High Power Soft-start (inrush current limiter) for toroidal transformers – assembled, tested

 84.00 110.40 Incl. VAT
 70.00 92.00 Excl. VAT

Fully assembled and tested Soft-start (inrush current limiter) for toroidal transformers


It is well known that when power amplifier is switched on, the initial current drawn from the Mains is many times higher than it’s nominal value and could reach hundreds of Amperes. Anyone who has a large power amp – especially one that uses a toroidal transformer – will have noticed a momentary dimming of the lights when the amp is powered up. The current drawn is so high that other equipment is affected. The main reasons for that are:

– Transformers will draw a very heavy current at switch on, until the magnetic flux has stabilized.
– At power on, the filter capacitors are completely discharged, and act as a short circuit for a brief (but possibly destructive) period

In-deep explanation of these phenomena can be read at Rod Elliott’s site

In order to mitigate the problem, a relatively cheap circuit for “soft-starting” the mains transformer was designed and successfully offered in the store during the last couple of years. However, this board is limited in performance up to 3000VA (at 230V AC). In order to solve this limitation (and because of the numerous customers’ requests), the present circuit board was designed and brought to your attention.

The Block-schematic of the board can be seen below:

Block-schematic of High Power Soft Start board

As you can see from the picture, the DC voltage for powering the logic circuitry and the relay is ensured by a versatile Zettler ZP03SXX00WE series AC/DC converter, which can work in wide range of AC Mains voltages (85 – 265V AC). This way the problem with the various AC Mains voltages available all over the world is elegantly solved.

Time-delay circuit is based on the well-known timer NE555. Delay time is approx. 3.5 sec. which is enough for settling the mains transformers.

Control logic of the Soft Start HP

The output MOSFET transistor serves as a gate for turning ON/OFF of the output power relay. The carefully chosen  relay is American Zettler’s solar AZSR-235 DPST series, which overall current capability is up to 70A (2 x 35 Amperes rated contacts)!!!

As a ballast resistor is chosen the TELPOD RTS-01-100 100W thick-film series resistor, which is characterized by a high power at small size, high resistance to a temporary overloads and very low inductance. The ballast resistor is mounted on a suitable Fischer Electronic SK489 series heatsink.

TELPOD’s RTS-01-100 high power ballast resistor mounted on a Fischer SK489 heatsink

Of course, for such a power unit the AC Power IN and OUT terminals are as important as the power relay and the ballast resistor. That’s why, the Wurth High-power PCB-to-wire connectors from WP-THRBU REDCUBE series was implemented onto the board.

Wurth’s WP-THRBU REDCUBE High-power PCB-to-wire connectors

It is well known that entire Soft-start logic starts its work after the Power ON-OFF switch (located in front of the AC IN terminal of the board) is turned ON. From this moment the delay time is counted and upon its end, the Main relay shunts the ballast resistor.
While for the less powerful systems there are a wide variety of Power ON-OFF switches, when things come to working currents of 20A and above, the number of descent, relatively cheap and (relatively) small ON-OFF switches becomes very limited. In order to address this problem, a new High-Power Soft start board was designed. It has an incorporated power relay (from the same high quality American Zettler’s solar AZSR-235 DPST series), which is controlled by a small switch and acts as a High-current Power ON-OFF switch:

Block-schematic of High Power Soft Start board with Power Switch ON-OFF relay

It can be seen from the Block-schematic, that the SW1 switch powers ON the Zettler AC/DC converter and thus powers ON the switch relay K1 and the soft-start logic. The power relay K1 immediately shorts its contacts and this way connects the ballast resistor R and the Main bypass relay K2 to the AC Mains.
The fully assembled board can be seen below:

The High-power Soft start module with Power ON-OFF switch relay

To the ends of the black wires is connected the operational (small) ON-OFF switch which controls the Power switch Relay K1.

– Control logic AC mains working voltage: 80V to 300V
– time delay for relay engagement: approx. 3.5 sec. (can be changed upon request)
– possibility to work with very high-power toroidal transformers: 3000 – 7000 VA
– max. current handling of the board – 50A
– current consumption with relay(s) turned on ~ 30-50 mA

PCB thickness: FR4 1.55 mm.
Copper thickness: 70um (2oz).
Surface finish – HASL (Hot Air Solder Leveling).
The PCBs are two-layer design with solder mask coating on both layers for best durability. The high-power traces are carefully designed and doubled on the both PCB copper layers so that the desired working currents to be firmly guaranteed.

What the buyer will receive:
The package will contain 1 pcs. Fully Assembled and tested PCB for HIGH-POWER Soft Start (inrush current limiter).

The unit will be shipped with Priority AirMail Registered (or equivalent courier service where applicable). The tracking number will be sent to the customer after shipment.

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Fully assembled and tested High-Power Soft-start (inrush current limiter) for toroidal transformers
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Additional information

Weight 0.25 kg
Power Switch Relay

Without Power Switch Relay, With Power Switch Relay