When the overload condition is set, the overload relay disconnects the high voltage from the amplifier, and other timing circuits are activated automatically. Overload Protection for High Voltage Power Supplies Mark Mandelkern, K5AM 2008 March 26 rev. 2008 December 12 Table of Contents Introduction . . . 1 Features . . . 2 Power supplies . . . 3 Modular construction . . . 5 Circuits . . . 10 Primary circuits . . . 11 High Voltage circuits . . . 13 Control circuits . . . 15 Step-start circuit . . . 17

Overload protection. An overload circuit must deal with any possible arcing or other shorts in the amplifier; it must prevent destruction of expensive power supply components, and it must protect the amplifier. A good high voltage relay is essential. For high voltage disconnect, one power supply described here uses a Jennings vacuum relay rated at 50 kilovolts and 50 amperes; the other supply uses an open relay with 2-inch contact spacing. The circuits provide high voltage disconnect, timed high voltage capacitor discharge, and power supply shut-down. Reliability. A power supply ought not to break down just when a rare DXCC country begins to be heard in your area, or in the middle of a contest. Op amp metering. This method virtually eliminates any possibility of meter damage. Heavy-duty construction. After an 8 hour RTTY contest run, at 1500 watts, the heavy-duty transformer is just slightly warm. The 5 kV capacitor is oil-filled, non-PCB. The wire-wound resistors are operated at less than 50% of rating. One supply is built into a sturdy aluminum cabinet, with casters and a lift-up top panel; the other supply is built into a steel cabinet, with casters, hinged lid, and drop-down front panel. Convenience. A heavy-duty power supply is best built into an enclosure separate from the amplifier. This requires nothing special other than a good high voltage cable. Other opinions will be encountered. Factory-built amplifiers nearly always have internal power supplies; the reasons probably include cost, portability, ease of installation, prevention of unsafe handling by the inexperienced, and safety regarding the high voltage cable. None of these factors is decisive for the experienced homebrewer. Only the safety issue requires special attention; these supplies use high-quality coax with unique connectors for the high voltage cable. The weight limitation imposed by an internal power supply nearly always results in an amplifier with limited capabilities for 100% duty cycle operation, such as RTTY and WSJT. Remote control. A small control head on the operating bench near the amplifier can provide full metering and control of a power supply. The bulky power supply cabinet can be placed out of the way, under or behind the bench. The remote units used here include metering, ON/OFF buttons, and neon indicators. The power supply panels have duplicate buttons and indicators for local control. Step-start. This circuit, using an op amp timer, is an improvement over the usual circuits. Variac tolerance. The high voltage may be adjusted with a variac on the 240 volt line, without causing malfunction of the control Overload Protection for High Voltage Power Supplies, K5AM 2 circuits. Independence. A high voltage power supply should be usable with any amplifier having demands within its capabilities; a restrictive one-to-one amplifier-supply correspondence is undesirable. A single- operator station can have several amplifiers simultaneously connected to a single high voltage power supply; each amplifier draws current only when it is transmitting. This idea is no more complicated than having several items connected to one 13.6 VDC power supply, or several lamps connected to one household circuit. Each amplifier on line has full internal metering; contains its own filament, bias, screen, and control power supplies; and requires no separate B-minus connection to the power supply. The B-minus lead from the rectifier in the power supply runs directly through a metering shunt to ground. A special B-minus connection to an amplifier for metering purposes is inconvenient, unnecessary, and potentially unsafe. Even grounded-grid triode amplifiers can be configured for this independence; a differential op amp circuit can be arranged to measure plate current as I b = I k - I g where I k is cathode current and I g is grid current. Power supplies This article will describe two power supplies which incorporate the features listed above. Although the supplies are not intended for exact duplication, details will be given as fully as possible, so that some of the circuit ideas might be applied to similar projects….

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