AVO 8 - does the transformer step UP low AC voltages?

[6SN7WGTB]

Seeing that the AVO has a low V AC scale that is completely linear, does the transformer step UP low AC voltages to overcome rectifier non-linearity/voltage drop?

[The Philpott]

I can't see any other reason why the 2.5v, 10v and 25v AC ranges use the transformer.
Can anyone confirm?
My High Resistance Avometer (a precursor of the Model 8 design) has a separate scale for the lowest ACV range...and this particular meter does not actually have a transformer.

Dave

[pmmunro]

The short answer is that the transformer is used to overcome the non-linearity of the rectifier but it is used on both alternating current and alternating voltage ranges. This may be clearer from the simplified circuit diagram of these ranges. It is not immediately obvious, at any rate not to me, how the exact circuit values were calculated so I suspect that they may result from some experimentation around theoretical values.

The circuit has not only to take account of the inherent non-linearity of the rectifier but also variations in characteristics between rectifiers.

The AVO manuals describe the transformer as a "current transformer" and since the meter is essentially based on a current indicating instrument this seems reasonable.

PMM

[6SN7WGTB]

Splendid, thank you.

This all came about from explaining to a friend the complexity of AC measurement and how it gets done in things like my RF millivoltmeter, although my explanation of that was probably less than 'thorough'...!

[Roger Ramjet]

Hi, would I be correct in assuming that the transformer shown on the schematic is a Autotransformer - noting that it does not appear to have a secondary winding ? Rog

[pmmunro]

Roger,

Yes, the current transformer in the Model 8/9 range is an autotransformer. This is also true for later designs, including the Model 10 and the Models 16 to 21 range.

Earlier designs of Universal Avometer, Models 7, 40 and their predecessors, the 20, 34 and 36 range Universal Avometers used current transformers with separate primary and secondary windings.

PMM

[Roger Ramjet]

Originally Posted by pmmunro

Roger,

Yes, the current transformer in the Model 8/9 range is an autotransformer. This is also true for later designs, including the Model 10 and the Models 16 to 21 range.

Earlier designs of Universal Avometer, Models 7, 40 and their predecessors, the 20, 34 and 36 range Universal Avometers used current transformers with separate primary and secondary windings.

PMM

Thanks for clarifying PMM, looking at he schematic further I can see the 10A AC range does indeed have a separate winding with just one leg connected to AC common. This is quite interesting & I intend to have a look inside an old AVO I have to see what the transformer looks like.

Out of interest & not being sure of the model no.... did the AVO MULTIMINOR also have a similar set up inside ?

Rog

[Herald1360]

A current transformer will essentially act as an AC constant current source- it will attempt to drive its secondary current through whatever load it is presented with be it linear (a resistor for example) or non linear (a rectifier with Vf drops). Its primary will present a similarly linear or non linear load on the source feeding it so the meter may not present a constant ohms per volt loading on AC voltage ranges.

[The Philpott]

'Avo Multiminor' does not have AC current ranges.

I have an Avo Model 8 MkIII that was unresponsive on the 10A AC range, it turned out that the (quite chunky) winding on the outside of the transformer had come adrift at a solder joint. Had it been one of the lower ranges within, it's unlikely that i could have fixed it, but as it was it was just a case of re-wetting and adding more solder.

I can provide photos of the transformer of an early Model 8 if that helps (i have one that is permanently 'open' with all the guts visible)

Dave

[The Philpott]

-The winding can be clearly seen, but any tracing needs referral to the circuit diagram as per usual! If we were to rewire a Model 8 for instructional purposes we would use different colours of insulation in the style of a Precision Avometer..

Dave