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:

https://www.buerklin.com/datenblaetter/C090525_TD_de_eng(1).pdf?ch=18598

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

http://everycircuit.com/app

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 f_3dB= 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

130

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

132

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