Combined module Soft Start (inrush current limiter) & DC Blocker (trap, filter) – assembled in case

Fully populated and tested Soft start & DC Blocker/Filter/Trap combined module.

From:  150.00

  • USD: 179.21$ - 262.83$


Sometimes a module for soft starting the big toroidal transformer (limiting the inrush current) along with blocking the DC component in the Mains is needed. And here is offered a combined Soft Start & DC Blocker module.

Here I’m offering you a fully assembled Combined module Soft Start & DC Blocker in metal case.

The PCB with Combined Soft Start module and DC Blocker is described here:

Specifications of the Combined module:
– AC mains working voltage: 230V or 120V (or some other one by demand)
– time delay for relay engagement: 3.5 sec.
– possibility to work with wide variety of toroidal transformers: 300 – 3000 VA
– current consumption with relay turned on ~ 100 mA

PCB for the schematic is with dimensions 110×110 mm.
PCB thickness: FR4 2.0 mm. Copper thickness: 70um (2oz). Surface finish – Nickel Gold (ENIG).
It is one-layer design with solder mask coating on both layers for best durability. Silk screen with parts layout is placed on the top of the PCB.

Schematic is designed to work with either 230V (European) or 120V (United States) AC mains. This is done by changing the capacitive divider’s value. If there’s need for work with different AC Mains voltage this can be calculated on request and tested with Variac.

Soft start part of the module utilizes a 20A power Omron G4A-1A-E relay. DC Blocker uses large 33000uf electrolytic capacitors. The overall power that module can handle is approx. 3000VA.

Note: Since the capacitive divider value and the value of resistor in series with transformer’s primary winding depend on the AC mains voltage applied to schematic, let me know your variant – 230V or 120V AC. In case the AC mains voltage in your country is different than the noted above, let me know and the optimal value for capacitor will be find.
PCB is populated with all possible paralleled resistors (7 pcs.) which form the series resistor. This corresponds to the max. handled transformer power (approx. 3000 VA). In case your transformer is smaller than the mentioned above, contact me prior to purchase and the optimal resistor’s value will be picked up. Otherwise you will get the full resistors set and could tune it’s value by un-soldering (cutting) unneeded ones.

The case is made by 1 mm. steel sheet black powder coated.
There are 4 small rubber feet underneath the case.
On the front side of the case are located 2 AC outlets which can be 5 different types – Standard European (German Schuko type), Euro-American type, French type, United Kingdom type and with IEC C14 outlets.

Note for United Kingdom customers (mainly): Due to specific of the UK wall plug, attaching it to the Blocker can be uncomfortable sometimes when DC Blocker’s AC Outlet is in the standard position. Hopefully it can be easily rotated to 90 deg. clockwise or anti-clockwise (see attached pictures).

Since sometimes a direct cable connections to the output of the board is needed, a special solution is offered: Combined SS+DCB module without Output sockets but with cable glands for direct cable attachment to the board – see pictures in the gallery. Cables with outer diameter of 7 – 12 mm. can be attached to the Board via cable glands.

Since the Soft Start module limits the current through toroidal transformers in the first 3-5 seconds, there must be a power switch to turn ON the power of the board (and the whole system after it) .
A SCHURTER KD14.1101.151 combined AC Inlet+fuse holder+power switch (  is used for this task.

There is a possibility for inserting a common mode filter immediately after the KD14 power switch and the module’s board. A Schaffner FN2090-16-06 ( common mode filter is offered as an option for mounting in the box.

All parts used are freshly obtained from reliable sources (Mouser, Farnell, Digi-Key etc.).

Note: Since some of AC outlet types are not widely used in Europe, please ask for availability before committing the order.

What the buyer will receive:
The package will contain 1 pcs. FULLY ASSEMBLED and tested Combined Soft Start module plus DC Blocker in steel case.


Weight 1.4 kg
AC Outlet

EU Schuko, UK Plug, US Plug, French Plug, Cable Glands, C14 Outlet

Common Mode Filter

Present, Not Present