When a transformer is abruptly connected to the mains supply it takes a large current that decays exponentially; this is called the inrush current (or sometimes the turn-on surge, or even the 'inductive surge') and it is highly inconvenient as it can be much greater than the normal current drawn, even at maximum output into the lowest rate load impedance. This inrush current is not a danger to the transformer, which has a big thermal mass, but it can and will blow primary fuses and trip house circuit-breakers. With small and medium-sized transformers the problem is not serious, but it does mean that you have to be very careful in sizing the fuse or fuses in the primary circuit, making sure that they have a high enough rating so their life is not impaired by repeated inrush currents.
With a large transformer (say bigger than 500 VA for a toroid) the inrush becomes large enough to trigger domestic overload protection. Since most houses now have magnetic circuit-breakers rather than wire fuses in the mains distribution panel, this is not as inconvenient as it used to be, but is still thoroughly annoying and will quickly earn you the enmity of your customers.
The inrush issue has to be taken very seriously as it can cause problems that only show up when the unit is out in the field. There is anecdotal evidence that circuit-breakers in Germany, while nominally rated the same as those in Britain, actually respond somewhat faster, so a design that has received careful checking in one country may cause serious trouble in another.
Inrush current is most conveniently measured with purpose-built instruments such as the Voltech power analyzer range. A cheaper method is to use a current transformer (typically of the 'giant clothes-peg' type) clamped around one of the primary connections, and connected to a digital oscilloscope; this is naturally only cheaper if you already have a digital oscilloscope.
It is characteristic of inrush current that its peak value varies widely from one switch-on to the next, as it depends crucially on the point of the mains cycle at which the transformer is connected. If you're unlucky the transformer core briefly saturates and a big peak current is drawn by the primary. For this reason repeated tests – possibly up to 50 – have to be done before you are confident you have experienced the worst case. This often has to be spread out over some time to avoid overtaxing inrush suppression components.
Toroidal transformers typically take greater inrush currents than frame types, due to their lower leakage reactance. There is a component of the inrush current that is due to the charging of the power-supply reservoir capacitors, but this is usually small compared with the transformer inrush. As a rough guideline, if your transformer is bigger than 500 VA you should consider using inrush suppression. If in doubt, then at least make provision for adding it to the design in the development phase.
The inrush current is controlled by making sure there is enough series resistance in the transformer primary circuit to keep the flood of amperes down to an acceptable level. The two main ways of doing this are to use an inrush suppressor component, or a relay that switches resistance into circuit for starting.
Inrush suppression by thermistor
An inrush suppressor component (sometimes called a surge limiter) is a giant thermistor whose resistance drops to a low value as it is heated by the current passing through it; they are usually of the disk type. The inrush suppressor is inserted in series with the transformer primary. The thermal inertia of its mass causes the resistance to drop relatively slowly, so the inrush current is restricted.
Because of their thermistor action, these components run very hot in the low-resistance state (about 200ºC) and must be mounted with caution to ensure they do not melt the plastic of adjacent components. The component leads must be left long enough to avoid thermal degradation of the solder joints with the PCB, and if these leads are insulated it must be with a high-temperature material such as fiberglass sleeving. They are also likely to burn the fingers of service personnel – it is only polite to put a HOT warning on the PCB silkscreen.
Inrush suppressors require a cool-down time after power is removed. This cool-down or 'recovery' time allows the resistance of the NTC thermistor to increase sufficiently to provide the required inrush current suppression the next time it is needed. The necessary time varies according to the particular device, the mounting method and the ambient temperature, but a typical cool-down time is about one minute. This is usually specified by the manufacturer as a thermal time-constant with values ranging from 30 to 150 seconds, the longer times being for the larger and more highly rated versions.
Inrush suppressors are available in many different sizes. The quickest design method is to select a few types that can handle the maximum current in the primary circuit, and try them out to see which is the most effective at controlling the peak inrush value.