[TenTec] PA current control

[Kevin Purcell]

Curiously this system is not used in early TT rigs -- Argosy, which  
has a trip in the PSU or a Airpax. And the Omni D (but I'm replying  
on memory for that).

I take it you have tested you hypothesis by running your Paragon at  
full drive into a dead short :-) If not, why not? :-)

As I said before the PA transistors will die if they go into runaway  
so anything that prevents that is a "good thing" (tm).

Perhaps TT were using a belt and braces on the Paragon? Did they keep  
both this design and current limited or current tripping PSUs on  
later rigs? Paragon II, Omni's?

Do the later rigs (Scout, Argo V, and similar) foldback power in a  
similar manner?

Curiously,
Kevin

On Sep 23, 2007, at 8:36 PM, Bob McGraw - K4TAX wrote:

> 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 at pobox.com>
> To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>
> Cc: "Kevin Purcell" <kevinpurcell at pobox.com>;  
> <ken.d.brown at 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 at pobox.com
>>
>>
>>
>> _______________________________________________
>> TenTec mailing list
>> TenTec at contesting.com
>> http://lists.contesting.com/mailman/listinfo/tentec
>
>

--
73 DE N7WIM / G8UDP
Kevin Purcell
kevinpurcell at pobox.com