Respectfully, current limiting in the final amp is exactly how
high-level plate modulation in an old AM transmitter works. Your audio
amplifier, which is required to generate a significant amount of power,
is transformer coupled into the plate lead of your final amplifier.
This allows the audio to vary the voltage and current in the final
amplifier turning it into a mixer. That's how you get the audio
sidebands plus the carrier that is a high level generated AM signal.
Flat-topping in a linear amplifier is actually nothing more than a form
of current limiting and it too generates spurious responses by making
the amplifier non-linear.
Before you do anything to modulate the current and voltage being fed
into a linear amplifier you should have a two-tone audio generator and
a spectrum analyzer of some kind to monitor your signal.
tim
On Mon, 29 Jan 2018 09:49:00 -0700
"Arv Evans" <arvid.evans@...> wrote:
Maybe part of the point has been missed at several levels. The
proposed current
limiter is not much different from current limiters in many commercial
power
supplies. It is not active until some current value has been
reached. Up to then
there is no current limiting or "modulation" or "compression". Yes,
the in-series
current sensing resistance does allow an output voltage drop until
voltage across
this resistance reaches about 0.65 volts to forward bias the sensing
transistor.
If your power supply output is not stiff enough to hold a steady
voltage this could
add to the sensing resistance and cause voltage droop and in really
bad cases
compression of RF output.
Question...is RF compression all that bad? It is what most transmit
ALC circuits
do. It would also be possible to make the proposed circuit driven by
reflected
power instead of current. Then we could call it a transmit ALC to
protect the
IRF510(s).
It would be fairly simple to add a latching function to the limiter
design so that it
totally shuts down output when Imax has been reached. This is
possible but would
then require a RESET button or power cycling to restore power to the
RF PA stage.
It is also possible to change the proposed design to put the current
sensing ahead
of the control MOSFET and make that MOSFET into a voltage control
circuit. This
was my initial approach but the component count became just too high
to be an
internal modification for transceivers. It would be more suitable for
converting
an unregulated power supply to a regulated one.
For those who are using modified PC power supplies it is possible to
change the
current sensing so that the switching regulator would shut down if
Imax of 2 to 3
amperes is exceeded. This approach would require power cycling to
restore operation.
Problem with using fuses is that they have a short but measurable
heating period
before they go open. When they do open they spark internally for
another short
period. Both the delay and the spark are potential problems for
circuit components.
Poly-fuse or resettable fuse devices are intended to address a
different issue.
Some are equivalent of FAST-BLOW fuses, and some are just conventional
resettable circuit breakers.
Suggestion here is that "
*if you can come up with something else that is adequate, *
*please submit a circuit drawing and explanation of how it works*" We
could all
benefit from that.
Arv K7HKL
_._
On Fri, Jan 26, 2018 at 11:27 AM, Tim Gorman <tgorman2@...> wrote:
A current "limiter" which modulates the current, i.e. limiting the
current no matter the drive level, is like applying an ALC to the
exciter. Improper design can result in compression of the signal
which can then generate spurious responses in the output.
If the current limiter is of a "drop-out" style which requires
resetting then this won't happen, it will just shut down the
exciter. In that case, however, it doesn't provide much efficiency
over a plain old fuse!
tim ab0wr
On Fri, 26 Jan 2018 05:32:08 -0800
"n5ib_2" <n5ib@...> wrote:
One other possibility to consider to protect against overcurrent
situations is to use a current limiting power supply, or add a
current limiter to an existing power supply or battery.
The current limiter circuit whose link is at this site:
<>
will go up to 2.2 amps, and includes a low-dropout voltage
regulator that also provides reverse polarity protection.
There is a version, in a different package, of that same regulator
chip that has an added "tracking" feature - being able to parallel
multiple chips to increase the available current. Details are on
the datasheet in the document.
Jim, N5IB
