Soft Start

The role of the Soft Start is to prevent current peaks through the primary coil of a toroid transformer during start up.

Due to initial magnetization of transformer core, it can happen that protection fuse burns out. Therefore, it is recommended to limit the initial current through the primary coil. Current limit is done by connecting power resistors in series with the primary coil of the transformer.

After a short time interval a "bypass" relay is activated, it shortcuts the power resistors and lets the current through primary coil to flow regularly.

Thermal fuse has the role to break the circuit in case of overheating.

Photo triac gets activated in zero which diminishes noise generated in mains.

The advantage of this Soft Start circuit is the solution to the common problem when a bypass relay fails in case of which there are two scenarios: 1. Thermal fuse breaks the circuit, 2. Total loss of AC power supply due to the relay failure. In both cases the circuit performs well and safe.

The PCB for Soft Start is carfully routed with special attention given to high power tracks.

Warning: The Soft Start circuit deals with high voltages. Myth Electronics takes no responsibility in case of improper handling, installation or usage of this device, nor it is responsible for any direct or indirect damage caused by this device.

The Soft Start circuit is a non-profit project started at HiFi Forum and it was moved by the authors to this site for easier maintenance.

Gallery:

Download:

Soft Start schematics (40kB)