Electrical Technology

The current in the primary circuit builds up to a maximum, limited by either:

1. The overall resistance of the circuit, or,
2. The ability of the circuit protective device to sustain the current, at which point the device operates and the current drops to zero.

What happens in the secondary circuit is dependent upon what happens in the primary circuit, and what is connected to the secondary circuit.

What happens in the secondary circuit is dependent upon what happens in the primary circuit, and what is connected to the secondary circuit.

In fact there is a transmission from primary to secondary ONLY if the primary current is variable. This means that ONLY during the phase of current increase or decrease a current will be present at the secondary which ever its own properties are. The current increase or decrease in the primary depends on its inductive impedance. What was mentioned above is valid for the steady state after connection and end of connecting phase.

Absolutely correct.

The original poster seemed to lack curiosity on this point.

There is a smoke in either

a) DC source

b) Fuse - if unfortunately connected

with or without a big bang depending on the size of the source

H approaches ∞ and μ approaches 1 (or 4∏ x 10^-7 if you prefer). The current will be limited only by the resistance of the primary.

WARNING: Do not hold the wires while disconnecting. Do not play with the secondary. Be alert for leaks of the Magic Smoke from the dc source.

In addition to all the above, I'm going to add my two cents worth of explanation. Since you asked the question, "What happens when a DC source is connected to a transformer?," I'm going to assume that you don't know much about electricity or transformer action.

Now, that being said, transformer secondary voltages and currents only change when the primary voltages and currents change. The instant the primary is connected to the DC source, the secondary voltage changes, roughly with the turns ratio, since the current/voltage changed from zero to some value. Conversely, the secondary voltage changes when you disconnect the DC source because the voltage/current changed from some value to zero.

Without getting too detailed about rates of change of current, transformer DC resistance and impedance, that's what happens.

NO the current in the secondary is zero after the steady state of the primary was reached!

I didn't say anything about the current in the secondary after steady state has been reached. What I said was, "Conversely, the secondary voltage changes when you disconnect the DC source because the voltage/current changed from some value to zero.," and it does.

Unfortunately, the Guest did not specify how the DC source was connected to a Transformer or whether the Transformer was connected to an AC source. So, thinking outside the box a little:

One consideration (connected directly or in parallel with no load on the secondary):

• DC power will not pass through the transformer except for a very brief time when it is first connected. After that the winding inside the transformer looks like a near short circuit to the DC power supply. Only the resistance of the transformer winding would limit the DC current flow.
• If positive going pulses were fed into a transformer, as in the output from a full wave bridge of an AC signal, the first pulse will go through to the secondary side of the transformer. Part of the second pulse may pass through but very quickly the core will reach a point of saturation where it will no longer pass any energy. At the same time, it wants to return that energy very quickly when you attempt to interrupt the DC current.

Another consideration (Connected in series - more often than you might think):

• This can be done under isolated conditions to create a DC offset to the AC power output. The DC energy can be filtered from the combined voltage with a series capacitor. Likewise, the AC energy can be filtered out with a series inductor. But there are substantial costs associated with mixing these two power sources. And transmission of the DC energy is not enhanced by mixing these two sources. The DC portion is still subject to the resistive losses. And lastly, it reduces the capacity of any transformer which would dissipate the DC portion as heat, since it will not pass through and only magnetizes the core in one direction.

The telephone industry is one who mixes AC and DC signals a lot. I think there are other applications where this may be appropriate, but it is always done with care and precision. Lack of isolation from ground (or earth) always results in problems. Then again, isolation to ground (or earth) can result in problems. The trick is to understand what and why you would do such a thing!

This is favourite viva voce question in college.Any way I will give the answer.The Impedence in AC Circuit consisting of reactance and resistance will imit the current.When a DC voltage (Assumed to be of same value as AC Volts) the winding resitance only has to limit the current.So this being less than the impedance excesive current flow and burn out will result.