Amplifier Protection
or, "Protecting the Family Jewels"
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Just like your house and premises, it simply makes good sense to want to protect your select family of valuable radio jewels. Failure to do so can cause un-necessary monetary expenditure as well as personal trauma. With a little planning, and application of some rather simple, inexpensive measures, many catastrophic, jewel-destroying failures can be averted.

Using the 4N33 Optocoupler:

The 4N33 optocoupler can be used in various circuits for amplifier protection. It can be used to protect cathode, screen, grid, and HV power supply circuits. The isolation voltage is 5.3KV, making it ideal for use in fast trip protection circuits. Such a circuit in the screen grid circuit of a tetrode, for example, is a must! If plate voltage fails and screen voltage is not immediately cut off, the resulting high screen grid currents will damage or destroy the tube. The circuit is relatively sinmple. When the device "senses" a preset over-current, the LED internal to the 4N33 lights off and biases the base of the internal transistors such that they conduct (switch "on"). This effectively connects the collector to the emitter, providing a path through which to energize a "Trip Relay". This relay is wired to provide an instant disconnect from a power supply or keying circuit. A number of protection schemes can be built around this device.

The 4N33 is a 6 pin chip. Pin 1 is the LED anode and pin 2 is the cathode. Pin 4 is the transistor (switch) emitter, pin 5 the collector and pin 6 the base. Only pins 4 and 5 are used to drive a relay. Pin 4 is normally grounded and pin 5 provides a negative return to the relay control circuit. The base connection is not needed.

Referring to the diagram at left, voltage is "sensed" by a resistor wired in series with the monitored circuit and parallel to the meter. If the meter used is calibrated for the monitored circuit, the sense resistor can be in series with the meter, as shown at right. The value of this resistor is dependent upon the amount of current flowing in the circuit being monitored. A variable resistor is wired in series with the 4N33 LED to set the point at which the transistors will conduct, or "trip". The 4N33 will switch "on", or "trip" when 1.1mA of current is allowed to flow between pins 1 and 2, causing the LED to emit light which, in turn, biases the transistors "on". This provides the ground path for the Trip Relay which energizes and disables a potentially damaging situation. The relay control circuit can use whatever voltage is required by the relay coil; 12 and 24 volts DC are most the common and are reflected on the diagram. The voltage drive circuit can be either negative or positive. Just be sure that the 4N33 LED anode (pin 1) is always on the positive voltage side of the "sense" resistor. This principle can be applied to a circuit which can have current flow in either positive or negative directions. The circuit below shows how to monitor and protect a circuit for which current is monitored by a "zero center" meter.

This circuit would be used in a circuit in which the possibility exists for current to flow in either positive or negative directions. Note the "zero center" meter. An example of such a circuit might be a PA screen grid circuit; in this case, current exceeding separately adjusted positive and negative values would cause the 4N33 transistors to conduct, energizing the "trip relay, and shutting the PA down. Shutdown could be accomplished either by unkeying it or turning off the power supply, or both.

Wiring the Trip Relay Contacts:

Trip relays are DPDT. One set of contacts is used in a "latch" circuit which must be manually re-set for the protection circuit to return to untripped condition. The other set of contacts is used for protection control. This circuit is shown at right in un-tripped condition, with the "Trip Relay" unenergized.

The "latch circuit" is comprised of the lower set of relay contacts, the reset pushbutton switch (normally closed, momentary open), and the LED with it's series resistor. When the protection sensor is tripped, the relay energizes, and the bottom set of contacts provides a return to ground for the relay. This also provides ground return for the LED circuit (the value of "R" is dependent upon relay voltage), which is used to show which protection circuit was tripped. To reset to normal, untripped condition, after the problem which caused it to trip has been cleared, the reset pushbutton is pressed, opening the ground return, and allowing the relay to de-energize.

