is there a way how to make the oscilloscope display the current waveform?
books said it only show the voltage waveform
thanks

 

You can use an ammeter that outputs a specified volt per amp output.

Look at http://www.tek.com/products/accessories/oscilloscope_probes/current.html

 

If you place a resistance in series with the current, then you get a voltage waveform that is dependent on the current (E = IR).

 

If you place a resistance in series with the current, then you get a voltage waveform that is dependent on the current (E = IR).

This is the standard method, but there are a few wrinkles.

You need a low value resistor, so as not to disturb the circuit too much. Look to make the ohms law voltage a few hundred millivolts. Make sure the resistor has enough power capacity for the expected current.

If your ac signal has a frequency greater than a few hundred hertz then you must use a non inductive resistor. Since low value resistors are often wire wound it is worth checking the type. Non inductive wire wounds are made by winding either in a zig zag or by doubling the wiring and winding on a former.

 

Direct current would be better monitored with an ampermeter, so assuming you want to see alternating current on the oscilloscope; pass the current carrying conductor inside a coil and connect the oscilloscope to the coil terminals. The more coil turns, the more sensitive to small currents.
Miguel

 

This is the standard method, but there are a few wrinkles.

You need a low value resistor, so as not to disturb the circuit too much. Look to make the ohms law voltage a few hundred millivolts. Make sure the resistor has enough power capacity for the expected current.

If your ac signal has a frequency greater than a few hundred hertz then you must use a non inductive resistor. Since low value resistors are often wire wound it is worth checking the type. Non inductive wire wounds are made by winding either in a zig zag or by doubling the wiring and winding on a former.

What do you mean 'non-inductive'? There will always be parasitic inductance, albeit, it can be low: ~nH. Believe it or not, using a 0.1Ω current sense resistor with a current shunt monitor will have nH worth of inductance and will kill accuracy when the signal is a sine wave.

 

parasitic inductance

I think we are talking powerline frequencies here or at highest a few hundred hertz.
Parasitic inductance is a phenomenon important at multi - MHz to GHz range.

If you are really interested, I first came across a description of non inductive emitter resistor construction in the article by Dr Bailey about his famous amplifier. If you double the resistance wire and wind the bifilar result around the former each half of the winding produces an equal and opposite self inductance, thus cancelling the overall inductance of the resistor.