Help Needed sizing a small transformer.

Discussion in 'General Electronics Chat' started by tekati, Apr 12, 2015.

  1. tekati

    Thread Starter New Member

    Mar 8, 2010
    I am building a circuit board that will have a lot of functionality with both Analog and Digital signals as well as operating (8) 5V relays.

    I am going with two 780X series regulators 5v at 1.5AMP with heat sinks one for the main board and a second to power a Raspberry PI (using for display, internet, etc) and one 3.3 LD1117 800mA regulator for some needed 3.3v stuff on the main board. I would like to find a transformer small enough to fit on the main board and output around 9-15V but I am unclear on how to size them. The regulators will be able to handle 15V which might be better as far as transformer size is concerned.

    Do I simply calculate VA for full load on the regulators figuring in efficiency and do a formula on that to size a transformer or is there another way I should be looking at this?

    Thank you in advance for any help and guidance
  2. crutschow


    Mar 14, 2008
    No, you don't want a 15v transformer since the current is fixed by the load and the transformer VA rating will thus increase directly with its output voltage (and dissipate more power in the series regulators).

    For best efficiency you want the transformer voltage to be as low as possible while still generating sufficient voltage to power the regulators with adequate voltage overhead.

    A 7805 requires about 2.5V above 5V for proper operation. If you use Schottky diodes for the bridge rectifier, add about 1V and and another 1V for ripple, giving a desired peak transformer of about 9.5V (6.7Vac RMS).

    The LD1117 is a low drop-out type regulator which requires 1.2V maximum above the 3.3V output. Again adding 1V for the bridge and 1V for ripple gives a total required voltage of about 6.5V peak (4.6Vac RMS).

    Thus you need a transformer with at least 6.7Vac output. The current should be derated by about 60% for a rectifier-capacitor filter on the transformer output, so the required VA would be 6.7V * (1.5+0.8)A / 0.6 = 25VA.

    If the transformer you select has a higher voltage, then the VA required will go up proportionally.
    Last edited: Apr 12, 2015
  3. tekati

    Thread Starter New Member

    Mar 8, 2010
    Perfect information. Makes for a fairly large transformer but it is what it is. I was actually going to use two 7805's but I will use a single LM1085 5AMP regulator provided the ripple is not too bad as it plays serious havoc on analog sensors :(

    So I will recalculate using a derating of 60% which is the exact info I needed.