# Transformer power on both sides

#### EFDG

Jun 11, 2016
33
Hello,
If we have transformer operating on pure Sin wave with load as AC motor.
How to calculate Voltage and current at both side with oscilloscope?
as there will be voltage lead the current.
V=?
I=? on both sides?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,516
How to calculate Voltage and current at both side with oscilloscope?
An oscilloscope is not an instrument to 'calculate' voltages or currents. It is meant to measure.

Voltage can be measured directly. Be careful with high voltages. Special high voltage oscilloscope probes or voltage dividers may be necessary.

Current can measured only indirectly by an oscilloscope. Put a (small) resistor in series with the wire where you want to measure the current. Measure voltage across the resistor and determine I=V/R. Again be very careful when it comes to high voltages.

#### EFDG

Jun 11, 2016
33
An oscilloscope is not an instrument to 'calculate' voltages or currents. It is meant to measure.

Voltage can be measured directly. Be careful with high voltages. Special high voltage oscilloscope probes or voltage dividers may be necessary.

Hello,
Can 't we measure with curve in oscilloscope?

Put a (small) resistor in series with the wire where you want to measure the current. Measure voltage across the resistor and determine I=V/R. Again be very careful when it comes to high voltages.
This can done for DC but in case of AC what to do?
what is the use of current transformer ?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,516
Can 't we measure with curve in oscilloscope?
I didn't say you can't. Of course, an oscilloscope is used to display a waveform and to get measurements from this display.
This can done for DC but in case of AC what to do?
This works for AC, too.
A current transformer, as the name says, transforms one current (typically a high current)= into another currrent (typically a low current) for easier measurement. Wgen using an oscilloscope you will still need to convert the smaller current to a voltage using e.g. a resistor.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
you will still need to convert the smaller current to a voltage using e.g. a resistor.

A current transformer requires an appropriate load. If it doesn't have one, the voltage is undefined. Typically this is a resistor of a certain value. This has the convenient effect of converting the current to a voltage for you.

beware any thoughts that the current transformer doesn't load down the circuit you're measuring. The only real advantage is isolation (ok, there are others...)

hall effect devices can also be used to convert a current into a voltage. They may well place a smaller load on the circuit.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,516
If it doesn't have one, the voltage is undefined
Not only undefined, but potentially dangerously high, depending on the specific parameters of the primary circuit and the transformer used.
Sometimes, when the load can be disconnected from a current transformer, special connectors are used which short-circuit the transformer's secondary when the load is unplugged to prevent an overvoltage.

Note that this is neither required nor good when using a voltage transfomer!

#### EFDG

Jun 11, 2016
33
This works for AC, too.
ok, yes it is working in AC i have seen in simulations
If we want to calculate by are under curve then how to do?
i mean how to define?
In DC the voltage is same for all time periods but in case of AC the voltage level change with respect to Time.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
mathematically you can determine this, however if you have a definite phase relationship between voltage and current (the simplest is that they are in phase) then you can simply base your calculations on the RMS value of the AC voltage and the power factor.

Which of these are you familiar with?

#### EFDG

Jun 11, 2016
33
i am familiar with in phase when there is phase difference i cant calculate.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,516
Starting on this page there is a series of tutorials on AC voltages, currents and their relationships. At the bottom of the page the next pages of the series are linked.

#### EFDG

Jun 11, 2016
33
is capacitor and inductor are linear components?

#### EFDG

Jun 11, 2016
33
ok, i have sen it many times .

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
i am familiar with in phase when there is phase difference i cant calculate.

Jun 11, 2016
33
ok, i will do.

#### EFDG

Jun 11, 2016
33
Hello,
Current can measured only indirectly by an oscilloscope. Put a (small) resistor in series with the wire where you want to measure the current. Measure voltage across the resistor and determine I=V/R. Again be very careful when it comes to high voltages.

The AC static meter at house use lot of circuit with embedded system big SMD circuit
does it work with same method?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
if you're talking about a meter used for billing electricity use, they measure power, not current.

#### EFDG

Jun 11, 2016
33
ok, How they measure power?
active or rective or both /real

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Typically domestic customers are billed for real power and large commercial customers for apparent power.

#### EFDG

Jun 11, 2016
33
Typically domestic customers are billed for real power and large commercial customers for apparent power.

Hello,
How to measure it??