Is this filament winding open or not?

[lassoharp]

I have a used PT on the bench with a 6.3V CT filament winding. 

When I measured DCR between each leg and CT I get normal (0.1r) readings.  When measuring DCR across the full winding I almost invariably get 'open'.  I did once manage to get a normal (0.2r) reading.

When running full AC into the primary I get 3.1V between each leg and CT and when I measure the full winding I've gotten 3 different voltages - 7.17V, 8.56V, and 9.07V.  I suspected a loose connection but no amount of prodding and pulling affected the voltage readings - they stayed steady.

Yet the DCR is showing open . . . (except for the one time I got a normal reading)

I checked the winding with an AC Z meter and it reads way higher than I think it should.  186K vs 289r for each leg to CT indicating an open.

So, best I can tell there is something not quite right.  Apparently a true physical open in the winding and I don't fully understand the situation of the AC getting through - induced where the insulation has broken down?

I plan on putting a string of tube loads on it later this afternoon and see what happens.

Any transformer veterans ever seen this before?  I assume it's not a good idea to count on this winding still being reliable in circuit, or yes? 

 

[JohnRoberts]

That measurement does not compute.

If A to B is .1 ohm  and B to C is .1 ohm. A to C could be less than .2, but never more than .2 ohm.

There clearly should be a low resistance path back through the center tap, equal to the sum of each leg.

Perhaps an intermittent bad connection inside the transformer.

JR

 

[lassoharp]

If A to B is .1 ohm  and B to C is .1 ohm. A to C could be less than .2, but never more than .2 ohm.

Yep.  The AC Impedance reading was high too.  A section of wire may have been on it's last leg.  I've tried several more times and can't get anything but open.


Any ideas as to why I'm getting the AC voltage readings I'm getting?

 

[shabtek]

you may be getting that ~8v because you have a sensitive meter, there is emf in the core, and there may be an intermittent connection in the winding--(i'd bet it is not hooked up to anything) try loading the secondary a little (with 1 filament) then see your reading drop down to a useless level

 

[bruno2000]

If A to B is .1 ohm  and B to C is .1 ohm. A to C could be less than .2, but never more than .2 ohm.

Yep.  The AC Impedance reading was high too.  A section of wire may have been on it's last leg.  I've tried several more times and can't get anything but open.


Any ideas as to why I'm getting the AC voltage readings I'm getting?

No load?
Best,
Bruno 2000

 

[JohnRoberts]

AC voltage measurements sum based on amplitude and phase relationship, so 1+1=2 is only true for identical phase/polarity.

Transformers should not offer much phase variation, which could be from a bad connection and light load. 

JR

 

[lassoharp]

Yep,  No load.  As said, I plan on connecting some real filament loads today.

I'll post results.


Also, the no load voltages I got are fairly higher than anything else I've measured on PTs and filament transformers.  The factory current rating for this particular winding is 6.3 @ 5A.  I have several other 5A s that all measure around 6.70 + or - a few hundreths.  8A or bigger measuring about 6.8 to 6.9V unloaded.  Different iron will be slightly different but 7.17V (lowest) doesn't feel correct. 8.5V and 9V are even further off.

 

[lassoharp]

try loading the secondary a little (with 1 filament) then see your reading drop down to a useless level

You nailed it shabtek.  It will not so much as power a single 150ma tube.  Drops to Nil - zero volts.

Also had tried several other loads up to 2.1A.  Same thing.  Filament winding is history.

 

[emrr]

I have seen B+ windings on two occasions that read open with various meters, yet have continued to function for years, show proper AC voltage on both sides, etc.  The happy side of that mystery, still unexplained. 

 

[peters]

Some meters may have trouble reading resistance of coils with high levels of inductance, especially digital meters with auto scale capability.

I have seen this on larger choke coils.

--Peter