Soft Start + DC Blocker for mains

Soft Start + DC Blocker for mains

Large (300 VA and above) Toroidal transformers suffer from high in-rush currents if they happen to have mains applied when the AC voltage cycle passes through zero volts. On a 1 kVA transformer, this can exceed 100A. Added to this will be an additional high current draw as the reservoir capacitor bank charges up – this can be many 10’s of amps for two to three seconds with typical 22000uF to 50000uF per rectified rail power supplies. To prevent this problem and to cater for power being applied at any position on the AC mains cycle, some form of current limiting is required. In the SSDCB, a 10 or 20 Ohm Ametherm thermistor is used to limit the inrush current to ~10A for 3~4 seconds, after which it is bypassed by a power relay.

An additional issue with large transformers, and especially toroidal types, is that caused by imbalance in the AC mains voltage sinewave halves. This is caused by uneven loading of the mains half cycles and the net result is effectively a DC offset. On a large transformer, the primary DC resistance can be as low as 1.5 Ohms (measured on a 1.2 kVA transformer). With just a small DC offset of 1V, this causes the transformer BH curve to be shifted off centre, resulting in very high peak flux densities in the direction of the DC imbalance which manifests as audible ‘growling’ of the core. In bad cases, this can result in core saturation potentially causing the equipment fuse to open.

The solution to this problem is to block the DC offset using a full wave bridge rectifier which is bypassed by two high value (in the SSDCB 10 000uF) capacitors. Since the capacitor voltages are clamped by the bridge rectifier, the voltage rating need only be a little greater than the peak voltage across the rectifier.

The maximum continuous mains current draw on the SSDCB is 2.3A RMS and this limit is imposed upon by the DC blocking capacitors. This figure is for the fully derated 105C 5000 hour 100/120 Hz supply frequency.

Here is a short YouTube video demonstrating DC on the mains and how the SSDCB blocks it

Download the PDF presentation here (c. 1 MB)

Here is the BOM (.xls file)

You can buy a DSTHP PCB for the SSDCB here. The boards are shipped with tracking and will take 5-7 working days for delivery