# How to separate induced voltage from terminal?

1. Jan 4, 2009

### Cspeed

I have a circuit set up like so: All in series, induction coils with internal resistance 22 ohms, attached to a resistor of 40 ohms.

When AC current is induced in the coils at a certain rate, the voltage dropped across the resistor is .14 V AC. The current, which should be the same throughout the entire circuit, is .003 amps AC.

I am trying to find out how much voltage is induced in the coils (as if there had been no internal resistance, nor a resistor). The .14 V seems to be the "terminal" voltage, so I'm guessing that more than .14 V are induced in the coils.

2. Jan 4, 2009

### Staff: Mentor

If the source of the changing flux through this secondary coil is loosely coupled (i.e., not a tightly-coupled transformer), then the load impedance on the secondary circuit will not affect how much flux is generated by the primary coil.

The changing flux in the secondary coil generates the secondary voltage via

V(t) = -N d(Phi(t))/dt

and that voltage across the total secondary impedance is what generates the secondary voltage. The finite output impedance of your secondary winding (its DCR) causes a voltage divider, and you see less than the total V(t) across your load resistor.

So yes, I think you are thinking about this correctly. You have the load resistance, coil resistance, and load voltage, so you can calculate the secondary coil source voltage from those, just as you do for any signal generator with a finite output impedance. BTW, What test is usually done to measure the open circuit source output voltage?

3. Jan 4, 2009

### Cspeed

I haven't looked into impedance (Z) much at all, so I don't think I'll be able to understand until I do so. Is it necessary for me to understand Z so that I can analyze this circuit?

I'm not sure if I understand correctly, but do you mean the voltage output by the induction coils? That would equal the voltage across the resistor, or if you simply attach a voltmeter in series.

I would like to analyze this by looking at a single cycle of the AC. I want to find the change in magnetic flux is during the first half of each cycle (it might be increasing) and then what the change in flux is during the second half (descreasing, equal in magnitude). If I can find the emf, then I can find the delta flux, because I know the time. If I assumed that the emf were just the "terminal" voltage (.14 V), then I'd be ignoring a big part.

My values for AC are likely rms.