The transformer above (Avel Y236907 800VA 45V+45V Toroidal Transformer), for example, will try to draw over 100 Amps on the first cycle of 60 Hz Power.
To keep a large transformer from being damaged at turn-on (and to keep it from saying "ow"), or to keep a breaker from popping, you put in an inrush current limiter circuit. This Instructable will detail how to do that.
The circuit I use contains a thermistor, a relay, and some resistors, a capacitor and a couple transistors.
This particular one here is the Amtherm MS35 20010 and can be bought from Digikey (570-1026-ND). Its nominal resistance is 20 ohms, and as temperature rises, it drops to very low resistance.
Now you might say "Great, just put this baby in series with my AC input line and I am done". In some cases you might be able to do this but the power amplifier I am designing is a 200W per channel set up and, though the pyrotechnics might be fun to some, being nominally averse to smoke and crispy electronic components, as well as slightly paranoid, I decided to add a relay to the circuit.
In addition, when someone turns off a device that has been on for a while, the thermistor will be very hot. If they then turn on the device immediately after shutting it off, the thermistor will dutifully say "I'm hot and tired, and I am not providing you any resistance". Your transformer will say "Ow" once again.
When power turns on, all current flows through the thermistor. But if we put a timing circuit in that closes the relay a few seconds after turn-on, the thermistor would do its job limiting current initially. But the relay would then take it out of the circuit by by providing an essentially zero resistance path through itself, and around the thermistor. The thermistor will then say "Thank You", cool down or just stay cool,and thus be ready for the next power turn on.
So how do we create that timed delay?
Two transistors were used so that a very small amount of current at the base of Q1 would be guaranteed to drive transistor Q2 into saturation, switching it on. A side benefit of having 2 transistors is that the normal .65VDC from base to emitter of the transistors becomes 1.3VDC now that there are two transistor base to emitter junctions in the circuit.
For my purposes, I have 12 volts DC available in my amplifier, sourced through a simple regulator circuit supplied from an independent low power transformer.
Resistor R1 and Capacitor C1 provide the timing delay. When power is turned on, and the 12 VDC comes up, current flows through R1 to charge capacitor C1. Over time, the voltage on the capacitor reaches the 1.3V or so needed for transistor Q1 and Q2 to turn on.
When power is turned off, capacitor C1 drains to zero through R2 so the circuit is ready for the next turn on. Care must be taken calculating the value of R2 - the voltage divider of R1 and R2 must always allow for greater than 1.3VDC or else the transistors will not turn on.
The DC voltage driving the timing circuit can be any voltage, including a DC source generated from the output of the power transformer that we are controlling the inrush current to. For example, if the output of the transformer was 36VAC, you could construct a simple zener diode DC source to drive the relay and a timing circuit with different component values. You might want to simplify your life by going to a 24VDC coil relay.
The delay this circuit injects is 2-5 seconds, depending heavily on the value of C1. Electrolytic capacitors have very wide tolerances (such as +10% and minus 40%) so they are not useful for precise timing circuits but certainly useful here. You should always just experiment with components to insure you get the right delay.
Hopefully this helps your large transformer have a happy day!