Voltage measurement

I would like to measure an induced voltage of 160mV generated when a current of 30mA pass through the differential transformer owing to a load.

I am imaging that I should isolate it by using a OpAmp and at the same time, amplify it 7.2 times, in order to obtain 1.2Vac. At this point I should rectify the signal so I can read it by an ADC.

Does anyone know how to measure this low voltage?
How I can avoid the saturation of the differential transformer?
How a multimeter works? the 160mVac were read by a digital multimeter.

How I must handle the ground or earth.


In the figure, at the upper right corner is the output (two terminals) where the induced voltage is read by the digital multimeter, when inside the transformer and in one wire of the AC voltage flows a current of 30mA of course a resistive load demands that flow.

The only data I have from the Txformer is 8 ohm, it is linear and works up to 700 volts.

Can you help me what can I do?

I connected a resistor of 99k between the two terminal then I put the multimeter, but the values changed. I have to sample this signal so I have to rectified this before sampling.

What special consideration I have to take into account about the diff. txformer.

Thanks in advanced

That is a current transformer, not a differential transformer. It is designed to operate into a low resistance load. When you use a high resistance, it is probably not accurate. Use an opamp full wave rectifier circuit to convert the AC to DC accuratly. You will find circuits in National Semiconductor Application Note AN-20 or LB-8.

I have seen the AN you mentioned, they are pretty good, I think the full-wave rectifier is what I need, how ever to I really do not know it this rectified signal is isolated from the transformer.

If an OpAmp has a high impedance at input, what is the difference betweet using a high resistance and an OpAmp.


About the input, what do I have to do with the other end terminal.
One terminal from the Transformer goes to the circuit and the other one, to the ground of the circuit or where?


thanks in advanced

You don't need to isolate the transformer from your circuit - the transformer already provides isolation between your circuit and the Mains voltage.

Russlk mentioned that you actualy need a LOW resistance between the transformer terminals in order to measure the current in the transformer. You can then hook your opamp on to the resesitor and measure the voltage across the resistor.

You can just connect one transformer terminal to your ground, or signal ground depending on how your op amp is set up. Which terminal you connect to ground depends on whether you want your singnal inverted or not.

bmcculla said:

You don't need to isolate the transformer from your circuit - the transformer already provides isolation between your circuit and the Mains voltage.

Russlk mentioned that you actualy need a LOW resistance between the transformer terminals in order to measure the current in the transformer. You can then hook your opamp on to the resesitor and measure the voltage across the resistor.

You can just connect one transformer terminal to your ground, or signal ground depending on how your op amp is set up. Which terminal you connect to ground depends on whether you want your singnal inverted or not.

Click to expand...

I was thinking he might need a diff amp, since the signal will be small, and there may be a significant amount of voltage coupling from the AC line (mains), although the low resistance load on the output of the transformer may take care of that.

Some comments recommends me to measure current and then convert it into a voltage.

Then what am I measuring with the multimeter? I put it to measure AC voltage in the range of 2 volts, so i got 160mVac.

Why I cannot do the same with OpAmps. I appreciate your comments, but I really do not want to damage another transformer. :roll:

Does anyone can help me to read this App Notes.
Here there are a multimeter, but I am not sure of using it when i read that I must read Current to convert it into Voltage

Is there any recommendation about the Differential Amplifier?
Does it load the Transformer?

I don't believe there is any danger of damage to the transformer. You do not want to build the digital voltmeter, but the AC/DC converter of Figure 2 of An-202 will be useful. The output if that circuit will be input to the ADC. The differential amp is good, the load on the transformer is small, if that is what you want. C1 is not needed but a capacitor at the output of the diff amp would be a good idea.

I am setting up the circuit that is in AN-202, I just need the AC to DC configuration, the three opamps, that do the AC to DC.

I found some strange, a multimeter does have a ground that comes from the battery, but I am using a transformer with central tap, so to setp up the opamps I need +/-12 volts, so, what about this earth/ground?

Does it will affect the small signal coming from the transformer?

What you recommend me to do? I have a deal here does I need a float ground or the ground from the central tap is ok.

Digital voltmeters have limited common mode range; they cannot measure a voltage that comes too close to the power supply voltage. If you use plus and minus power supply voltage, ground will not be a problem. One of the voltmeter inputs can be ground, the other will be the DC voltage.

I am surprised I did the dif. amp. showed above and the resistor from the transformer is 1 ohm bigger than before, is it possible?.

The resistance was 8ohm before connecting it to the Dif. Amp.

Am I wrong? or it was modified.


It will be better if instead of using 100k in the dif Amp I replace them by 1M ohm. just to reduce the load at the transformer

I don't understand: what resistance are you talking about, and how did you determine the value?

The transformer showed in the picture has a resistance of 8 ohm, before I connected it to the Diff Amplifier circuit it was 8 ohm, but after I proved it with the circuit the resistance of the transformer changed at 9 ohm, I don't know why. is it possible?
I measure this resistance with a multimeter

The resistances used in the diff amplifier showed in the picture are 100k, however, due to the load the circuit may cause to the transformer I changed them to 1M ohm.

Does it can help to prevent any load to the transformer?

:idea:

The resistance of the transformer did not change, I don't know why you are getting a different measurement. One Megohm is probably too high for the op amp unless you are using a CMOS type.

thanks you very much.
How you consider the maximum resistor value on a CMOS or JFET opamp.
I am using a high speed J–FET input quad operational amplifiers (incorporating well matched, high voltage J–FET and bipolar
transistors in a monolithic integrated circuit). The TL084CN.

how you choose the value. 10k or 100k in this case, which is better?

Does anyone know why using this design, and no signals at input I see with an oscilloscope (in real life) a signal of more that 8 volts at a frequency of 60 Hz)

I am feeding the TL084 with +/-9 volts. and I do not know why in the simulation using circuit maker I have to put a line from the coming signal to the ground, without it the simulation does not run?

What do I have to do with in real life?