Strange transformer problem.

I was called today to assist the burner repair guy with an electrical prob lem in one of the apartment houses I work in. He wasn't able to get correct voltages at one of the zone valves.

I heat my home entirely with wood, so first let me say that what I know abo ut heating systems could fit into a very small vessel. But I offered my ass istance as an electronics technician.

There was a bank of 24 volt transformers, each feeding two apartment's zone valves. And two of these apartments had no heat. He told me that the trans former was a 40 volt unit, he went on to say that it was good, and attempte d to demonstrate this by measuring across each secondary terminal and groun d. When doing this he measured 40 volts from each secondary screw to buildi ng ground. I had to look twice at this to believe it. There was indeed from each screw to ground 40 volts!

So my next best guess was that perhaps it is a 40 volt unit and with a cent er tap to case ground. So half expecting to see 80 volts across the seconda ry I read 0 volts. However I did confirm the rating on the label. It was a

24 volt non tapped transformer and an ohm meter check between each secondar y terminal and ground showed infinity. And interestingly the same check acr oss the two secondary terminals showed 1 ohm.

We replaced the transformer with a new 24 volt unit and restored the heat i n the two apartments. I asked for the transformer. I may have a chance to a utopsy it after the guy shows it to his boss. Whether I'll notice anything would remain to be seen.

This problem seems to defy all logic. Does anyone have any thoughts as to w hat could be going on here? Lenny

Reply to
captainvideo462009
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I was called today to assist the burner repair guy with an electrical problem in one of the apartment houses I work in. He wasn't able to get correct voltages at one of the zone valves.

I heat my home entirely with wood, so first let me say that what I know about heating systems could fit into a very small vessel. But I offered my assistance as an electronics technician.

There was a bank of 24 volt transformers, each feeding two apartment's zone valves. And two of these apartments had no heat. He told me that the transformer was a 40 volt unit, he went on to say that it was good, and attempted to demonstrate this by measuring across each secondary terminal and ground. When doing this he measured 40 volts from each secondary screw to building ground. I had to look twice at this to believe it. There was indeed from each screw to ground 40 volts!

So my next best guess was that perhaps it is a 40 volt unit and with a center tap to case ground. So half expecting to see 80 volts across the secondary I read 0 volts. However I did confirm the rating on the label. It was a 24 volt non tapped transformer and an ohm meter check between each secondary terminal and ground showed infinity. And interestingly the same check across the two secondary terminals showed 1 ohm.

We replaced the transformer with a new 24 volt unit and restored the heat in the two apartments. I asked for the transformer. I may have a chance to autopsy it after the guy shows it to his boss. Whether I'll notice anything would remain to be seen.

This problem seems to defy all logic. Does anyone have any thoughts as to what could be going on here? Lenny

I would hazard a guess that the secondary winding had shorted into the primary winding.

Gareth.

Reply to
Gareth Magennis

VERY strange. My guess is there was a failure of the primary that shorted a spot on the primary to the secondary. (This should NEVER happen!) The primary was open, the secondary was being energized from a spot on the primary where there was a lot of inductance, therefore it didn't go all the way to 120 V. That would explain all the readings you got, but this sort of short should NOT be possible. You may find a non-infinite resistance reading between one of the primary wires and the secondary.

It is also possible that there was enough capacitance between primary and secondary to give the 40 V reading without an actual short.

Jon

Reply to
Jon Elson

snipped-for-privacy@gmail.com pretended :

What did you measure the 40 volts with ? Digital volt meter I suppose. Why did the Tech (Electrical?) tell you they were 40 volt units when the label said 24 volts?

Since end to end of secondry is 1 ohm, of course the volts to ground will be the same both ends. :-? It is most likely the primary is open and all the volts are just coupling from the line to the secondry. You did not tell us the continuity of the primary. :-?

--
John G
Reply to
John G

** The reading is normal.

** There is about 100pF of capacitance from primary to secondary.

The primary winding is all at 120V AC since the neutral end is not onnected - due to corroded wire, open thermal fuse etc .

The meter has a 10Mohm input impedance.

