Hi all, refer to the picture i attached, i would like some advice from you all regarding the diode current of the rectifier.

The current observed at diode should be DC current right?

In this case, does it means that current flowing from transformer secondary winding is also DC current?

However, as far as i understand, the transformer can only works on AC voltage.

Really appreciate if somebody can provide some advice on this matter.
Thanks very much!

 

i think it is not near dc yet, and yes the transformer only works with AC

 

i think it is not near dc yet, and yes the transformer only works with AC

Then what shall be the diode current? AC or DC?

 

If we assume an ideal diode (which is quite close to a real diode). The current will only flow in one direction through the diode. So the current through the diode will be DC. To be 100% correct a pulsating DC

 

but for the transformer, the current from secondary winding is DC current too?

 

Sometimes people misunderstand the meaning of AC and DC.

When one say Alternating Current one would assume this to mean current that reverses direction whereas Direct Current should mean current that flows in one direction only.

However, the terms are also used to refer to frequency components in a signal such as when a DC voltage is superimposed (added) to an AC signal.

A transformer responds to a change of current, not just a true AC signal.
The current through the diode is DC, but it is also a half-wave DC signal.

 

Yes the current in the secondary windings will be pulsating DC current by the definition of DC.

 

Given this is a classic choke input rectifier-filter it would help if you acquainted yourself with the many websites which discuss the concept in substantial detail.

 

That's why we used to joke about "AC fuse" and "DC fuse". We make fun of our new technicians that any fuse blown before the rectifier should be replaced with an AC fuse and that after the rectifier should be using DC fuse. Actually they are the same fuses.

Allen

 

Thanks for all your precious advice. So any current that passes through the power switches such as IGBT or diode, no matter i measure it before the power switches or after, it should be DC current. Am i getting the right concept?

 

When you say "measure" you are now venturing into an area of great uncertainty.

Let us assume you wish to measure the current in such a circuit. Because we are looking at a fluctuating current (whether it is AC or DC) you cannot do this accurately with any type of meter. You will require a meter that measures true RMS value and not all meters are designed to do so.

There are only two ways to determine RMS (root mean square) that I can think of:

1. measure the heating effect of the current (as in a hot wire ammeter)
2. digitize the waveform and calculate the RMS value (the square root of the sum of the squares)

 

That's why we used to joke about "AC fuse" and "DC fuse". We make fun of our new technicians that any fuse blown before the rectifier should be replaced with an AC fuse and that after the rectifier should be using DC fuse. Actually they are the same fuses.

Allen

A simple Google search for "ac fuse vs dc fuse" brought up a plethora of hits. The attached is only the first in the long list. There IS a difference between AC rated and DC rated fuses.

 

Wow, it really hits me by surprise. I wonder if I go to an electronics shop and ask for an AC fuse, what will the shop assistant give me...

Allen