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

 57.60 Incl. VAT
 48.00 Excl. VAT

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

In stock (can be backordered)


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 circuit for “soft-starting” the mains transformer (also known as inrush current limiter) is widely used.
There are plenty of variants for completing the task but the mostly adopted one is with resistors in series of the primary winding which are shorted by relay after desired time (a few seconds).
In order to do the job many schematics rely on external DC supply voltage. Such approach however suffers from lack of flexibility since it’s not a-priory known what DC voltages would be available in the given power appliance. The best way could be to use small-sized autonomous mains transformer (10 – 15VA) but it’s heavier and expensive than the solution used here.
In the proposed schematic this problem is overcame by use of capacitive divider which is cheap, light and does not get hot. The only drawback is that the board is not galvanic separated from the mains network but since the series resistors and relay’s contacts are connected to the AC mains in all schematics the CAREFUL OPERATION with the soft-start boards is needed in all situations.

Soft-start’s block-schematic can be seen on the attached images.

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.
Since the voltage after capacitive divider is dependent of current flowing, it will wary between the initial period when the relay is not engaged and after it is turned on. That’s why a voltage regulator for feeding the time-delay circuit is used.
The values of all parts along with voltages and currents are carefully calculated so that relay will hold on engaged state even upon large variations of AC mains voltage.

– AC mains working voltage: 230V or 120V
– 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 110×60 mm.
PCB thickness: FR4 1.55 mm. Copper thickness: 35um (1oz). 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.

As was stated above 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.

On the pictures below you can see the bare PCB as well as fully populated one.

Note: Since the capacitive divider value and 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 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.

What the buyer will receive:
The package will contain 1 pcs. Fully Assembled and tested PCB for 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 Soft-start (inrush current limiter) for toroidal transformers
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Additional information

Weight 0.25 kg