Transformer Calculator

Use this calculator to decide the best transformer to use for your Superpower supply. Given the values you enter, it computes the ripple, decides the drop out voltage based on selected Superpower type and load current, sums everything and calculates the minimum Vrms of the transformer.

Calculator fields have only minimum validation so if the Vrms result has something bizarre (like "NaN"), recheck your input values. If Vrms is negative, Load current exceeds the capability of the selected Superpower type.

Superpower type
(select to set drop-out)
Drop out voltage
Regulator output voltage (volts)
Max Load Current (amps)
Line frequency
Rectifier Diode Drop (volts)
Rectifier Filter Capacitance (µF)
Line voltage variation (in %)
Safety margin (in %)  

Transformer 2ary Vrms
Transformer min. VA
  Regulator input Vpeak
Regulator Dissipation (Watts)
  Max ripple (Vpk-pk)
Max heat sink °C/W

Heat Sink

The value in the Max heat sink °C/W box shows the maximum thermal resistance for a heat sink on a Superpower with the Vin shown in Regulator input Vpeak with the other values as given. To see the heat sink needed for a different Vin, change the value in Line Voltage Variation until the Regulator input Vpeak equals the Vin you will use in your application. The heat sink calculation assumes a 75°C temperature increase of the regulator.



  • Linear power supply with transformer/rectifier/filter caps/Superpower
  • Transformer has sufficient power that it does not sag under load (use safety margin to account for sag)
  • Vdc of rectifier output is minimum value + safety margin
  • Full wave center tapped rectifier follows the transformer.
    For a bridge with no center tap, double the diode drop
  • Transformer output voltage is specified as Vrms
  • Filter capacitance is entered in µF
  • Select the power line frequency for your locale
  • Regulator dissipation assumes nominal line voltage but allows for a drop of line variation % without losing regulation
  • Rectifier diode drop allows entry of Si, SiC or other diode drop
  • Heat sink allows 75°C temperature rise due to power dissipation


Special thanks to a customer whose suggestions helped us improve this know who you are!


How can Vrms be less than Vout?

Transformers are specified as Vrms. Full wave rectified and filtered transformer voltage is, with no load, approximately Vpeak, which is Vrms X sqrt(2). So Vrms is lower than the Vpeak required at the regulator's input, and with low output current requirements, may be lower than regulator Vout.

Why is Vrms negative and almost 1000?

The calculator does this when Load Current exceeds the capability of the selected Superpower type.

What kind of capacitors should I use for a rectifier filter?

Use the electrolytic capacitor of your choice. The most important issue for regulation is to have sufficient capacitance to prevent ripple that goes below Vout+Vdropout.

Should I bypass the filter capacitors with ceramic?

Yes, a 0.1µF ceramic cap at the Superpower Vin terminal helps reduce high frequency noise and RF. This amount or more capacitance should be placed at the Vin terminal to ground to prevent possible low level oscillation at some load current values. This does not affect the calculation very much.

How much filter capacitance should I use, can I use too much?

More filter capacitance is better, it reduces ripple. When the room lights start to dim as you switch on the power supply, you may be reaching the point of "too much." Or maybe you should run a separate mains wire for your audio system :-).

Can this calculator be used for any voltage regulator?

It can be used for any series voltage regulator if you know the drop-out voltage. For Superpower Type choose Custom regulator and enter the drop-out value for your regulator at the given load current.