19-12-2016, 08:51 AM
The article by Peter VK3YE in December Amateur Radio magazine titled "Tune for maximum brightness: An LED current indicator" has a fundamental flaw in its simplicity :" There is no mention of the RF power limits which apply to this design.
The problem arises when there is RF power substantially higher than Peters typical transmitter power of 5 watts or 10 watts. The schematic reveals that there is essentially a 1 turn primary to 3 turn secondary current transformer in series to the antenna wire. The LED is wired directly across the secondary winding BUT two extra components should have been added... a diode and a resistor.. so that it can cope with higher power levels.
The resistor is in series in one leg of the lead from the secondary winding and the diode is in parallel with the LED but connected diode cathode to LED anode, diode anode to LED cathode i.e. in parallel with the LED but in the reverse direction. The resistor value is typically going to be in the range 100 to 500 ohms, the diode a 1N914, 1N4148 or similar type.
The purpose of the diode is to prevent the LED from excessive reverse voltage and subsequent breakdown.. and then it will no longer light and the user will wonder if the transmitter or feed cable is at fault. The purpose of the series resistor is to limit the current to the diode/LED combination so that excessive current doesn't cause LED failure.
A final note: be aware that the permanent inclusion of this design on the antenna feed wire could cause transmission of harmonics of the original frequency ( as does the inclusion of any non-linear device after the transmitter output filtering)
The revised circuit is:
[attachment=35]
The problem arises when there is RF power substantially higher than Peters typical transmitter power of 5 watts or 10 watts. The schematic reveals that there is essentially a 1 turn primary to 3 turn secondary current transformer in series to the antenna wire. The LED is wired directly across the secondary winding BUT two extra components should have been added... a diode and a resistor.. so that it can cope with higher power levels.
The resistor is in series in one leg of the lead from the secondary winding and the diode is in parallel with the LED but connected diode cathode to LED anode, diode anode to LED cathode i.e. in parallel with the LED but in the reverse direction. The resistor value is typically going to be in the range 100 to 500 ohms, the diode a 1N914, 1N4148 or similar type.
The purpose of the diode is to prevent the LED from excessive reverse voltage and subsequent breakdown.. and then it will no longer light and the user will wonder if the transmitter or feed cable is at fault. The purpose of the series resistor is to limit the current to the diode/LED combination so that excessive current doesn't cause LED failure.
A final note: be aware that the permanent inclusion of this design on the antenna feed wire could cause transmission of harmonics of the original frequency ( as does the inclusion of any non-linear device after the transmitter output filtering)
The revised circuit is:
[attachment=35]