In order to generate a DC voltage with, for example, 24V/DC from 230V AC voltage, two different methods can be used: Generating a low AC voltage with a transformer and subsequent rectification with a rectifier (unregulated power supply) or generating a DC voltage with the aid of complex electronics (regulated power supply). Both variants have advantages and disadvantages and above all effects on the connected consumers. 

Right away: It becomes very theoretical. If the details and backgrounds don't interest you that much, just scroll to the very end and read our conclusion. But now let's get started:

How an unregulated power supply works

A transformer converts 230V/AC into 24V/DC, which are then rectified with a bridge rectifier and smoothed with a capacitor. In the unloaded case, however, the resulting voltage is not 24V/DC, as might be expected, but 24 * 1.41 = 33.8V (open circuit voltage). This voltage only drops significantly under load. That's why the voltage specifications on these power supplies are often given in ranges, for example 21 - 28V/DC. In addition, this type of power supply is still dependent on voltage fluctuations in the power grid (230V +-10%). Thus, 207V to 253V may well be present at the input of the power supply. Example:

Ventilation power supply, output voltage 21 - 28V/DC at max. 1A. We make measurements with a primary voltage of 207V, 230V, 253V. The output is loaded with 0/0,5/1A. At the output we measure the rms value of the output voltage and the ripple. This (example) power supply has internally a transformer, which generates secondary 18V, so the theoretical DC voltage is 25.4V (unloaded, at 230V input voltage).

What is ripple?

Ripple is the peak-to-peak voltage of the AC voltage superimposed on the DC voltage. On power supplies you often read Vpp (Volt Peak-Peak), also on drives you can find such information (e.g. max. 2,4Vpp). For unregulated power supplies the ripple has a frequency of 100Hz (mains frequency * 2).

Measurement results unregulated power supply

Input	Load [A]	Output	Ripple [Vpp]
207V	0	26,2V	157mV
230V	0	29,3V	174mV
253V	0	32,4V	203mV
207V	0,5	21,1V	3,25V
230V	0,5	23,8V	3,3V
253V	0,5	27,1V	3,4V
207V	1	18V	5,6V
230V	1	20,9V	5,8V
253V	1	23,7V	5,9V

 

Oscillograms

The following pictures show well that with increasing load of the output the ripple increases and the rms voltage decreases. The screenshots show the measured values at an input voltage of 230V.

Output voltage unloaded:

Output voltage, 0.5A load:

Output voltage, 1A load:

Ripple at 1A load:

Functionality of a regulated power supply

In this design, the input voltage (230V) is rectified and "chopped" back into an AC voltage by means of a semiconductor, which in turn is transformed down to the desired output voltage via a transformer. After rectification and screening, this is then available. The output voltage is measured continuously and any necessary corrections to the generated voltage are made before the transformer. The AC voltage generated by means of semiconductors has a much higher frequency than that of the mains voltage, which is why the transformers can be smaller in size.

Switching power supplies, in addition to the size and weight advantage over conventional power supplies, also have much better efficiency. The output voltage is practically independent of the current and has a very low ripple component. Our test candidate is the LNT 2406 ventilation power supply, which can deliver a current of 6.5A at 24V. The output voltage is galvanically isolated from the input, just like the conventional power supply. By the way, regulated power supplies are short-circuit proof on the output side.

We have also carried out corresponding measurements here:

Measurement results regulated power supply

Input	Load [A]	Output [A]	Residual ripple [Vpp]
207V	0	24,38V	275mV
230V	0	24,38V	275mV
253V	0	24,38V	324mV
207V	1	24,35V	450mV
230V	1	24,36V	500mV
253V	1	24,36V	525mV
207V	1	24,34V	550mV
230V	3	24,34V	600mV
253V	3	24,32V	650mV
207V	6	24,29V	750mV
230V	6	24,29V	850mV
253V	6	24,30V	800mV

Oscillograms

The following pictures show very well the constant output voltage with increasing load. Also the ripple of the output voltage increases only minimally. You can also see the high frequency at the ripple, which is in the kHz range in contrast to unregulated power supplies. Here we only show the recordings at high load with 6A.

Output voltage, 6A load:

Ripple at 6A load:

Conclusion

You should only buy unregulated power supplies if a) you need exactly this type as a replacement for an existing system or b) if you are sure that the drive to be connected can handle this voltage quality. However, this requires some expertise.

Basically, you can say that "old" drives - i.e. drives that have only very simple electronics - are very likely to work with this. Drives of newer generations with complex electronics (microcontrollers...) on the other hand need a stable and smooth supply voltage.

Please also pay attention to the information on the type plate: There are drives which are specified with 24V/DC +-10% and drives which are also approved for 24V/DC +20% -10% (as a rule, these are then suitable for SHEV).

In case of doubt, please contact us, we will certainly find a solution and the right power supply for your application (ventilation power supply, ventilation control center).