Adding an overvoltage protection circuit to the front-stage power amplifier of the FM transmitter

The exciter of the FM302E-I FM transmitter uses the HPB-1210 motherboard of Japan's NEC company. The carrier is directly modulated using phase-locked frequency stabilization and frequency synthesis technology. The pre-stage power amplifier (BLF-177 field effect tube) is directly driven by the exciter, with a maximum output power of 150W. It passes through the circulator to the final stage and serves as the driving stage of the final tube power amplifier. The pre-stage power amplifier power supply uses a 4NICK48 integrated power supply. Unfortunately, this stage does not have an overvoltage protection circuit and a DC voltage indicator, which sometimes causes this stage to fail: the exciter output is normal, this stage has no power output, and the final stage has no power output. After investigation, it was found that the BLF177 power amplifier tube broke down and burned the ring. The load of this power supply was disconnected, and the output end of the integrated 48V power supply was measured to be 84V. The cause of the failure was a failure in the voltage stabilization part of the 4NICK48 integrated power supply, and the 84V fault voltage broke down and burned the power amplifier tube BLF177. Because the power supply is fully sealed, it cannot be repaired.

The integrated power supply and power amplifier tube are relatively expensive, and the loss is very large. Therefore, it is necessary to add an overvoltage protection circuit to the power amplifier tube at this level, especially since our station is located on a high mountain. During the summer thunderstorm season, lightning often enters the power line and burns out the transmitting equipment. Adding an overvoltage protection circuit can ensure safe and high-quality broadcasting, reduce the suspension rate, and save a lot of money.

We use the existing components in the library to design the overvoltage protection circuit shown in the figure, and install a terminal board. The wiring is simple, the control is simple, the cost is low, and the experimental work is reliable, which is very practical. The working principle of the protection circuit is introduced as follows, and the schematic diagram is shown in the figure.

In the figure, JX is the terminal board of the protection box, which is fixed on the box. JX-1 and JX-2 are connected to 220V AC power supply; JX-3 and JX-4 are connected in series in the AC contactor JC4 coil circuit that controls the high voltage II gear of the transmitter, and this contact is used to control the high voltage II gear on the machine; JX-5 and JX-6 are connected to 48V voltage sampling. K is the working switch (OFF/ON) of this box. When the protection circuit is activated, this switch acts as a reset switch, which is closed and then closed. T is a transformer with input of 220V and output of AC 15V. It is used as the power supply of this device. The three-terminal voltage regulator LM7812 outputs +12V DC voltage. P is the newly added 48V voltage indicator (full scale 100V). V1 is a switching transistor (3DK9E), RB1, RB2, and RW are bias resistors (also sampling setting circuit), RB1=13kΩ. RB2=180Ω. RW=4.7kΩ. J is a small electromagnetic relay. Model: JRC-19F/012M. D5 protection diode (2CP20), D6 light emitting diode (red Φ5mm), SSR is an AC solid state relay (model: JGX-2F).

Protection control principle: RB1, RB2, RW form a 48V sampling voltage divider circuit. When the power supply of the power amplifier is normal +48V, adjust the potentiometer RW. Make the base voltage of V1 0.5V. The switch transistor V1 is cut off, the collector of V1 is at high potential, the relay J is not attracted, its normally closed contact (4, 6) is connected, the input end of the solid-state relay SSR receives a 12V voltage and turns on, the high-voltage II gear controls the AC contactor (~220V) JC4 to attract, and the high-voltage II gear enters the normal working state. When the 48V power supply of the power amplifier exceeds 52V (the working voltage of this field effect tube is 48V. The maximum working voltage is not greater than 52V, so the protection control voltage should also be set to 52V). The base of the V1 switch tube is 0.65V. This tube is saturated and turned on, the collector is at a low potential, the relay J is energized, and the normally open contact of J (4.8) is self-protected (because the 100W power amplifier power supply is controlled by the high voltage II gear. After the high voltage II gear is blocked, the self-protection function is to prevent the 48V sampling circuit of the 100W power amplifier power supply from failing, causing the relay to alternately turn on and off); the normally closed contact (4, 6) of the electromagnetic relay J is disconnected, the solid-state relay SSR input end loses the 12V voltage and is cut off, the JC4 coil is de-energized and released, and the high voltage II gear is blocked. Then the 100W power amplifier power supply is blocked to play a protective role. After the cause of the fault is found and eliminated, the switch K is turned off and then closed. The protection circuit is reset to enter the next real-time protection state.

After assembly, conduct a simulated protection experiment first. Find a 1:1 220V isolation transformer and a voltage regulator. Assemble into a rectifier and filter power supply, the voltage output range is adjustable between 0-100V, as a 48V sampling voltage and simulated 84V fault power supply, connect the protection circuit for testing. Adjust: RW so that when the sampling voltage is 48V, relay J does not attract, the normally closed contact (4, 6) is connected, the AC 220V 25W incandescent bulb connected in series to the output end of the AC solid-state relay is turned on and lights up, and the high-voltage II block simulates normal operation; when the sampling voltage is greater than 52V, adjust RW so that when the sampling voltage is 52V, relay J attracts, the normally closed contact (4, 6) is disconnected, the output end of the AC solid-state relay is cut off, the incandescent bulb is extinguished, and the protection circuit simulates normal protection. Then repeatedly change the output of the voltage regulator: test whether J works normally, and through experiments and measurement results, it can work according to the design requirements. Put the protection device on the machine to work. It has been working for many years and has never had any malfunction, and has played a protective role for the pre-amplifier many times.

The connection between this overvoltage protection circuit and the transmitter: The wiring board Jx-1 and JX-2 of the overvoltage protection circuit are connected to the AC 220V input, and are connected to the transmitter wiring terminals X1-1 and X2 respectively. JX-3 and JX-4 are connected in series to the JC4 line package circuit; JX-5 and JX-6 are connected to the positive and negative ends of the solid-state power amplifier power supply using the 4NICK48 integrated power supply (that is, connected to the transmitter plug XS4-14 and XS4-12 pins) as 48V sampling voltage. The voltage meter head is also on the JX-5 and JX-6 terminals.