Which transformer voltage should I choose?

Updated: Jul 14

Choosing the right transformer-voltage is important, as we try to avoid heat on one hand, need enough power in each situation on the other hand. That's why many customers choose a "custom made transformer", which we deliver with the needed secondary voltage matching your application for a very reasonable price. Especially for higher current applications it's necessary to fine-tune the transformer-voltage, so that regulators need to "waste" as less voltage as possible due to small voltage differences between the input and output voltage.

To make your decision easier, which transformer voltage can be chosen, you will find some measurements of the ALS-HPULN power supply below:

The chart describes the often needed output voltages of 3,3VDC, 5VDC etc.

Using different loads (Ampere), different transformer voltage should be used. The reason is:

Let's assume a device with a real power need of 3,3VDC / 2A


Transformer 1 with secondaries of 3,6VAC:

3,6VAC x 1,414 = 5,09V (after rectification)

5,09V - 0,8V = 4,29V (diode voltage drop)

4,29V - 3,3V = 0,99V (voltage difference to "waste")

0,99V x 2A = 1,98W (of heat)


Transformer 2 with secondaries of 4,7VAC:

4,7VAC x 1,414 = 6,65V (after rectification)

6,65V - 0,8V = 5,85V (diode voltage drop)

5,85V - 3,3V = 2,55 (voltage difference to "waste")

2,55V x 2A = 5,1W (of heat)


With this example we see additional ~3,1W of heat using a transformer of 1,1VAC difference. That heat is avoidable using a transformer matching the application needs.

We made this measurements in our lab, using a variable transformer fed with exactly 230VAC mains power, and using an electronic load (long term load here) and the ALS-HPULN power supply. The transformer voltage was adjusted to a voltage level, where no ripple was measureable anymore and regulation worked seemless.


It should also be mentioned, that the regulator itself needs a Vdifference of 0,26V to be able to regulate, we recommend 0,5V here. That should be taken into account.

The chart should be used to support you, and as there are always tolerances we can not guarantee or take responsibility, that the measurements and results will be the same at customer's setup due to tolerances (such as mains voltage, wires, transformer etc.). For example: If your mains voltage differs from 230V (or 115V), the secondary voltage will differ by the same ratio.


Using the SLS-HPULN module with its two regulation stages makes it necessary to calculate additional transformer headroom, as both stages consume power. For "normal" use we recommend to set the first stage 0,8VDC higher, then the second stage, what means in case of 5DVC output to set the first stage to 5,8VDC. We used the headroom of +0,8VDC in each of the folllowing measurements:

Necessary secondary voltage is lower using the LSIB-HPULN power supply, due to its negligible voltage drop resulting in alternative and quasi lossless rectification:

Although the LT4320 based rectification works with lower voltages, we respect the minimum operating voltage of 9V. That`s why we publish a minimum transformer voltage of 6,4VAC, while the rectification worked with <6VAC as well with low current. Due to unnecessary generated heat, we recommend to choose the transformer voltage well, and with a low difference between input and output.

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