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Upgraded Amplifier Controller

Over time, others using the All-Purpose Amplifier Controller, which this board was developed to replace, suggested some enhancements. This board has all of the features of the original (combines the functions of a relay sequencer, amplifier bias switch, temperature-controlled cooling fan and a high-VSWR cutoff switch); it also incorporates some of those suggested changes, which are:

  • The ability to completely lock out the main power to an amplifier under high-VSWR conditions

  • Additional current capacity from the on-board 12v regulator - now up to 500ma is available

  • Increased fan current handling - this one can handle fan currents up to several amps

  • Feeds for panel indicator LEDs, which we all love to put on the front panels of our amplifiers.
     

Ok, let's examine the high-VSWR lockout enhancement first:

The previous version would react to a high-VSWR signal by cutting off the bias to the amplifier, and releasing it again a couple seconds later. This is adequate protection under most conditions; the fact that some amplifiers can continue to develop some output power without bias suggested a more complete shutdown might be even better.

The new board can completely power down the amplifier it controls while simultaneously commanding it's on-board sequencer into bypass mode; it will keep things in this state until manually reset (cycling the power off for a couple of seconds will reset the system; alternatively one can use a momentary-contact pushbutton switch connected across R2). The lock-out circuit was designed to operate a gate feeding the main power to the amplifier under control. This gate can be a mechanical or a solid-state relay. Here's how it works:

Q3 is normally on during closure of PTT (transmit), and supplies power to the sequencer timer.

When Q2 receives a large enough signal (negative-going) from an SWR sensor, it allows Q1 to conduct, cutting off Q3 and latching itself permanently on. D5 (the LED) is not only an indicator of a lock-out condition, but also serves as part of the latching gate.

With Q3 cut off, PTT is disabled, and the sequencer timer backs the system off (in sequence) into bypass mode.

The power to the amplifier is cut off as the sequencer backs off into bypass mode (the recommended connection for the power gate to the amplifier is the "sequence 2" port of the on-board sequencer).

Alternatively, the disable port of Q4 can be used to initiate a more immediate shutdown of the amplifier power source.

The next item is the improvement to the on-board 12v supply. One of it's original functions was to supply power for a 12v fan to cool the amplifier under control, but it's capacity was limited to small fans (150ma or less).

By switching the chip over to a DPAK 7812, the power dissipation was improved, and the extra current available is now about 500ma, enough to operate up to two larger-size fans.

The fan switch is now capable of handling a lot of current; this was accomplished by switching the fan power return instead of switching the power feed from the 12v regulator.

So, if one must manage fan currents in excess of 12v @ 500ma, the way to do it is to feed the fan(s) from a higher-current 12v supply, or from 28v, and run the return line to the fan enable port. This port is switched to ground by a transistor capable of handling several amps.

One final note: there are ports on the board to feed indicator LEDs for main power, PTT (transmit) and lock-out. The return for power and PTT is ground, and there are two pads for the lock-out LED, one for cathode (-) and one for the anode (+).

The board can also be operated from 12v; the only changes recommended for 12v operation are to eliminate the 12v regulator chip (U1), jumper the 28v pad to the 12v pad, and change R6 to 1.5K (the feed for the power indicator LED).

Here is a link to the full schematic

Here's the link to a high-resolution photo of the board (for component placement)
If you are building this project from a kit I supplied, please note the positioning of the board spacers included in the kit; because there are traces on the back side of the board, it must be elevated slightly above conductive mounting surfaces. The spacer under the 7812 regulator also assists with heat transfer away from the chip.

The following diagram shows a typical use for the board, outlining some of the possible connections.

Note: There is a new revision of this board available (V5):

This latest rev of the amplifier control board has some circuit simplification, and a minor improvement to the fan control circuit. Specifically: 

  • The extra kill switch at Q4 (on version 4 boards) has been eliminated, as this was a back-up function found to be unnecessary.
  • The fan control circuit has been simplified, and a small modification made to the high-temp trigger circuit. Explanation:

When the temperature of a typical heat sink rises to about 110F, the fan would turn on and cool the heat sink surface (where the fan sensor is usually located); as the temperature dropped just below 110F, the control board would sense this, and shut off the fan. Seconds later, the heat stored in the interior of the heat sink would make its way to the sensor and trigger the fan on again. Depending on the location of the sensor, this re-cycling of the fan might happen 2 or 3 times.

Not really a problem, but this was an annoyance to some users; so the latest rev (v5 series) adds R15 and R14. These components create a hysteresis gap that causes the sensor to keep the fan on until the heat sink temperature drops about 5 degrees lower than the turn-on temperature. This allows the heat stored in the interior of the heat sink to be drawn off, which eliminates the fan stuttering.

The sensor adjustment trimmer should be set for a voltage of 2.7v at its control arm (also pin 12 of the control IC). This corresponds to the recommended fan force-on temperature of about 110F. Fan release will happen at about 105F. Of course, the trimmer can be set to an alternate temperature of the userís choice.

It should be noted that it is still normal for the fan to run continuously during transmit. The circuit described above will only force the fan on continuously when the average heat sink temperature rises above 110F. The revised schematic is shown here.