WA2LTD
12-12-2006, 10:12 PM
Linear Amplifier Inrush Current Control and Soft Starts
Thomas G. Siglin – WA2LTD
Part One
Ka-Thunk… The noise my linear amplifier makes when I turn it on. It makes that sound because a large amount of current is momentarily drawn when the tube filaments are cold, electrolytic capacitors are in a discharged state and the power transformer is at a resting state of flux(1).
To understand inrush current in linear amplifiers, let’s look back at circuit design philosophies and available components in earlier times. #We would find many tube, transformer and capacitor manufactures. These parts were readily available and prices were reasonable. The circuits themselves used vacuum tube or mercury vapor rectifiers with choke filter inputs and lower value capacitance. These construction methods tended to keep inrush currents in the manageable range.
Today, with present design trends favoring capacitor input power supplies and solid state rectifiers, inrush currents are substantial, sometimes approaching peaks of ten times the normal operating current. #With Eimac® tubes and Peter Dahl® transformers selling at a premium, it would be well to practice operating methods that ensure long useful lives from these components.
A check of most ARRL Handbooks® on linear amplifier design will include a series resistance in the primary of the power supply, i.e. 20 Ohms, which will be bypassed through relay contacts. This additional resistance limits the maximum inrush current to about 6 Amps. #Building this circuit onto a chassis during initial amplifier construction is a simple task. Adding one to a commercial amplifier is more trouble but within the grasp of many hams. What poses a problem is voiding approval agencies, i.e. underwriters’ Lab ® certifications or warranty terms from the manufacture. To avoid these problems, I chose to build my inrush current limiter in a 4x4 inch electrical box with an outlet and switch that I can simply plug my amplifier into.
To construct a simple 120 Volt inrush current limiter, use a heavy AC cord/plug and install it though a Romex® clamp on a 4X4 electrical box.. The white neutral wire of the cord is connected to the white screw of the 120 VAC -15 Amp duplex outlet. The Black hot wire will attach to a screw on the AC switch. At the other screw of the switch attach a jumper to the brass screw on the duplex outlet. Now install a 20 Ohm 20 Watt resistor between the two switch screw terminals. Connect the green ground from the cord to a green screw on the outlet, switch or with a ground screw to the box. As you attach the cover with the switch and outlet to the box, dress the wires away from the resistor. Test your work for safety before using it.
To use this limiter, plug your amplifier into the outlet on the new box. Set the switch of limiter to OFF. Plug the limiter’s cord into a wall outlet. Turn on the amplifier. After a few seconds, flip the switch on the limiter box ON. (Note: if you do not turn ON the box’s switch and use the amplifier to tune or transmit, you will burn out the limiter’s resistor!) You now have an inrush current limiter. When you turn off the amplifier, also turn off the limiter’s switch for the next cycle. If you need a schematic or pictorials, please send an eMail to: tsiglin@SIGTOMICS.com
Part two will improve on the above inrush current limiter. Soft starting will be introduced later.
® are trademarks of the respective owners
(1) http://www.allaboutcircuits.com
Thomas G. Siglin – WA2LTD
Part One
Ka-Thunk… The noise my linear amplifier makes when I turn it on. It makes that sound because a large amount of current is momentarily drawn when the tube filaments are cold, electrolytic capacitors are in a discharged state and the power transformer is at a resting state of flux(1).
To understand inrush current in linear amplifiers, let’s look back at circuit design philosophies and available components in earlier times. #We would find many tube, transformer and capacitor manufactures. These parts were readily available and prices were reasonable. The circuits themselves used vacuum tube or mercury vapor rectifiers with choke filter inputs and lower value capacitance. These construction methods tended to keep inrush currents in the manageable range.
Today, with present design trends favoring capacitor input power supplies and solid state rectifiers, inrush currents are substantial, sometimes approaching peaks of ten times the normal operating current. #With Eimac® tubes and Peter Dahl® transformers selling at a premium, it would be well to practice operating methods that ensure long useful lives from these components.
A check of most ARRL Handbooks® on linear amplifier design will include a series resistance in the primary of the power supply, i.e. 20 Ohms, which will be bypassed through relay contacts. This additional resistance limits the maximum inrush current to about 6 Amps. #Building this circuit onto a chassis during initial amplifier construction is a simple task. Adding one to a commercial amplifier is more trouble but within the grasp of many hams. What poses a problem is voiding approval agencies, i.e. underwriters’ Lab ® certifications or warranty terms from the manufacture. To avoid these problems, I chose to build my inrush current limiter in a 4x4 inch electrical box with an outlet and switch that I can simply plug my amplifier into.
To construct a simple 120 Volt inrush current limiter, use a heavy AC cord/plug and install it though a Romex® clamp on a 4X4 electrical box.. The white neutral wire of the cord is connected to the white screw of the 120 VAC -15 Amp duplex outlet. The Black hot wire will attach to a screw on the AC switch. At the other screw of the switch attach a jumper to the brass screw on the duplex outlet. Now install a 20 Ohm 20 Watt resistor between the two switch screw terminals. Connect the green ground from the cord to a green screw on the outlet, switch or with a ground screw to the box. As you attach the cover with the switch and outlet to the box, dress the wires away from the resistor. Test your work for safety before using it.
To use this limiter, plug your amplifier into the outlet on the new box. Set the switch of limiter to OFF. Plug the limiter’s cord into a wall outlet. Turn on the amplifier. After a few seconds, flip the switch on the limiter box ON. (Note: if you do not turn ON the box’s switch and use the amplifier to tune or transmit, you will burn out the limiter’s resistor!) You now have an inrush current limiter. When you turn off the amplifier, also turn off the limiter’s switch for the next cycle. If you need a schematic or pictorials, please send an eMail to: tsiglin@SIGTOMICS.com
Part two will improve on the above inrush current limiter. Soft starting will be introduced later.
® are trademarks of the respective owners
(1) http://www.allaboutcircuits.com