We developed the TDMM quite well and achieved a good measurement accuracy. So far, we only measure DC voltages and flows. A change voltages should be added today. With a shunt you can also measure alternating currents. AC voltage measurement is not a very trivial thing. The shape of the AC voltage plays a role for our measurements, as is the frequency. Do we find a clean sinus course? What happens if our AC voltage has a rectangular form or comes as a triangular vibration?
Used components
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All resistance |
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TL074 operational amplifier |
Electronic needs; Div. Provider |
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Potiometer 100 kΩ |
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Elko 10 µf |
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Condensator 1 µF |
ELKO range or electronics requirement for non-polarized C |
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Power supply |
First a few words about the "theory" in advance, guaranteed with only very simple formulas.
Sinus change voltage
Let's start with the Sinus AC voltage, as we know it from the power grid. Our power grid delivers 230 V SINUS - AC voltage with a frequency of 50 Hz. The effective value is meant by "230 V". If you place this voltage on an ohmic resistance, then the heat output on the resistance corresponds exactly to that of 230 V DC voltage. That means "effective value". If the resistance has a value of 100 Ω, the heat output is p = u2/R >> P = 2302/100 >> P = 529 watts, regardless of whether it is connected to the power grid or on a DC source.
Why this distinction between effective value and other measurement variables?
Let's take a look at a sine voltage in practice. So she looks at the oscilloscope:
On the vertical y-axis, our sinus swings back and forth over 6 scale units. The zero line is exactly in the middle. Each scale unit corresponds to 200 mv. So our Sinus 600 MV swings up and 600 mV around the zero line. This AC voltage has an amplitude of 2 x 600 MV, i.e. 1.2 Vpp, One speaks of "tension peak-peak" or German "tension pointed tip". The so -called "vertex value" uP = ½ * upp.
This information is in a fixed relationship with Ueff, the effective value. This fixed relationship is easy to determine.
Ueff, = UP / √2
In our case: ueff, = 1,2 / √2 >> Ueff, = 0.85 V
That applies only For a sine change voltage. With the circuit declared here, we only measure the Ueff. If we UP want to calculate, we multiply ueff with √2 ( ~1,41).
What is frequency?
The sine wave in our picture constantly changes polarity and needs a certain time. In our case, it has her zero passage - on the far left in the picture. The complete vibration ends in the third zero passage, five scale units further on the right. The displayed scale value of the x-axis is "2 ms". That is the time per scale unit. 5 x 2 ms = 10 ms. A vibration runs during this time. One speaks of "frequency" f = 1/t. frequency is the duration of the vibration period and is given in Hz: f = 10 ms >> 1 = 100 Hz.
The measurement results of the circuit are frequency -dependent. It delivers usable values up to a frequency of ~ 10,000 Hz. Up to this frequency, the display is sufficiently linear. This is sufficient for many applications.
Crest factor
One question is still to be answered: Which measurement results can you expect if we do not have a sinus vibration? I would like to refer to external sources, e.g. to Wikipedia, which explains the concept of the crest factor here: https://de.wikipedia.org/wiki/Scheitelfaktor. It boils down to the fact that - depending on the signal form - you multiply the measured value with a factor.
The TDMM precision rectifier
We put the precision rectifier either on the Breadboard or build a small additional device for the TDMM: If you choose to build a small box, you can use the 4-fold socket that we have connected our test tips with "Speak button" at the TDMM. There is already the DC voltage input.
The box still needs a power supply. Either internally, or as here - external (diode plug on the right).
In the box itself there is a white socket as a common mass for all alternating voltage measurements and three green sockets for the measuring areas up to 15 V, up to 150 V and up to 450 V.
In the choice of measuring areas, they are basically free. More on that later.
The circuit
Perhaps the view of this circuit for one or the other reader is a bit unusual, but I would like to encourage you to go through the circuit diagram with me from left to right:
You can see a symbolic signal source in the bottom left in the picture. It is used for circuit simulation. Because with the Open Source Project Kicad 8 you create a circuit, simulate it and create it if necessary. Even a printed board with it, everything in a consistent work process.
The signal source delivers 500 MV AC voltage with 440 Hz to the non-polarized capacitor C1. He keeps all of the DC voltage shares away from the rectifier. The signal reaches the first buffer level U1A via R1. There is the reinforcement 1: 1. The level serves as an impedance converter. With "Quadopamp" the TL074 denotes that I can recommend for this application. All four operating amplifiers are housed on a single 14-pin chip. There are many other 4-way opamps that are also suitable.
