Discussion on question by user1247: Can't understand why oscilloscope requires incomplete circuit in order to measure voltage in secondary winding of very low inductance transformer

Feeds

21:39

Q: Can't understand why oscilloscope requires incomplete circuit in order to measure voltage in secondary winding of very low inductance transformer

If I build (a very poor) transformer (for pedagogic use) using a few windings of wire for the primary and secondary coils, and use a signal generator on the primary side, I can measure the voltage on the secondary side with an oscilloscope probe, but only if I don't ground the other side of the s...

transformer

Eugene Sh.

It sounds like you transformer doesn't work, and the measure you see when not connecting the ground is the noise.

user1247

No, it's pretty clearly the actual signal, with a perfect sinusoid exactly matching the frequency of the signal generator (which can be varied), with about the expected frequency-dependent voltage as well.

Eugene Sh.

Is the secondary winding connected to anything but the scope?

BTW, it can be very well the noise induced by the primary, so it is perfectly matching it.

JRE

No doubt you are seeing the signal. But it is probably getting to the scope by some way other than through the normal transformer coupling.

Trevor

Yes but with no ground from the scope you are picking up the noise of the primary signal AC coupled through the capacitance between the coils.

Frosty

21:39

What waveform do you see when you connect the scope to the primary side? It sounds like you are trying to drive your signal into nearly a dead short (very small DC resistance and almost no inductance).

user1247

The secondary winding is not connected to anything else. It is a very simple setup, with just the secondary winding, and one lead from it going to the scope.

brhans

With no grounding on your secondary I'd guess you're seeing capacitive coupling from primary to secondary windings.

user1247

I have a resistor on the primary side to make sure it's not shorted, and I can drive up the inductive reactance of the primary by increasing the frequency on the signal generator. If I hook up the scope to the primary, things look perfect.

Yeah, I'm not so confused about what I'm seeing with no grounding on the primary. What I'm much more worried about is why I can't see a signal if I DO ground the other side of the primary!!!

Eugene Sh.

Whats your core is made of? What is the windings are made of? Are they insulated? What is the ratio?

user1247

Adding a core doesn't seem to change things much, so I've mostly just tested with an air core. The windings are magnet wire, insulated. I've tried various ratios, but generally something like 1:5 step-up.

If I put a 5V sinusoid into the primary, I can get up to a 10V sinusoid out (with a 1:5 turns ratio, and upping the frequency), again with the proviso that I DON'T ground the secondary

Transistor

21:39

It sounds as though you have a hopeless transformer but with reasonable capacitance between the primary and secondary.

Trevor

How big a resistor on the primary?

user1247

100 ohm resistor on the primary

I agree Transistor and Brhans that it may be capacitive coupling, but I find it really weird that the transformer is really THAT hopeless that an oscilloscope with microvolt precision can't pick anything up

I've measured the signal using the oscilloscope on both the primary and secondary sides

Trevor

Post a picture of the thing with something that gives us an idea of scale.

user1247

I can later, but it's really just a primary with 5 loops (diameter of a few cm) right on top of a secondary with 25 loops (also a few cm) with magnet wire.

Eugene Sh.

This "on top" makes me suspicious...

user1247

21:39

Meaning that the oscilloscope doesn't pick up a signal if the two coils are far apart, but it does (just like expected), when the two coils are situated right next to each other so that the magnetic flux from one goes through the other.

Eugene Sh.

The magnetic flux will "wrap" around the boundary of the primary coil right away without having a core.

user1247

But if they aren't separated by any gap at all I don't see how that is going to be too problematic?

Eugene Sh.

I guess it is something that needs to be properly calculated, or at least estimated using some information we don't have.

Anonymous

1) what is the diameter of the turns in the windings? 2) what frequency range you have tried 3) have you insulated wire in your windings?

user1247

1) The diameter is about 2 or 3 cm. 2) I've tried frequencies between 1 Hz and 100000 Hz. 3) The wires are insulated.

Eugene, the inductance of the coils are about what I expect from the simple N2 mu A / l induction formula, around 20 micro H. I guess the question is about an air-core transformer with 2 inductors in the ten micro H range.