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# How to get voltage reading using current transformer?

#1
11-26-09, 05:54 PM
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Join Date: Nov 2009
Posts: 2
How to get voltage reading using current transformer?

I have wore my eyes out searching the web for an answer and have come up short.

I know that by using a current transformer you can monitor volt, amp, and frequency, but I am not sure how you get those values.

Well, the voltage more specifically, I found article after article that pretty much says you get a current through the secondary that is proportional to the current through the primary.

One article furthermore says that if I have a turns ratio of 1:10 and use a burden resistor of 1ohm that I should get .1 volts on the secondary for every 1amp in the primary, and that by increasing the resistor value it would proportionally increase the voltage on the secondary, ie. 10ohm resistor results in 1 volt on the primary, etc.

I do understand that there are other factors here, just using the above as an example, but lets say I have my primary that is a 120vac line, and I was to get a voltage on the secondary that was proportional to the AMPS in the primary using my example above, it seems I could certainly determine the amperage, but how do you convert the ".1v per 1amp" into the voltage present in the primary, ie. 120vac?

The formula volts = amps * resistance in ohms doesn't seem to apply to my example above, if .1v in secondary equals 1amp in primary using a 1ohm resistor, then primary volts = (1 * 1) * 10 since turns ration is 1:10 ???

The article I got the example numbers from also provides a formula: V=(I/N*R)
Ok, V=volts, I=current(amps?), and R=resistance(burden resistor), but nowhere in the article is N defined, nor can I find the meaning of N as it pertains to electricity.

Please I am pulling my hair out over this, so frustrating when I find so many results on the web but none of them seem to answer this question.

Note: yes I do know to NEVER open the secondary circuit of a current transformer while there is current in the primary

Last edited by HazardTW; 11-26-09 at 06:25 PM.
#2
11-27-09, 03:26 PM
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Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,495
First off, you do not use a current transformer to measure the voltage in the circuit. You use a voltmeter connected between the high and low voltage points of the circuit, namely the so-called "hot" terminal and the so-called "neutral" terminal.

A current transformer is used to measure the current, i.e. the FLOW (or load) of the circuit. Generally current transformers are constructed so that the current in the conductor the CT surrounds is some multiple of five greater than the current that flows in the secondary of the current transformer. A typical current transformer may be designated 100/5 meaning that when one hundred amperes are flowing through the primary conductor there will be five amperes flowing through the CT secondary. NOTE, it is CURRENT (amperage) that you measure from the CT secondary, NOT VOLTAGE! The main reason for using a current transformer is to allow a (relatively) low-cost low range ammeter to measure high currents.

You CANNOT (under any ordinary circumstances) use a current transformer for to measure voltage in the primary circuit. Nor can you measure frequency by use of a current transformer.

#3
11-27-09, 07:19 PM
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Join Date: Nov 2009
Posts: 2
I thank you for your reply, my fruitless searches were leading to believe that this was the case.

I must be misunderstanding things an acquaintance told me, he said he was using a current transformer to monitor the mains and generator voltage in his equipment room, and I know the unit that monitors these reports them as voltages, ie. 240V on the mains.

It is possible that the voltage range he is getting on the secondary of his current transformers he is interpreting as a scale of 240V rather than a scale of the amperage in the primaries.

I was also thrown off by a panel device I found online that reports volts, hertz, and amps. The only connection I saw were from the current transformer it uses, further digging after reading your reply turned up that the unit ALSO requires being physically connected to the primaries that it is monitoring, apparently the current transformer is just for... current

While I am disappointed since I would love to be able to detect the voltage level without having to tap into a primary, I am relieved that my vain search for the answer is over... I can sleep now, thank you!

p.s. I finally found out what "N" meant in the article I was referring to in my original post, found the answer elsewhere, it represents the turns ratio of the transformer.

#4
11-29-09, 01:18 PM
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Location: Wet side of Washington state.
Posts: 18,495
There are several "power monitors" available that measure voltage, amperage, watts/kilowatts and frequency. Few are really suitable for residential use. Not knowing what your acquaintance is using or the device you saw somewhere on the Internet I cannot offer any comment.

#5
12-02-09, 04:06 AM
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Keep in mind, too, that no transformer is 100% efficient.

#6
12-05-09, 07:25 PM
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Location: Northern Virginia
Posts: 4,240
"N" refers to the number of turns in each coil, with the turns ratio equal to N1/N2 (N1 = number of turns in primary coil, N2 = number of turns in secondary coil). Not sure what you are trying to do, but figured I could answer one of your questions.