U1b is the rectifier. It delivers its output signal via R5 to U1C, which works as an integrator. R7 is used for fine comparison. The last buffer/ amplifier level with U1D follows. There U1D delivers a DC voltage that exactly ueff, corresponds. It is passed on to the difference in the TDMM. DC_OUT lies at the U+ difference input, the mass of the TDMM.
DANGER: It also follows that the TDMM and the TDMM rectifier need separate power supplies. Otherwise, another circuit concept would have been necessary. But it fits well for us.
Practical structure of the TDMM rectifier
The U1A has a JFET input with an input impedance of 1012 Ω. This is a very high value, which is why the touch with a finger can completely overlook the rectifier. Since we generally do not need this very high impedance, a 1MΩ resistance is switched to mass in the entrance group. It could also be 10 MΩ if you want a higher input impedance. Especially when building Breadboard, you need a little tact on this part of the circuit. The resistance should be placed quite close to the pins of the TL074.
And here is the Breadboard:
You can use an ELKO for the input capacitor, but I recommend a non-polarized capacitor of good quality. The resistors come from the AZ resistance range.
Supply voltage
On the circuit diagram, I specified as operating voltage +/- 5 V. The Breadboard is +/- 15 V. It makes sense that the operating amplifier cannot give a higher voltage than its own operating voltage. If you want to measure alternating voltages up to 15 V. Choose +/- 15 V. The operating voltage affects the function of the circuit little because you can compensate for any deviation with the compensation potentiometer. Only the measuring range depends on the operating voltage. The supply voltage should no longer be changed after comparing. Their stability is rather uncritical.
Separate power supplies - humming loops
Digital multimeter usually have a single power supply, a battery or battery.
We have chosen a modular structure here that needs more space, can be expanded and offers special options, such as the voice output. We get to know more functions.
I chose this path because we may want to measure alternating voltages that have a connection to the protective earth (PE). Our laptop is also connected - probably via detours (e.g. connected USB devices, other measuring devices). And we have already conjured up interference signals and/ or a nice grumbling loop in our measuring structure and see puzzling measurement results.
So please: The TDMM rectifier has its own supply, preferably from batteries / batteries.
We present a simple, inexpensive variant right here.
Commissioning and comparison
In order to put the TDMM rectifier into operation, we need a sinus generator, or in the simplest case a small mains transformer that is on the secondary page 6 V.eff gives. We also need a classic digital voltmeter, which we connect on the secondary side of the transformer to see a comparative value for the output voltage of the transformer.
Now connect the TDMM rectifier to the TDMM and equalize with the 100 kΩ potentiometer the display of the TDMM on the display of your digital voltmeter. This is not a premium method, but completely sufficient for day use.
Expand the measuring range
If you look at my small TDMM rectifier (2nd photo from above), you will see input sockets for tensions ≤ 15 V, ≤ 150 V and ≤ 450 V. Since I like to repair tube radios, I need the area up to 450 V, because the tube devices deliver anode voltages of e.g. 275 V.eff.
These measurements are carried out with a simple resistance network and are also safe due to the high pre -resistances. Since 10 MΩ with 1% accuracy are well available, I have realized the necessary 20 MΩ by switching two resistors.
9 MΩ 1% you usually only get as designated measurement resistors.
In the "150 V" measuring range, the measured value of the TDMM is multiplied by factor 10, for "450 V" with a factor of 30.
Double power supply
If you want to operate the TDMM rectifier with a +5 volt voltage source, you can easily implement the negative -5 volt operating voltage with a single chip and a few other components. This is explained here: With +5 V we do - 5 V
If you want to use the full measuring range of +/- 15 V, I recommend using two modules of the type Mt 3608. This module has been presented here so often and has been explained that I can keep my execution very short.
On the entrance side I recommend two batteries of 6 V each. Since the TL074 needs only a little electricity (type ≤ 20 mA), the MT3608 can be set cleanly to 15 V. Then please make sure that a minus output of one of the MT 3608 is connected to mass. With the second MT 3608, make it the other way round. There the plus output is to mass, so that the operating voltage is available at the minus output.
DANGER: They actually needed two batteries. The reason for this is that the converters are not earth -free. The GND (minus) entrance is directly connected to the GND (minus) output.
Now I wish you a lot of fun with the TDMM and its new skills. Next time it is about how to bring the measurement data of the TDMM into the network, in home assistants or on the mobile phone and how MQTT works.
Until then,
Your Michael Klein
Source information:
The stated circuit is further development of a proposal
taken from the website: https://www.electronicdeveloper.de/
