Somebody please explain what I'm doing wrong

[alexwhittemore]

If you could, take a look at the picture I've attached.

Yesterday I found a microwave being thrown out, so I immediately harvested its transformer. HV win! I was playing around with it today to see what it could do, so I hooked a function generator to the primary and scoped that vs the secondary. The function generator is on scope channel 1 via aligator leads (so 1x, 1Mohm input), the secondary is on channel 2 via a 10x probe, and another 10x probe is metering the secondary on the multimeter.

Here's my confusion. If you'll notice, the secondary is reading 20Vpp on the scope = 200Vpp. The meter, via the 10x probe, is reading 5.5Vrms = 55Vrms. But last I checked, 100V / 1.414 = 70.72Vrms. So assuming the secondary output is, in fact, a good sine wave (and it looks so), one of those two measurements is wrong. I trust the scope, but I also trust the meter, which leads me to ask if my measurement setup is somehow flawed.

[IanB]

...another 10x probe is metering the secondary on the multimeter. ... The meter, via the 10x probe, is reading 5.5Vrms = 55Vrms.

Is the 10x probe on the multimeter specifically designed for that meter, or is it a scope probe? It is highly unlikely that a scope probe will correctly couple to a normal multimeter. The multimeter input arrangements (resistance, capacitance) are different.

[Rufus]

A x10 probe is only x10 when feeding a 1M ohm input and, at moderate frequencies up, when compensated to match the capacitance across the 1M.

Your meter is about 10M ohm and 100pF which would make a x10 probe about x2 at dc, and probably about x100 at high frequency. You seem to be running at 14kHz (way high for a big iron cored transformer) and it looks like the probe is about x13 there.

[alexwhittemore]

You guys were right, I hadn't considered that the probe is only 10x driving a 1Mohm load. When I pulled the probe out and used a clip lead, everything looks as expected.

As for the frequency, it's actually half that. Note that the cursors are set up to get a peak to peak voltage measurement, which is actually a half cycle. So the 1/T measurement is high by a factor of two.

You're right, the transformer is meant for 60hz, but it happens that the system is resonant at that frequency of almost exactly 7khz.