The AM Forum

There's a thin line between being loud and splattering (only a slight percentage of modulation) and this simple circuit will help identify when that line is being crossed.

Circuit Description:

An NE555 timer is used to convert a very brief trigger to a more visible 0.5 second indication. Power for the circuit can come from any convenient supply but should be limited to 12 volts at the IC for proper peak detection. I use this in a Viking Ranger, so the 6 volt A.C. heater supply and a #51 bulb were chosen. You can also substitute an LED with the appropriate dropping resistor, such as a 2.2k, which works well when powered by a 6 volt heater circuit. The 470k resistor and 1 mFd capacitor set the "on" time for the indicator. I found 1/2 second to be good for catching the eye with an incadescent indicator. Lower either value for a shorter time constant.

On the input trigger side, the 5 volt zener diode, D2 (1N914), and the 1M resistor ensure that the trigger voltage at the input of the IC remains between 0 and +5 volts. Either point A or B would be connected to your modulated B+. Both work well, so it's up to you.

At A, choose the resistance and wattage of R1 to drop the modulated B+ sufficiently so that the maximum current rating of the protective zener diode is not exceeded on positive modulation peaks. The indicator is triggered when B+ drops below 5 volts, which is -99% for a 500 volt supply.

If B is used, no modulated voltage is passed by D1 until it drops below 5 volts. R2 is used to protect D2 during excessive negative voltage peaks. The value is non-critical and can be between 1 and 10k depending on your supply voltage.

Notes:

1) Be sure to use a high voltage resistor or several regular resistors in series for R1 and R2 to reduce the risk of arcing.

2) Some amplifiers reach -100% before the modulated supply drops to 0 volts. The circuit can be modified to accommodate these designs.

On my single MOSFET transmitter, I use a 3-diode keep-alive circuit to eliminate my negative peaks.  I would like to build something like this though just as a monitor to ensure I don't have any.  Under load, my Vdd is 34 volts.  My negative peak limiter supply is 5 volts. 

How would I modify this circuit to work for the lower voltages than plate modulated rigs?  I'm guessing that I'd be concerned if my modulated voltage dropped below, say, 2 volts dc.  So, would I just change the zener to 2 volts instead of 5 if I used connection point B?

Jon
KA1TDQ

Hi Jon,

The 555 functions like a comparator such that when the trigger (2) voltage drops below the threshold (6) the output (3) goes high until the condition reverses, and that's timed by the 470k / 1.0 uF pair.

Two ways to increase the triggering voltage are:

1) Use a resistor divider circuit so that the desired triggering voltage is reduced to the actual triggering voltage based on your supply voltage (VCC). This is just a matter of putting a resistor in parallel with the zener, and since you're dealing with relatively friendly voltages, you could even use a potentiometer to adjust the sensitivity, ie, the indicated percent negative peak level.

2) Increase the supply voltage (VCC) but to no more than +15VDC and use a series resistor on the output since that will increase as well. The output can sink or source up to 200mA, which is perfect for anything from LEDs to incadescent bulbs to relays.

Clark