Negative peak limiter for Johnson Ranger and other transmitters

[AB2EZ]

Attached is the schematic of a negative peak limiter that I am presently using with my Johnson Ranger. It works very well... and can be used with other transmitters provided the components are selected to handle the peak inverse voltages (diodes), and the power dissipations (resistors).

Of note:

The basic design is a combination of the 3-diode negative peak limiter that has previously been posted by WA1QIX (Steve), and a simple voltage divider to produce a negative peak limiting reference voltage that is around 9% of the average B+.

Of note:

On negative peaks, the capacitor across the 10k ohm resistor will begin to discharge (to supply the additional current being drawn from the negative peak limiting supply, via the diodes)... but since the percentage of the total time that the modulation hits negative peaks is very small, the negative peak limiting voltage will not change very much.

The 10k ohm / 110k ohm voltage divider, in combination with the 100uF capacitor, has a long enough time constant to average out the modulation of the modulated B+ supply.... so it is not necessary to use a separate, unmodulated B+ line to drive the voltage divider. However, this does place an additional 100k ohm load on the modulator. If that is a problem, you can use a separate line from the unmodulated B+ (instead of the modulated B+) to drive the 10k ohm /110k ohm voltage divider.  

As always... there are high voltages involved, so be careful with both the physical design and the installation.

Stu

[w4bfs]

hi Stu .... this simplifies the previous art in not requiring a seperate keep-alive supply ... if you want a visual indicator of negative peak activity break the line from the positive terminal of the 100uF to the junction of the anodes and put in a led in parallel with a 220 Ohm or so resistor in the broken line and as the cap discharges toward the pa stage the led should flicker ...

[steve_qix]

hi Stu .... this simplifies the previous art in not requiring a seperate keep-alive supply ... if you want a visual indicator of negative peak activity break the line from the positive terminal of the 100uF to the junction of the anodes and put in a led in parallel with a 220 Ohm or so resistor in the broken line and as the cap discharges toward the pa stage the led should flicker ...

The self-powered "keepalive supply" works very well with transmitters such as the Ranger, where the H.V. supply is on all the time.  If the H.V. supply is keyed, another source of DC for the "keepalive" supply should be used to ensure the "keepalive" is there when the transmitter is brought into operation.

[w4bfs]

oops .... took a second look ....need to move the led / resistor combination to between the anodes otherwise the led fires during all negative excursions .... just want firing of the led during keep-alive .... sure wish we had a schematic capture mode during these discussions ...

[PA4WM]

....need to move the led / resistor combination to between the anodes otherwise the led fires during all negative excursions .... just want firing of the led during keep-alive ....

I don't understand that. Keep alive current only flows when the negative peak droppes below the 90 ish %. Isn't that the only action the limiter does?
Or is there a difference between "all negative excursions" and the "keep alive action"?

Martin

[w4bfs]

....need to move the led / resistor combination to between the anodes otherwise the led fires during all negative excursions .... just want firing of the led during keep-alive ....

 is there a difference between "all negative excursions" and the "keep alive action"?

Martin

absolutely !

the keep-alive action occurs when the cap discharges thru the 3 diodes directly to the pa stage

[PA4WM]

Offcourse there is...

Sorry, I misunderstood "excursion". Should use the dictionary more often...

Martin