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 transformer 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 on the circuit board. The mains voltage selection is accomplished with wire jumpers inserted into a 4-pin connector. These jumpers and the connector are provided with the ISS module.
All connections to power switches and other controls external to the board are done at low voltage levels (3.3-12 V).
If you are curious about the design considerations of the ISS, I suggest reading my Ultimate Guide to Soft Start Design.
The main features of the ISS are listed below.
- On-board handling of all mains-related circuitry (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 an ARM Cortex M0+ micro controller.
- Supports three soft start delay settings, selectable by jumper: 250 ms, 500 ms, and 1.0 s.
- Support for two types of power switches: Toggle and momentary switches. These can be low-voltage types as 3.3 V signalling is used.
- Child-protection feature, enabled by jumper, requires the momentary power button to be held for 1.5 seconds to turn on or off the power.
- 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). The brightness of each LED can be set independently using two trimpots.
- +5 V always-on auxiliary power supply (100 mA, max.)
- Low-asserted FATAL input, which allows for emergency shutdown of the power. This input is compatible with the nFATAL output provided by the Purifi 1ET400A and Hypex NC500.
- 2.65 × 3.50 inch (68 × 89 mm) board footprint. The assembled module measures approximately 35 mm in height.
- The ISS module is built on a gold plated PCB, assembled using lead-free solder, and is RoHS compliant.
- 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 up to 2 kVA.
