Techniques for measuring true power

[bfishman]

Hello folks,

I'm building a simple step-up transformer at home for fun and learning, and am looking for some advice on measuring true power on the isolated side.

To do so, I must know the phase angle between voltage and current. So to observe current on my scope, I know I can measure the voltage across a known-ohmage resistor.

The isolated side of my xfmr is simply the secondary coil in series with a resistive load (say a power resistor). If I measure the voltage drop across this resistor using my (earth-grounded) scope, thus grounding out the circuit, will this affect the performance of the secondary coil? I'm trying to picture the current flow before and after grounding. In an isolated state, where does the current 'go'? Does it flow back into the inductor, and in doing so impede/assist the surrounding b-field? Does grounding the secondary coil reduce/increase this effect? Would this increase/reduce back-emf?

I know I could use a differential probe to measure across the resistor, but I don't have one of those.

Because of my concerns with influencing the system by grounding it, I'm considering purchasing an AC wattmeter, and letting it measure everything for me (I'm assuming they're battery-powered and isolated, not earth-grounded...). But before I do so, I wanted to ask for the advice of some experienced tinkerers. My current goal is to measure 'real power', and see what (if any) effect upon that I can have by varying properties of the core, trying different windings, etc.

And please, if I seem to be making any incorrect assumptions in this post, please feel free to offer a correction, I won't be offended. The entire purpose of this project is for personal education and enjoyment.

Edited post @21:37 GMT to clarify the question a bit

[The Electrician]

You haven't told us what the voltages and currents at the primary and secondary are.  You haven't told us what the operating frequency is.  Those are the sorts of details that matter.

[bfishman]

Sure!  Up to 60 vpp (using a small class a/b amp running off a lead acid battery), the primary coil has an inductance of about .5 H, 2.9K Ohms resistance. I'm starting off with a 1:2 step up, the secondary is essentially two perfect matches of the primary wired in series and clamped together. Operating frequency will be low freq audible range but adjustable, probably around 200 hz to reduce the inductor's reactance. The resonant frequency of the coil due to parasitic capacitance is above 60khz.  So at max voltage and 200hz, I'm expecting around 17mA, assuming I've done my calculations correctly. I may need to increase current to produce a larger magnetic field.. I understand I could do this by placing a large enough capacitor in series, although I think it would have to be fairly large. Would placing a resistor in parallel with the coil increase the current flowing through it?

[AG6QR]

The isolated side of my xfmr is simply the secondary coil in series with a resistive load (say a power resistor). If I measure the voltage drop across this resistor using my (earth-grounded) scope, thus grounding out the circuit, will this affect the performance of the secondary coil? I'm trying to picture the current flow before and after grounding. In an isolated state, where does the current 'go'? Does it flow back into the inductor, and in doing so impede/assist the surrounding b-field? Does grounding the secondary coil reduce/increase this effect? Would this increase/reduce back-emf?

Current flows in loops.  Consider an isolated circuit, consisting of a transformer secondary and any kind of load: if you ground any single point on that isolated circuit, the ground wire has not created a loop.  No current will flow through the ground wire.  And the ground wire will have no effect on the performance of the isolated circuit.

If you later make a current path between some second point and ground, then you may cause current to flow through ground wire, as the ground wire is now part of a loop.  This is something to be aware of, especially if dangerous voltages may be present -- there's a chance of creating a shock hazard.

But in an isolated circuit, as long as you only clip your ground lead to one point on the circuit, there's no need for differential probing or using isolated instruments, etc.

[IanB]

The isolated side of my xfmr is simply the secondary coil in series with a resistive load (say a power resistor). If I measure the voltage drop across this resistor using my (earth-grounded) scope, thus grounding out the circuit, will this affect the performance of the secondary coil?

No. Not in the slightest. Assuming it is a regular low voltage transformer and the secondary you are measuring is not connected to any other equipment that might upset the isolation.

Since you want to measure power you will need to measure voltage and current on the scope at the same time. Here you must be careful to connect both ground clips to the same point in the circuit, say, one side of the transformer. Otherwise you will cause an unintended short circuit between the two ground clips and that will upset the circuit (if not worse).

[bfishman]

fantastic! Thank you for the loop explanation and the warning about grounding two separate points in the circuit. It makes perfect sense. Up to this point I have been dealing with a non-isolated system on the primary side, as my signal generator is earth grounded, and have been very cognizant to maintain a single common ground, but I had not considered the situation on the isolated side which both of you described. Thank you!

it looks like I'll use my scope to measure the secondary voltage and current, the latter being measured across a series shunt resistor. I'll have to be okay with only measuring one side of the xfmr at a time, but that's not too hard of a pill to swallow considering the cost of differential probes and/or current probes.

Another question - aside from increasing voltage or decreasing the copper resistance of my coil, how might I increase the current that flows through it? Parallel resistor?capacitor? If the latter, would the capacitance actually have to be greater than 1 F in order to reduce the circuit's impedance, or am I misinterpreting the math there?

Thanks again, this forum has a great community.