Frequency Changes Affecting The Output Voltage Of 555 Seen on Multimeter

Thread Starter

phantomII

Joined Dec 23, 2019
15
Hi, I'm trying to make this circuit and have some problems. I replaced the 180k resistor with 200k POT. When I increase the frequency, the voltage value that I measure with multimeter (no true rms spec.) increases too. I have observed same stuff in Proteus but the amplitude of the wave stays same in Proteus' oscilloscope . My question is this; what I measure with multimeter and which data is reliable?
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DickCappels

Joined Aug 21, 2008
6,649
There are various kinds of voltmeters. I assume what we are talking about digital voltmeters. Few until recently measured RMS voltage and many of those that do today are not intended for wideband use. Meters without RMS converters (RMS-to-DC) use a diode or ideal diode detector to convert the incoming AC to DC. Apparently you are demonstrating that the detector in your meter is not wideband.

Since you are changing the duty cycle when you adjust the variable resistor that will probably have a large effect on measurements with most types of digital voltmeter. That, combined with the frequency response of the transformer can make for huge changes in the measurements.

In other words, what you observe is not a surprise.
 

dl324

Joined Mar 30, 2015
11,897
I have observed same stuff in Proteus but the amplitude of the wave stays same in Proteus' oscilloscope . My question is this; what I measure with multimeter and which data is reliable?
An oscilloscope will give you better information about varying waveforms than a DVM. The DC setting on a DVM is for measuring DC voltage. If the measured voltage is alternating, who knows what the DVM is measuring. Even an AC measurement would be dubious.

The voltage is varying as you change the frequency because the off time is constant and you're varying the on time and frequency with the pot.
 

Thread Starter

phantomII

Joined Dec 23, 2019
15
There are various kinds of voltmeters. I assume what we are talking about digital voltmeters. Few until recently measured RMS voltage and many of those that do today are not intended for wideband use. Meters without RMS converters (RMS-to-DC) use a diode or ideal diode detector to convert the incoming AC to DC. Apparently you are demonstrating that the detector in your meter is not wideband.

Since you are changing the duty cycle when you adjust the variable resistor that will probably have a large effect on measurements with most types of digital voltmeter. That, combined with the frequency response of the transformer can make for huge changes in the measurements.

In other words, what you observe is not a surprise.
I got it now but why Proteus' oscilloscope doesn't show the response of the transformer?
In real can i observe the voltage changes using oscilloscope?
 

Thread Starter

phantomII

Joined Dec 23, 2019
15
An oscilloscope will give you better information about varying waveforms than a DVM. The DC setting on a DVM is for measuring DC voltage. If the measured voltage is alternating, who knows what the DVM is measuring. Even an AC measurement would be dubious.

The voltage is varying as you change the frequency because the off time is constant and you're varying the on time and frequency with the pot.
You mean varying of the voltage is a normal process? so why i couldnt see the voltage changes on the oscilloscope in Proteus although it is seen in voltmeter?
 

dl324

Joined Mar 30, 2015
11,897
You mean varying of the voltage is a normal process? so why i couldnt see the voltage changes on the oscilloscope in Proteus although it is seen in voltmeter?
No. You're getting inaccurate readings because you're trying to use a DVM. If you use an oscilloscope, you'll see the waveform and the indicated voltages will be correct within the accuracy limits of the scope. Since you're using a simulation, the results are estimates.
 

DickCappels

Joined Aug 21, 2008
6,649
I got it now but why Proteus' oscilloscope doesn't show the response of the transformer?
In real can i observe the voltage changes using oscilloscope?
Maybe the transformer has a high enough magnetizing inductance so that the roll-off is far from the frequencies coming out of your NE555.

When using an oscilloscope you should get waveforms that are close to what you see in the simulations.
 

Thread Starter

phantomII

Joined Dec 23, 2019
15
No. You're getting inaccurate readings because you're trying to use a DVM. If you use an oscilloscope, you'll see the waveform and the indicated voltages will be correct within the accuracy limits of the scope. Since you're using a simulation, the results are estimates.
Thank you so much. Now somethings are clearer.
 

Thread Starter

phantomII

Joined Dec 23, 2019
15
Maybe the transformer has a high enough magnetizing inductance so that the roll-off is far from the frequencies coming out of your NE555.

When using an oscilloscope you should get waveforms that are close to what you see in the simulations.
Thanks for giving help. I should get an oscilloscope as soon as possible
 

dl324

Joined Mar 30, 2015
11,897
I should get an oscilloscope as soon as possible
You don't need to spend a lot of money. I bought a DSO138 for $20 from Ali Express after my daily use Tektronix Scope (15MHz SC502) stopped working. I have other scopes, but I use the DSO138 when I don't need better capabilities (higher frequency or more channels) to save wear and tear on my better CRT based scopes. IIRC, the DSO138 is good for up to 100kHz.
 
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