The control of PA current is derived from a R in the A+ line to the PA. The
voltage across the R is fed to the 2nd mixer board and driver which reduces
drive inverse to the PA current. The current never gets to max
destruction value.
Having a Paragon manual in front of me, the current sensing R is R1 {0.015
ohm 10W} located on the DC power board. This voltage is fed to U2a which is
a level shifter and its output feeds U2b to which the output if it exceeds
the Ic limit of 22 amps at full power, diode D25 is forward biased and ALC
actions results, therefore reducing drive to the PA stage. Thus PA current
will always be limited to a max of 22 amps for some 300 watts input
regardless of load Z. Yes, in theory, it will drive a dead short with rated
output. Remember the Tentec PA is not current limited. It is drive
limited.
The fuse, the circuit breaker and power supply limiting serve no purpose
except
in the event of catastrophic failure and thus exists simply for the
protection of
other components.
Big difference.
73
Bob, K4TAX
----- Original Message -----
From: "Kevin Purcell" <kevinpurcell@pobox.com>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Cc: "Kevin Purcell" <kevinpurcell@pobox.com>; <ken.d.brown@hawaiiantel.net>
Sent: Sunday, September 23, 2007 7:12 PM
Subject: Re: [TenTec] (no subject)
> The transistor "fry" because they heat up, their current gain
> increases and the collector current runs away.
>
> <http://en.wikipedia.org/wiki/Thermal_runaway#Bipolar_transistors>
>
> You aren't going to be able to heat them up in 17nS they are
> thermally too massive. The time constant in the power supply is not
> the determining factor. The thermal time constant of the system is
> the important issue.
>
> This is not an over-voltage (punch through) failure which would
> happen in the RF cycle timescale.
>
> I suspect (but don't know for sure) that this time constant is in the
> hundreds of milliseconds or possibly even longer. So a rapidly
> responding (but not too rapid) circuit breaker or fuse works as
> effective SWR protection.
>
> On Sep 23, 2007, at 1:32 PM, Ken Brown wrote:
>
>> I'm trying to understand this. What part of Ten-Tec's circuit keeps
>> the
>> PA transistors from frying in a half cycle at 30 MHz? Looking at my
>> Omni
>> VI PA schematic C25 a 33ufd capacitor on the PA board would hold
>> enough
>> charge to supply the PA board for well over 17 nanoseconds, by my
>> calculations.
>
> --
> 73 DE N7WIM / G8UDP
> Kevin Purcell
> kevinpurcell@pobox.com
>
>
>
> _______________________________________________
> TenTec mailing list
> TenTec@contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
>
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