Updated: Dec 6, 2019
Basic knowledge of electronics / physics is assumed (if not, let us support you).
Further you might be interested in reading a comparison between toroidal and EI transformers here.
The transformer is an important component that will step down the 230V mains (115-120VAC line for those in the US) down to a usable voltage. It’s important to remember, that transformers are specified at their RMS voltages. RMS means Root Mean Square which means the peak voltage is actually √2, or 1.414 times higher than the RMS value. So if you use a 12V transformer, your peak output voltage would really be 16,9V.
According to the LT3045's maximum specs, that's also, why 14VAC is the highest transformer voltage, which is possible to use with these regulators.
The transformer must be able to deliver the POWER that your circuit requires overall, including any kind of losses, which can occur in rectifying, leakage or regulating the voltage (transforming power into heat). Since the current in the transformer flows in pulses (no charge during 90% of the time, while only 10% of the time the capacitors are charged with 10 times of the nominal current) and since the power loss depends on the resistance of the copper wire on the power to square, ie (10% of 10 * 10) = 10 times higher than DC (which is not present at the transformer, but RMS), the required transformer could be even bigger.
Choosing the right transformer depends on
1. The voltage that your device / application needs
2. The current that your device / application needs
As a "rule of thumb" we recommend to use a >50VA transformer powering the medium sized HPULN-PS's, while the large sized HPULN-PS's should be powered with >80VA transformers for loads of >50% of the HPULN-PS's capability.
As also described in the FAQ, it's necessary to avoid unnecessary heat - a transformer's voltage should be well chosen matching your application's needs. Knowing your device's power requirements, a transformer should have enough power, also to have "headroom" left in situations, where your device reaches 100% of it's current requirement.
That means: Calculate your full power need and add some power-headroom, a factor of 1.6x - 1.8x is often recommended here.
The chart shall support you, comparing a 11VAC and a 12VAC transformer and the absolute maximum (measured) voltage/current ratio using an LSIB-HPULN power supply:
Additional info regarding measured results above: All measurement points were chosen as an absolute maximum, where no ripple is measureable (FFT) anymore. Your application shouldn't reach that maximum values. Measuring began with 6ohms, resulting in the values above. From technical aspects measuring ended with 2,5 /2ohms, partly NOT showing the HPULN's maximum voltage/current ratio.