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Astron's Automatic Reset Circuit for Linear Power Supplies By Kevin Custer W3KKC

General Description:
This web page describes the Astron "official" method to provide an automatic reset of an Astron power supply that has been tripped into overvoltage protection. They call it the OVPR-12V overvoltage reset circuit.

Concept:
Have you ever had to run to a remote site to reset an Astron power supply that was tripped into overvoltage protection? This situation happens when there has been a near lightning strike or when the power line has been surged. The Astron power supply has an excellent overvoltage protection circuit that will clamp the output of the supply to near 0 volts when the power supply's output has gone over the predefined voltage above normal. In most Astron power supply's the point of overvoltage is near 15 vdc. When an Astron power supply has gone into protect, it must be reset by turning off the mains power to the supply for at least 10 seconds. This gives enough time for the filter capacitors to bleed down so complete reset is possible. Protection in the supply is afforded by the use of an SCR to crowbar the output when an overvoltage situation has occurred. The circuit that fires the SCR is very fast, and remains in protect mode until mains power to the supply has been dropped for at least 10 seconds, or has been reset by the following circuit.

Construction:
The circuit described below will automatically reset an Astron power supply that has been accidentally tripped into overvoltage protect. The circuit can be built on a simple breadboard, or purchased directly from the Astron Corporation. A small circuit is used to reset the LM-723 regulator and clamping circuit in the Astron supply. This circuit is referred to by The Astron Corporation as the OVPR-12V overvoltage reset circuit, and is not referred to in their web site as of this writing. This reset circuit was designed by The Astron Corporation, and when I bought one in 1988 the cost was $20.00 plus shipping. This circuit is intended to be installed inside the case of the the power supply. The factory built unit has a small mounting tab that is used to mount the circuit board to the bolt in the hold down clamp for the large filter capacitor. The home made one can be secured using the builder's choice of method, even with double sided foam tape.

Circuit Description:

Refer to the above circuit schematic image above to understand the circuit flow.

Hookup of the circuit:
The circuit has four wires and is connected as suggested in the
Black and White PC board hookup image, or click here for a
Color PC board hookup image.

The following paragraphs were added by Bob WA1MIK:

In case you want to know where these wires connect electronically, the black wire goes to the negative side of the large filter capacitor (ground). The red wire goes to the positive side of the large filter capacitor. The yellow wire goes to the LM723 regulator pin 13 that goes to one end of C3. The white wire goes to the LM723 regulator pin 3, which is also the point on the edge of the circuit board labeled terminal 2, which goes to the main output terminal of the power supply.

If the power supply crowbar circuit activates, this reset circuit detects the loss of output voltage and momentarily grounds the LM723 regulator pin 13, which shuts down the LM723, thus removing the output voltage and allowing the crowbar circuit to reset. The normal output voltage is then restored several seconds later.

Parts List:
If you prefer to build the overvoltage reset circuit yourself, these are the parts you are going to need:
2 - 2N3904 general purpose NPN transistors,
1 - 1N4148 general purpose switching diode,
1 - 1uF 50 volt, Tantalum capacitor,
1 - 2k resistor,
1 - 5.6k resistor,
1 - 10k resistor,
1 - 39k resistor,
1 - 1 meg ohm resistor,
6 to 8 inches of #22 insulated wires: (Astron used white, yellow, black, and red)
and a small pc breadboard or perfboard to build it on

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