Measuring a PT Resistance, Primary and Secondary

[Gary_S]

How do you measure this? Been reading Merlin's book and in the chapter on rectification this is discussed. Would i just measure the wires on the primary side where they connect to the my mains switch? Same with the secondary winding, would i measure it via the standby switch?

[tubeswell]

To measure the DC resistance properly you can't have any caps between the windings and any other part of the circuit 'connected' to the ground return. (i.e.: the winding ends should be 'floating').

This won't tell you if your PT is working properly however. (It will only tell you whether each winding is still intact, and if measuring between windings, that each windings is still clearly not shorted to some other winding somehow)

To check whether the PT is working, you need to plug the primary into a mains AC supply (via a mains fuse and switch) and measure the VAC resulting across each secondary (and between each secondary end and its relevant CT - if it has a CT)

[Gary_S]

Thanks tubeswell for the info. So where exactly would i put my meter probes? I'll post up the schematic of my amp if it'll help, it's a Marshall 4010 JCM 800 combo.

  Gary

[tubeswell]

Am I correct in assuming that you obviously don't have much (if any?) experience working with electronics?

When taking DC resistance measurements, you attach the meter probes to each end of the thing that you are measuring the DC resistance of.

At this point I feel that I should caution you to be careful about working on tube amps. Tube amps contain potentially lethal voltages. If you happen to accidentally complete a path between a high potential and a low potential through your body, it may kill you. Learn about insulating yourself and about proper safety precautions before attempting any diagnosis or repairs on your amp. Or take it to a tech. I mean this in the kindest possible way.

[Gary_S]

Am I correct in assuming that you obviously don't have much (if any?) experience working with electronics?

No you're not correct in that assumption i have quite a bit of experience in amps and know only too well the safety procedures that are involved and how to drain the system etc etc etc.

When taking DC resistance measurements, you attach the meter probes to each end of the thing that you are measuring the DC resistance of.

  I know that, but a resistor is entirely different to a PT primary or secondary. My original post stated; take the measurement from across the primary winding where they connect at the mains switch? 

At this point I feel that I should caution you to be careful about working on tube amps. Tube amps contain potentially lethal voltages. If you happen to accidentally complete a path between a high potential and a low potential through your body, it may kill you. Learn about insulating yourself and about proper safety precautions before attempting any diagnosis or repairs on your amp. Or take it to a tech. I mean this in the kindest possible way.

I know all about safety procedures and so on. I'm asking for advice and you patently don't want to give it but you do want to trot out the "dangerous voltages inside" line.

I know about draining the caps and all that, i'm not asking about that.

If you don't want to be helpful then just don't reply. I'm on here because i want to learn everything myself, because i'm interested, i don't want to take it to a tech. The whole point is learning.

[HotBluePlates]

When taking DC resistance measurements, you attach the meter probes to each end of the thing that you are measuring the DC resistance of.

  I know that, but a resistor is entirely different to a PT primary or secondary. My original post stated; take the measurement from across the primary winding where they connect at the mains switch? 

Operation in use may be different, but resistance is still resistance whether you're talking about a length of wire (transformer winding) or a carbon/metal mixture sprayed on a rod and spiral-cut (resistor).

That said, a useful number for transformer primary/secondary resistance is a little more involved. It's also easier/safer if you do this with a transformer not installed in an amp (and obviously never plugged in or energized).

Step 1: Measure resistance of the high voltage secondary end-to-end and end-to-center-tap.
Step 2: Measure resistance of primary end-to-end.
Step 3: Note the voltage ratio of primary to the entire secondary (say 220v pri, 345-0-345v sec; ratio is 220:690 or 1:3.14)
Step 4: Find the impedance ratio primary to secondary (voltage ratio, squared; for our example ~9.84)
Step 5: Multiply the measured primary resistance by the impedance ratio to get "primary resistance, referred to the secondary"

For most calculations, you will use secondary resistance plus "primary resistance, referred to the secondary" because the latter better shows how the secondary voltage you're interested in is responding to the resistance of the primary.

Depending on the calculation involved, you may only be using half of the total measured secondary resistance. It depends on what you're trying to know and how the transformer is being used; I'm sure Merlin probably mentions what portion of total resistance to use if he presents calculations based on those facts.

[Gary_S]

Thanks HBP, i knew as soon as i saw the post was from you it would have sense in it and be useful to me. Lots of cool information in there.

Cheers mate, appreciate it.

[tubeswell]

When taking DC resistance measurements, you attach the meter probes to each end of the thing that you are measuring the DC resistance of.

  I know that, but a resistor is entirely different to a PT primary or secondary. My original post stated; take the measurement from across the primary winding where they connect at the mains switch?  

Sorry. Your question seemed to indicate that you didn't understand something as basic as measuring the DC resistance of a length of wire. That's why I assumed you didn't know that much.

Measuring the DC resistance of each winding will tell you whether:

1) each winding has continuity between both ends of the wire.

2) each winding is not short circuited to another winding.

The best way to check whether a PT is working is to connect the primary to an AC voltage source and then (carefully) measure the VAC across each pair of winding ends (and where there is a CT on a winding, between the CT and each end of the winding. If you're going to use main AC for the power source, then you should have a fuse in series with the phase/active mains wire and the primary winding.

[HotBluePlates]

Measuring the DC resistance of each winding will tell you whether:
...

Yes, but:

... Been reading Merlin's book and in the chapter on rectification this is discussed. Would i just measure ...

The issue of transformer winding resistance comes up in power supply design and has implications for power supply impedance and the inherent "regulation" of the power supply voltage. That is, the higher the transformer winding resistance (as well as higher rectifier voltage drop) the less constant the voltage of a power supply is in the face of varying current draw.

But I think you're contemplating measuring your amp's transformer winding resistance to see what's typical. That's fine, but the numbers may not have much meaning for you until you're ready to start thinking about power supply design.

[Gary_S]

But I think you're contemplating measuring your amp's transformer winding resistance to see what's typical. That's fine, but the numbers may not have much meaning for you until you're ready to start thinking about power supply design.

Ha! You know me HBP and are reading my thoughts!!  Yeah Merlin is talking in the book about pretty much the way you describe it; measuring the transformer when it's not connected in an amp. But i was thinking along the lines of testing the resistance when your PT is already in an amp. Purely as an exercise to see what the measurements would be and if they were in the ballpark of what they should be.

This for me at the moment is all hypothetical because there's nothing wrong with my amp. I'm only thinking of this because i'm reading that part of Merlin's book and thinking "how does that apply to my own amp? what can i learn from it? and is it measureable in a built amp scenario?"

[Gary_S]

Sorry. Your question seemed to indicate that you didn't understand something as basic as measuring the DC resistance of a length of wire. That's why I assumed you didn't know that much.

No worries tubeswell and no need to apologize. As far as amp and electronics knowledge; i don't know a smidgeon of what most of the guys on here know, however i know safety procedures and all the basics that are involved.

Transformer measurement and analysis is something i've not studied in depth yet. So  studying Merlin's book i'm just trying to absorb and understand what i'm reading and wondering how that would apply to a built up amp.