The "control circuit" is comprised of the upper set of contacts. Note these are closed, the typical "on" condition, in this circuit state. The trip relay can be used stand-alone, with each set of control circuit contacts adressing its own individual circuit, or several sets of control circuit contacts can be connected in series, all controlling one circuit (see diagram at right).

Implementing the 4N33 Protection Circuitry:

As of this writing, Mouser has 4N33ís listed in their catalog for thirty cents each. The protective circuit can either be built on a perf board, or one can etch one's own PCB. Either board is mounted directly on the back of the meter monitoring the circuit to be protected. The trip relay may be placed on the board with the 4N33 support components or mounted at another location.

At right, is the screen grid protect circuit for a GS-15B 23cM PA (click on picture for zoom). The Trip Relay is mounted on the perf board used to build the circuit. The entire assembly is mounted on the back of the screen grid current meter. The meter is a 0-1 mA meter shunted to read 0-10mA. Two 5KW pots are used, one to set the trip level, and one to calibrate the meter. Note that the 4N33 is in a socket. It is highly recommended that a socket be used; bad failures have been known to blow the 4N33 right out of the the socket and not caused any damage to the tube or associated components.

The relays are not critical. Any coil voltage will work from 6VDC to 24VDC. The 12VDC, DPDT relays most common at Radio Shack, mount directly on either perf or pc board. Larger size relays will also work, but require more mounting space and more current to operate.

A "sense" resistor in series with the monoitored circuit supplies the current to operate the 4N33. As noted above, a pre-set scale meter may be used with a "sense" resistor in series with the meter, or a lower capacity meter, like 0-1mA, can be used with a shunt serving as the "sense" resistor. Using a 0-1mA meter for grid and screen circuits, a resistor between 100 and 200 Ohms will let a trip current of 5mA be set. For cathode and higher current circuits, the resistor will vary from 1 to 3 Ohms, allowing settings for currents as high as 2 Amps. When setting the full scale value of a 0-1mA meter, a 5KW pot can be used to set the desired scale. A 5kW trim pot is also used to adjust the trip point of the 4N33. With lower currents a smaller sized tip pot can be used for both adjustments; either 500W or 1KW pots would be suitable for the lower currents.

Above is shown (left to right) grid, screen and cathode protection circuits for a TH-327 PA. The right-most meter is calibrated for the circuit it is monitoring, so there is no shunt resistor, and only one variable resistor (trip level set). Relays are mounted elsewhere in the amplifier. Note that standard 14-pin IC sockets are used. Standard 16-pin sockets could be used as well. There is no problem if 6-pin sockets are not available!!

Simple Power Glitch Protection:

Should you be in the transmit mode and the station power is interrupted or glitches, many bad things can happen. The most common being the quick switching of the pre-amp while still in the transmit mode. This usually results in a loss of the device in the Pre-amp. Bad things can also occur in the exciter-to-final amplifier chain. The Protection Circuit shown below in un-energized state will eliminate these bad things caused by power glitches.

The circuit consists of a DPDT relay. Any coil voltage will work as long as it will operate the relay. When S2 is closed and the momentary push button switch is pressed the relay coil is energized. The latching circuit using one set of contacts will keep the relay energized until there is a power interruption. The control circuit or PTT line uses the other set of relay contacts which are closed when there is voltage on the relay coil. Upon an interruption, the relay drops out, and the control circuit contacts open. The control circuit will remain open until S1 is manually pushed to re-energize the relay.


Two forms of protection for "radio Jewels" are presented above, amplifier protection circuits which can be built around the 4N33, and a simple power glitch protection circuit for protecting overall station. The 4N33 PA protection circuits presented are proven to be fast acting and most reliable, while providing the operator with Peace of Mind that his Family of Valuable Radio gear is Protected. Thanks to Darrell Ward VE1ALQ for his help and guidance with Protection Circuits using the 4N33.

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This Page Last Updated: 29 November 2003
Feedback: Paul S. Goble, III, ND2X
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