Making a Transformer

[Nitzans]

Hi all,
I'm trying to make a very basic transformer for a science project. The main purpose of this transformer is actually to estimate the number loops in the secondary coil, by measuring the induced emf. My question is: the impedance on my secondary coil is expected to be very high (~500 kohms), i.e. orders of magnitude higher than the impedance on the primary. Should I be worried about impedance matching? I know my questions and details seem a bit vague, so I'll happily supply more details if those are required. I should also mention that my knowledge in electronics is basic at best. If you're still undeterred, I'll appreciate your responses.
Thanks,
Nitzan.

 

[Resqueline]

Measuring on very high impedance sources you need to worry about the measuring instrument loading down the source.
It's called measuring error, but the real off-load value can be calculated - if you know the impedances involved.

 

[duke37]

500k is an unbelievable transformer impedance especially on a home wound transformer. Is this the impedance of the measuring instrument?
Show a drawing of your circuit and specify the voltage and frequency of the primar supply

 

[Nitzans]

I apologize, but as it turns out,I have a much more basic issue which is unclear to me, the answer to which must come before tackling the problem I mentioned in the first post. I know this simple equation for a transformer:

The question is: Is Vp the source voltage, or the induced emf in the primary coil?

 

[duke37]

VoltsSecondary/VoltsPrimary=NturnsSecondary/NturnsPrimary

The emf at the primary comes from the power supply.
The emf in the secondary is induced from the magnetic coupling.

The output impedance will have a component due to the secondary resistance and another due to the primary resistance. An ordinary mains transformer would likely have an output impedance of less than 1k and, for a hand wound transformer with only a few turns, less than 10R.

 

[Nitzans]

VoltsSecondary/VoltsPrimary=NturnsSecondary/NturnsPrimary

The emf at the primary comes from the power supply.
.

Sorry, but I'm still not clear though. Does VoltsPrimary = voltage of power supply, or does VoltsPrimary = emf at the primary ?

 

[duke37]

If the power comes from the mains at 230V AC then the volts on the primary will be 230V AC. You can call this emf if you like.

 

[Nitzans]

If the power comes from the mains at 230V AC then the volts on the primary will be 230V AC. You can call this emf if you like.

Does this rely on the assumption that coil resistance is negligible, and all the 230V go into inductance? Because if you input the expected current due to resistance (say ~100 ohms) in the primary into Faraday's equation, you get an induced emf which is much lower than 230V. Additionally, this voltage is a function of the coil dimensions and the ac frequency.

 

[duke37]

I think you are trying to punch above your weight. The voltage from the mains will not be affected to any significant degree by your transformer.
For a simple assessment, the voltage out vs. the voltage in is due to the turns ratio. If you wish to go complicated, you could spend a lifetime on transformer design.

Factors affecting the performance are primary resistance, secondary resistance, interturn capacitance, interwinding capacitance, inductive coupling to core, inductive coupling from core, primary inductance, secondary inductance, leakage inductance, Eddy currents in core, core non-linearity etc. etc. etc.

I use mains transformers for hobby purposes and find that the open circuit voltage corrected by resistive loss is good enough for me. The inductive current should be much less than the load current. High frequency transformers and radio frequency transformers are the same in principle but other factors such as capacitance and leakage inductance begin to dominate.

 

[Merlin3189]

Primary EMF

If the power comes from the mains at 230V AC then the volts on the primary will be 230V AC. You can call this emf if you like.

Yes, that's what I thought when I first read the question: I always take the PD across the winding as the input emf. But on reflection, don't we model the primary as a resistance in series with an inductance? So connect it to 230V AC and, say 1V is dropped across the R and the primary emf= 229V. It's just that for 'real' transformers, the resistance is so low compared to the inductance that we can ignore it. If Nitzans is winding his own, he may not get near this sort of ratio (large number of turns, because R=kN, but L=kN^2; good magnetic circuit) and the resistance may be more significant.

On another point, I would be a bit dubious about connecting a homewound transformer direct to the mains: I'd certainly think about what might happen if my transformer started to draw a current limited only by the resistance of my primary winding (10A, 20A, 50A... more?) and take precautions accordingly.
Using a Variac (if they still exist?) to start with a low Voltage and work up, would seem like a good idea.
For measuring transformers I have instead used a signal generator with a variable output (0-10V, up to 1A) and short circuit protection.
Apart from power dissipation considerations, V and A measurements scale up linearly. The only snag in this case is that you are (I guess) making a step down transformer (perhaps 230V to 10V?) which means you would be measuring less than 0.5V AC output, which is difficult to measure. Perhaps the sig gen output could go through a step up transformer (240V: 12V used in reverse) to create your driving Voltage. Even though you're now generating higher Voltages, you can start low and there's no chance of very high currents, as the generator will self limit.

Final point: if you are just interested in investigating turns and Voltage\ current ratios and you can use a sig gen as above, then you can save yourself a bit of winding by using a higher frequency. Every time you multiply the frequency by 10, you need 10x less turns. I'd guess that you could work at 5000Hz instead of 50Hz before the factors that Duke mentions become too significant.

Don.