# Transformer - open primary

Discussion in 'Homework Help' started by shespuzzling, Oct 19, 2010.

1. ### shespuzzling Thread Starter Active Member

Aug 13, 2009
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0
I have a homework problem that is related to a switch mode power supply. The problem has a DC battery connected to the primary of a transformer, with an ideal switch also on the primary side. The secondary side has a load.

My problem is with what happens when the current reaches a maximum on the primary side, and the switch is suddenly opened. The current can't fall immediately to zero, because you have an inductance, right? I can't visualize what would happen to the current after you open the switch. Any help would be appreciated.

Thanks!

2. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,668
890
This is where 'self inductance' comes into play. The voltage will drop immediately the switch is opened. we know:The coil will resist any change in current. Without voltage the magnetic field of the coil will collapse and the lines of force will 'cut' the inductor windings. This creates the current that the inductor supplies to the circuit. At collapse the induced current is aiding the circuit. At build up the induced current opposes the circuit current flow.

Plug that in and re-calculate.

3. ### shespuzzling Thread Starter Active Member

Aug 13, 2009
88
0
I'm not following you....the way I was looking at it, on the primary side with just a battery, closed switch, and primary transformer, the equation for the current is:

(with initial conditions: @t=0, i=Imin; @t=aT, i=Imax)

i(t)=(L/V)*t+Imin

So that is a linear equation, and when you reach Imax, you open the switch. What happens to the graph of the current on the primary side? What is the path for the current after t=aT?

4. ### DonQ Active Member

May 6, 2009
320
11
One word:

Mutual inductance.

(well, two words, but you get my point.)

The current can go to zero in the primary, the same way that the current can start from zero and become some non-zero value in the secondary. The point is that the combined current in the inductively coupled windings maintain the current equal to the magnetic field that had been generated just before the primary was opened. The collapsing magnetic field doesn't know the difference between one coil and the other. It (magnetic field converting to current across windings) just goes where it has the easiest path.