, it throttled back very nicely to keep Ic current within the design operating range.  Again, 13.8 volts at some 20 amps or 276 watts DC input.  Now admittingly, I didn't let it sit there for a period of time to see just how hot the heatsink would get.  Trust me on this one, dissipating 276 watts into the heat sink will get very hot and very fast.  My point, the circuit handled the shorted output without any component failure or trip of the current circuit in the power supply and it did not blow the back panel fuse.  The radio did what it was designed to do.

As to mis tuning..........on a port of my coax switch, I use a MFJ bridge to adjust my tuner thus I'm assured, once adjusted, that the radio and amp will be happy.  I also have a suitable dummy load for the amp so it can be tuned to full power without putting it on the air.  When I switch the amp and or radio to the tuner all will be happy and I've not put a signal on the air.  This is all per FCC requirements so as not to cause interference.  I then ask "are you operating your station legally?"  From what I hear on the air most hams are not.  {Pet peeve of mine}

Have I ever made a mistake?  Darn right many times but in every case it was an error on my part or in some cases a failure of a switching circuit that allowed the VHF legal limit amp to dump its power into the front end of the mast mounted preamp.  Frankly, there wasn't enough of the preamp board and circuit left to repair.  Accidents do happen and always will.  Expect that, but we can have circuits that prevent claustrophobic damage and we can be careful in switching things around.  In my case, I learned to use and now use a written check list.  That comes from my hours of flying as a pilot. As is said "there's old pilots and bold pilots, but there's no old bold pilots".  Which do you choose to be?

73
Bob, K4TAX