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.
|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.
|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
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.
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
Copyright © 2003 by ND2X and K5JL, all rights reserved