.... Phil

Reply to
Phil Allison

oblem in one of the apartment houses I work in. He wasn't able to get corre ct voltages at one of the zone valves.

bout heating systems could fit into a very small vessel. But I offered my a ssistance as an electronics technician.

ne valves. And two of these apartments had no heat. He told me that the tra nsformer was a 40 volt unit, he went on to say that it was good, and attemp ted to demonstrate this by measuring across each secondary terminal and gro und. When doing this he measured 40 volts from each secondary screw to buil ding ground. I had to look twice at this to believe it. There was indeed fr om each screw to ground 40 volts!

nter tap to case ground. So half expecting to see 80 volts across the secon dary I read 0 volts. However I did confirm the rating on the label. It was a 24 volt non tapped transformer and an ohm meter check between each second ary terminal and ground showed infinity. And interestingly the same check a cross the two secondary terminals showed 1 ohm.

in the two apartments. I asked for the transformer. I may have a chance to autopsy it after the guy shows it to his boss. Whether I'll notice anythin g would remain to be seen.

what could be going on here? Lenny

I never thought to look at primary resistance, or for that matter resistanc e between primary and secondary. That could be a pretty serious failure, es pecially if you weren't expecting it. If I get this thing back I'll do some more tests on it and let you guys know. Lenny

Reply to
captainvideo462009

oblem in one of the apartment houses I work in. He wasn't able to get corre ct voltages at one of the zone valves.

bout heating systems could fit into a very small vessel. But I offered my a ssistance as an electronics technician.

ne valves. And two of these apartments had no heat. He told me that the tra nsformer was a 40 volt unit, he went on to say that it was good, and attemp ted to demonstrate this by measuring across each secondary terminal and gro und. When doing this he measured 40 volts from each secondary screw to buil ding ground. I had to look twice at this to believe it. There was indeed fr om each screw to ground 40 volts!

nter tap to case ground. So half expecting to see 80 volts across the secon dary I read 0 volts. However I did confirm the rating on the label. It was a 24 volt non tapped transformer and an ohm meter check between each second ary terminal and ground showed infinity. And interestingly the same check a cross the two secondary terminals showed 1 ohm.

in the two apartments. I asked for the transformer. I may have a chance to autopsy it after the guy shows it to his boss. Whether I'll notice anythin g would remain to be seen.

what could be going on here? Lenny

I forgot to mention, yes we did use a digital meter to look at this. I tho ught about bringing my 260 in to look at it and probably should have just t o make sure I wasn't looking at noise. I will check it with a low impedance meter when I get it back. Lenny

Reply to
captainvideo462009

oblem in one of the apartment houses I work in. He wasn't able to get corre ct voltages at one of the zone valves.

bout heating systems could fit into a very small vessel. But I offered my a ssistance as an electronics technician.

ne valves. And two of these apartments had no heat. He told me that the tra nsformer was a 40 volt unit, he went on to say that it was good, and attemp ted to demonstrate this by measuring across each secondary terminal and gro und. When doing this he measured 40 volts from each secondary screw to buil ding ground. I had to look twice at this to believe it. There was indeed fr om each screw to ground 40 volts!

nter tap to case ground. So half expecting to see 80 volts across the secon dary I read 0 volts. However I did confirm the rating on the label. It was a 24 volt non tapped transformer and an ohm meter check between each second ary terminal and ground showed infinity. And interestingly the same check a cross the two secondary terminals showed 1 ohm.

in the two apartments. I asked for the transformer. I may have a chance to autopsy it after the guy shows it to his boss. Whether I'll notice anythin g would remain to be seen.

what could be going on here? Lenny

So this "cube", does it present a low impedance to the meter and maintain t he high impedance to the circuit, or is it the opposite? Is this an active or passive device? And if a passive device how can something like this not affect the reading? Lenny

Reply to
captainvideo462009

From the description, it's purpose *is* to affect the reading. Think about the case you described: if you had used a low impedance meter, you would not have gotten a 40v reading due to interwinding capacitance. Sometimes you *want* to load the circuit.

Reply to
Jerry Peters

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