Lab Experiments with AoE (Art of Electronics)


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Filter Application I: Garbage Detector

Problem Statement:
A standard power line transformer produces a lot of noise. In this experiment the noise should be made visible by the use of a simple passive RC-high-pass filter.   


The Circuit:
An excellent introduction to the subject, and a lab setup is described on page 83 in the reference book:

Thomas C. Hayes, Learning the Art of Electronics, Cambridge University Press, 2016

The print transformer „SPITZNAGEL Part Number 02206“ has a primary 220V voltage at 50 Hz (Europe) and a secondary 9.8V voltage. The datasheet is here:        

A simulation of the transformer with noise figure and an appropriate simulation can be found in EveryCircuit with user “rigo59”:


Lab Setup:
The print transformer is shown in Fig. 1. The secondary pins of the transformer are directly connected to the RC-high-pass filter, shown in Fig. 2 and simulated in EveryCircuit.


Experiment 1:
When the transformer is connected to the power line a short-term back-EMV pulse with a peak of 25V is measured with the digital volt meter. The non-load voltage is 9.8V. Unfortunately I’m not able to capture the pulse with the oscilloscope.


Experiment 2:

Now I measure the output signal of the transformer with the oscilloscope in both time (T) and frequency (F) domain.

T: The shape of the transient signal is close to what the AoE reference book describes as „ (it) look(s) … more or less like a classical sinewave“. Obviously there are 3-rd, 5-th, 7-th, 2k-1… harmonics in the signal, which gives the signal a kind of rectangular shape. See Fig. 130 for details.

F: The measured impulse response in the frequency domain is shown in Fig. 131. The scaling is dB(V) above frequency. At 50 Hz the voltage is 18dBV (7.9V). 


Experiment 3:

Now the passive RC high-pass is connected at the output pins of the transformer. I use a 3dB-frequency of f3dB = 1000 Hz, with R = 16.5k and C = 10nF. The damping of the filter is 26dB at 50 Hz. The impulse response in the time domain shows Fig. 132. The remaining signal has 500 mV (RMS). In the frequency domain the noise is clearly visible by comparison of Fig. 133.





Figure 1: Transformer with passive RC high-pass on breadboard





Figure 2: Passive RC high-pass connected to an emulation of a transformer, a 50 Hz generator signal superimposed with a 10 kHz noise signal





Figure 130: Output signal of the transformer looks more or less like a classical sinewave





Figure 131: Output signal of the transformer in the frequency domain





Figure 132: Output signal of the transformer behind the high pass in the time domain





139                    140



Figures 133: Output signals of the transformer at the input (left) and output (right) of the high pass in the frequency domain




Created by dl1go on March 24, 2016 - 14:00 |  Lab Experiments with AoE