Electronics Questions & Answers ← browse Q&As

why don't the primary coil in a transformer gets shorted .......?

In part, one of the factor, is because it takes time for electrons to accelerate (and they are in finite number and the nucleus of atoms try to keek them nearby, etc... ).

Imagine a long 1km long wire loop, one end connected to the negative side of a 5V battery. Assume we define the 0 volt at this point. All the wire is now at 0 volt. Next, connect instantaneously (as possible) the other end of the wire to a positive side of the battery. Then, at that moment, you won't have all the wire aware of that new voltage. The electrostatic field has to "walk" along the wire. It does it quite fast, indeed, but with a finite speed. Now, as it progress along the wire, imagine it reaches the 10m mark, then, between the 10 000mm mark and the 10 001 mark, there is a difference of ... 5 volt (in fact, probably between 2 volt and 4, but that is something else goins on here). But 5 volt, or 2 volt, across 1 mm offering almost no ohmic resistance that does not mean that an infinite amount of electrons will instantaneously stop everything that they were doing (related to temperature), but they may, on average, drift, slowly in comparison of the speed of light, on average. Some may even bounce (not a mechanical collision but due to a local stronger repulsion from other electrons) in the opposite direction but in general, they could let the region locally depleted of their presence, so, making a region attractive for electrons at the 10 001 mm mark (accelerating them), as well as influencing those electrons at the 9999 mm mark (slowing them back). They may even be pushed harder, from their back, by electrons newly influenced, but which were already moving, thermally, in the right direction. And thus, it takes time to reach an equilibrium where Ohm's law could then be verified (on average).

If you have a scope which can get into the 2ns range, you can experiment with a 30m long wire without too much problem. Terminate the wire with a variable small resistor. Place a probe close to the end point which will be tied to the +5V side of the battery, the other end tied to the resistor itself tied to the negative side of the battery. Set the first probe (you would need a scope with at least two probes) close to the would be at +5 volt end point, set the trigger mode to raising front at +1 volt. Place the second probe 10 meter away (making a large loop in the wire that is, since physically, your scope doesn't span a geometrical size of 10 m ) . Next, complete the loop with a momentary switch between the +5 volt of the battery and the intended end of the 30m long wire. Push the momentary button down. It may take some tries to get a nicely displayed result (in the nano second scale) and see the difference in time that it takes for the "signal" to reach the second probe. You should also be able to spot that the signal doesn't raise to 5 volt, but is probably close to 4 volt (or even as small as 2 volt), but to explain that is outside this explanation, EVEN if it shows that INITIALLY, the full voltage IS NOT what is SEEN by the real world, and the the Ohm's law has limitations about WHEN it is applicable, and definitively when you are too close to a "transient" (sharp event occuring). As for why placing the second probe at 10m of a 30m wire, part of the reason is to avoid the disturbance coming from the 0 volt side to reach that second probe (having 20 m to travel) before the signal from the 5V reaches it. And 10 meter, with a 2 ns scale, should be great enough to be able to measure the time span it took for the signal, while being "some" times larger than the sampling precision of the timing.

So why no short? Because it takes time to get a "steady flow", it is also a reason why not all the water get spilled insatantaneously out of a bucket, ... well, not in all cases at least :-)

I forgot to mention to have a single shot mode... about the trigger for the experimental part.

well, I think its something that has to do with inductive reactance too due to self induction in the primary coil

"inductive reactance" is the key to the whole thing. Just for background, every wire, coil, etc. has "inductance" and this has to do with the magnetic field produced by a current in the wire. Its unit is "henries" (L) . When an alternating current flows in the wire it is limited by the "reactance" (= 2 * pi * f * L) which is expressed in ohms (same as resistance) but is denoted by X. Ohms law applies here so when when an ac voltage V is applied the resulting current is I = V/X so there is no short circuit which should put your mind at rest. One big difference of X from R is that there is no power loss, hence no heating in an X so P is not = I^2*X . Of course in real life there is also some R but in a transformer or motor winding it is normally small compared to X.