Protect your rig against voltage over 15V !

Recently a friend of mine, who is the system-operator of a Packet Radio digipeater, experienced heavy and expensive damage with his system by a failure in the power supply.

Usually people pay attention to short circuits protection in the supply and have effective means to shut down the power supply to protect the connected equipment. But what happens when the voltage regulator or the final semiconductors (mostly one or more 2N3055) in the power supply itself dies ?

Well,- that's exactly, what happend to my friend. All of sudden there were more than 28V unregulated voltage instead of the regulated voltage of about 13V to 14V at the output of the supply.
Nearly all connected items died within seconds...

Many have already experienced, that also HF can affect a bad shielded power supply. Those are lucky, where the voltage drops, but often the voltage rises over the allowed 14V. There may be plenty of other reasons for the unwanted increase of output voltage...

This could have been prevented by only some more parts in the power supply. The most important point in this context is, that the shut-down of the voltage must happen as fast as possible,- every microsecond counts and decides whether the connected equipmet survives or not...
(The shut-down, steered by a mechanical relais, takes a much too long time).

From my point of view the best way to fulfill this job is the following:
The motto is: "short-circuit" instead of "high-voltage". A modern power supply mostly has a very effective short-circuit protection,- mostly with the popular L200 regulator. If you short-circuit the output, your power supply regulates the voltage down without a delay.

So,- why not short-circuiting, when the voltage rises over a specific level ?
That's exactly what this small tool does: If the output-voltage of the power supply exceeds the break-through-voltage of the Z-Diode (for exampe 15V), it begins to conduct and a current runs over the 1K resistor. The voltage drop over the resistor fires the thyristor (Gate positive against Kathode) and it becomes conductive without delay and causes the wanted short-circuit...

Use a heavy duty Thyristor (>30 A) and check your power supply for good short-circuit behaviour. For unmanned operation make sure that the switching closes the supply permanently down.

Three weeks after installing the new item, I tuned my HF-radio on a wrong antenna and confused the supply by too much direct HF,- voltage rises,- the safety device responds and shuts down everything, but no damage to the radio !
Tell me your personal opionion on this little tool,- have you probably got annother good idea ?

Best 73: Johannes / DL4EBJ