An Automatic (External) Reset Circuit
for Astron Power Supplies

General Description:
This circuit will provide an automatic reset of an Astron power supply that has been tripped into overvoltage protection.

Concept:
Have you ever had to run to a remote site to reset an Astron power supply that has been 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.

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 and mounted in a metal box for safety. A large enough enclosure should be selected to allow an AC outlet to be safely mounted. Use a line cord with a ground pin and ensure the ground wire has continuity to the switched outlet. It is not necessary to ground the box chassis from the AC Plug. The reset circuit uses a regulated 12.00 volt supply that operates independently from the Astron supply. This supply is built conventional brute force to hopefully withstand more abuse than the Astron. A relay is used to drop both sides of the line voltage to the Astron supply. Primary power to transformer T1 can be connected to the input line cord where it connects to the 4PDT relay. Ensure that the primary power is connected on the "line" side of the relay and not the "switched" side. Use fuses where appropriate.

Circuit Description:

Print this schematic

The basic concept of the circuit operates like this.
Upon initial power-up, RY-1 is in the relaxed position which allows current to flow from the brute force 12.00 volt supply through diode D8, a 1K pot, and a set of NC contacts of RY-1, to the coil of reed relay RY-2. The reed relay has a very large (33,000 uF) aluminum electrolytic capacitor connected across its coil. Because RY-2 is a reed type relay, it doesn't need much current to actuate. This allows the use of a relatively high series resistor to supply current to the coil. Although this series resistance is relatively high as far as actuating a relay coil, it is relatively low as far as a resistor / capacitor time constant. This is why it's necessary to use the large C4 capacitor. This resistor / capacitor circuit allows a time delay before RY-2 closes. The series resistance is provided by a 1K pot that determines the charge time of capacitor C4. After C4 charges up, to the trip voltage of RY-2, it closes allowing power to be supplied to the coil of RY-1 through diode D6, half of a "diode OR gate." At this point several things happen simultaneously. First, RY-1 closes allowing mains power to be supplied to the Astron. Secondly, capacitor C4 is discharged through R1 allowing RY-2 to be closed for only a short period of time. In this short period of time, if all is ok with the Astron, it will supply power through R2, and the other half of the "diode OR gate" D5, to the coil of RY-1 latching it on. If the Astron doesn't supply current to RY-1 the circuit will reset itself and the cycle starts all over again, however the time it takes for recycling is about half of the initial time because capacitor C4 is now partially charged and has not been fully discharged by R1.
A test switch allows the user to test the circuit while at the site. D7 is used to absorb the counter EMF that the coil of RY-1 produces when deactivating. RY-2 does not create much counter EMF as a function of a reed relay and what little is produced is absorbed by the low impedance of C4. C2 and C3 are Tantalum types that are soldered directly to the leads of the 7812 voltage regulator as indicated by a star on the schematic. Be certain to observe polarity of all capacitors. If this circuit is used near high levels of RF, install .01 uF bypass capacitors across C2 and C3, the input and output leads of the 7812 regulator.

Parts List:
T1, 120 to 12 volt power transformer rated @ 300 mA. or more, Radio Shack # 273-1385, 273-1365. If the 273-1365 is used do not connect the center tap. I use a 25.2 CT, Radio Shack # 273-1366 in "full wave center tap" configuration by only using two 1N4004 rectifier diodes.
D1 - D4, 1N4004 or equivalent, Radio Shack # 276-1103
D5 - D8, 1N4148 or equivalent, Radio Shack # 276-1122
R1, 220 ohm ½ watt, Radio Shack # 271-1111
R2, 47 ohm 1 watt, or 50 ohm 10 watt, Radio Shack # 271-133. Can also be 2, 100 ohm ½ watt in parallel.
Pot, 1K ohm potentiometer, Radio Shack # 271-280
C1, 4700 uF - 35 volt electrolytic, Radio Shack # 272-1022
C2, and C3, 1 uF 35 volt Tantalum, Radio Shack # 272-1434
C4, 33,000 uF - 16 volt Aluminum electrolytic, Nichicon UVR series Digikey # 493-1055-ND
VR-1, Three terminal voltage regulator, model 7812, Radio Shack # 276-1771
RY-1, 4PDT 12 volt coil relay, Radio Shack # 275-214
RY-2, SPST Reed relay 12 volt coil. Coil resistance is 1050 ohms, Radio Shack # 275-233
SW-1, SPST momentary, Radio Shack # 275-646 or equivalent.
Breadboard for mounting parts.
3 Prong line cord.
Grounded outlet.
Metal Box.
Fuses and holders where appropriate.

Notes:
Do Not substitute C4, obtain the exact part from Digikey http://www.digikey.com/ Online ordering is available and easy to use. Copy and Paste the Digikey #, exactly as indicated above, into their parts search to quickly locate it. If substitution is necessary, replace with exact like unit. A Sprague "80 D" model is an exact replacement. Also do not substitute the reed relay if at all possible. The 1050 ohm coil resistance plays a large part in determining the time constant. If the reed is substituted, ensure the 1050 ohm coil resistance.

Set the potentiometer for initial charge time of 28 seconds. This will produce a recycle time of approximately 14 seconds. This should be enough time for the Astron power supply to completely reset. I have found that the resistance of the pot ends up being about 800 ohms. Let the circuit "rest" with no power applied for several minutes to allow C4 to completely discharge for checking initial charge time. To check recycle time simply turn off the Astron's power switch or disconnect the + lead from the Astron, while supplying power to the reset circuit.

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This circuit was designed by Kevin K. Custer W3KKC on April 2, 1997