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P O L Y T E C H . N U

400Hz power supply lab setup

It's not that hard to obtain 28 VDC or 250 VAC 50 Hz. Almost any regular DC power supply or lab supply can create the desired direct current voltage. Also some 50 or 60 Hz signal is rather easy to obtain using a (variable) transformer and the power from a regular wall outlet.

But for a lot of avionics 115 VAC 400 Hz is needed. And sometimes even a three phase signal with a neutral. Creating a 400 Hz power signal takes some effort. Most of avionics need only one phase 115 VAC 400 Hz, so in thie article is not focussed on the three phase power supplies.

I tested several methods to create a 400 Hz power supply and I want to share here my findings. The findings published here are convenient for a lab/test bench environment. A dedicated 400 Hz power supply is most convenient, but takes more effort. Therefore a lab setup is arbitrary.

basic setup
The default setup create a 400 Hz 115 VAC power signal is by creating a 400 Hz sinewave, amplify this signal using an audio amplifier and increase the voltage by applying a power transformer a shown below.


Practically it will look something like this:


setup explained
1. The first step is to get a 400 Hz sine wave signal of approximately 2 Vrms. The easiest way is to create a 400 Hz (audio) signal using a lab function generator or similar instrument. The frequency is not that critical, but the shape of the wave should be a sine wave. A square wave has too much harmonics and therefore is not preferred. If no function generator is available, a simple DIY tone generator board will also work. A tone generator pcb is less accurate in terms of frequency and output signal, but works great for this purpose. It's cheap and small. The result should be an 2 Vrms 400 Hz sine wave. The output power of this tone generator has not enough power to drive the instrument though.

2. The next step is to amplify the 400 Hz signal. Since 400 Hz is in the audible range of 20...20.000 Hz, a regular audio amplifier can be used for signal amplification. The output impedance is usually 4 or 8 Ohms. The amount of energy to drive an instrument, depends on the instrument itself. Let's assume that a 'normal' audio amplifier can deliver 25 Watts of power. If for example 25 Watts of power is needed, the output current is 2,5 A at 10 VAC when connected to 4 Ohms. The power is there, but the output voltage is too low. 115 VAC is needed where 10 VAC is available. So the voltage has to be increased. The easies way is to use a power transformer.

4. Although power transformers are designed fot 50 or 60 Hz, for a test setup 400 Hz will also work quite fine. A regular 250:12 VAC transformer has a voltage ratio of roughly (250/12=) 21 times. So if the audio amplifier signal is fed to the 'secondary' 12 VAC side, the 'primary' side outputs approximately 21 times the input voltage. If the audio amplifier produces 10 VAC and is amplified 21 times, the output voltage is approximately 210 VAC. In real life the voltage is lower due to losses, but you get the idea probably. By reducing the audio volume knob, the voltage can be set to the desired level. Likely the desires output voltage is 115 VAC. Remind that the current is lowered by 21 times, so the output current is a little but above 100 mA. By using a more powerful amplifier the output current van be increased. Also by using a power transformer with different voltage ratio's, the setup can be optimised. I used a 240:24 VAC transformer and that works fine also.

5. Sometimes instruments need a three phase signal. If this is needed to power a three phase motor, it's possible to 'cheat'. By connecting two phases and one phase via a capacitor, the motor will also work. The capacitor introduces a phase shift so the motor will work. The capacitor value is depending on the motor specifications. The easiest way is to try different capacitor values to obtain the best results.


advanced setup
Since most of the audio amplifiers have two channels, it's convenient to use also the second amplifier. Wiring two amplifiers parallel is not advices. The better solution is to use both amplifiers in a push-pull configuration. Remind that the input signals of the audio amplifier have to be inversed compared to each other.