ISS: Intelligent Soft Start

The ISS is designed as a companion board to the Power-686 and Power-86 power supply boards. It is equally well suited for use with other power supplies, including switching power supplies, where a soft start circuit is needed.

Outline

Introduction

The inrush current drawn by power transformers, especially the toroidal power transformers commonly used in audio equipment, presents a serious challenge for many DIYers. When the power is first turned on, the transformer will draw significant current for the first few mains cycles as the magnetic field builds within the transformer core. The inrush current can reach several hundred ampere, which severely stresses the transformer. Furthermore, to accommodate the high inrush current, the mains fuse used to protect the transformer and connected circuitry must be grossly oversized, which limits the effectiveness of the fuse in the event of a catastrophic fault in the circuit. The Intelligent Soft Start (ISS) solves this issue by limiting the inrush current to a safe value using a NTC resistor. Once the magnetic field has been allowed to build within the transformer, the soft start circuit shorts out the NTC resistor with a relay.

For many DIYers, connecting the primary of the power transformer correctly presents a source of significant anxiety. The ISS minimizes this anxiety by offering a 4-pin terminal block where the primary is connected. The Design Documentation contains a detailed description of how to identify the primary wiring of the popular Antek AN- and AS-series power transformers and connect the transformer correctly to the ISS.

All the necessary connections for mains power switching and mains voltage selection are completed by three wire jumpers on the circuit board. All connections to power switches and other controls external to the board are done at low voltage levels (5-12 V).

Features

The main features of the ISS are listed below.

  • On-board handling of all mains-related circuitry (fuse, power switch, mains voltage selection, etc.)
  • Relay Saver Technology I: An electronic switch engages the soft start before the main power relay on power-up and disengages after the power relay on power-down. This minimizes the switching current and voltage across the relay contacts, thereby minimizing arcing and the associated contact wear and RF emissions.
  • Relay Saver Technology II: After the power relay is engaged, a simple circuit reduces the relay coil voltage by 60%. This is plenty to keep the relay engaged and reduces the power dissipated in the relay coil by 85%, thereby extending the life of the relay.
  • All switch and relay timing handled by logic ICs. No programming required!
  • Support for two types of power switches: Toggle and momentary switches. These can be low-voltage types as 5 V signalling is used.
  • Opto-isolated 12 V trigger input for power control. This input has been optimized for good noise immunity.
  • Standby/Power indication using a bi-colour LED (or two individual LEDs in anti-parallel).
  • +5 V always-on auxiliary power supply.
  • 6-pin connector for interfacing to protection circuits.
  • 2.90 × 5.00 inch (74 × 127 mm) board footprint. The assembled module measures approximately 35 mm in height.
  • Gold plated PCB. Designed and manufactured in Canada.

While the ISS has been designed for use with the Power-686 and Power-86, it is equally suited for use with other power supplies and power transformers.

The image below shows the fully assembled ISS circuit board. The parts cost of the ISS is just shy of $44. I have set up a project with Mouser Electronics for easy ordering of parts.

References

The HP 14570A Power Controller was a significant source of inspiration for me in the design of the Intelligent Soft Start. I encourage readers who are interested in the theory and design considerations underlying the ISS design to study the HP 14570A service manual and HP Journal linked to below.

Artek Manuals: HP 14570A Op/Service Manual. ($7.50 download).

HP Journal (12/1982).