Transformer question

there are usually two wires in the center tap... if they are not touching when you take your reading ...there will be infinite resistance...

On Sat, 25 Sep 2004 14:35:33 +0100, T N Nurse wrote:

|I have a 50 watt valve amp which seems to have blown its |output transformer. A new one is on order, but I was puzzled |by some of the reading I got off it and wondered if someone |could give an explanation. All the measurements were made |with the transformer *_completely out of circuit_*, both primary |and secondary. | |The transformer is from a push-pull standard guitar amp (Trace |Elliot 50 watt combo, 2 EL34s) and one of the valves was glowing |red hot before the fuse popped. Checking the bias voltages, I |had around -46v on the grid of each of the El34s, so that was |a reasonable figure. I then removed the output transformer |and did some resistance measurements on it. When measure from |the centre tap to either of the primary outer connections, the |reading was 69 ohms for each. But when I measured across the |primary outer connections, instead of the expected 138 ohms, |I got an open circuit. I rechecked it numerous times but with |the same result. I even removed the cables from their connector |and measure across the bare wires, but still the same result, |69 ohms from the centre tap to the outers, but open circuit |between the outers. | |On the basis of these bizarre reading, I assumed the transformer |is faulty and ordered a new one, but can anyone offer an |explanation as to why I got such resistance readings? I have a |vague recollection of similar results on a small 15 watt amp I |repaired many years ago and replacing the transformer fixed it |and it went on to give good service, but I would like to know |what is actually going on. Anyone?

Did you use a digital multimeter?

I have encountered some strange an incomprehensible readings when taking resistance measurements on highly reactive components such as transformers when measuring with my Gossen Metrawatt MetraHit 25S digital meter (a not too cheap instrument), and when this occurs I always revert back to my trusty old moving coil analogue meter (AVO model 7) and usually the correct expected measurement is obtained.

Ross H

snip

Hello, yes you've reminded me of that same experience. It's caused by the DMM sampling (frequency) and back EMF. Sometimes just shorting out a winding kills the back EMF so a reading can be made.

I liked the two separate windings with two wires not making contact for the centertap. Has the OP retested or checked for this possibility?

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its most likely your digital meter. my fluke i use at work will not properly read a field winding above a 100 ohms due to inductive reactions. it just sits there blinking now and then giving me an OL reading. try putting it on DIODE mode.

John Fields wrote:

Any chance there's more in there than a transformer? Like a couple of protective diodes (perhaps as snubbers) that are back-to-back when viewed from end-to-end? It would be odd, but not completely out of the question.

Thanks to everyone who replied to this. The problem has now been solved - intermittent fault in the probe lead. Grrrrrr!!!! New probe leads now give the correct readings.

, T N Nurse wrote:

Yes.

That's an interesting observation, and perhaps explains the previous weird measurement. I note that someone else has said that their expensive Fluke doesn't like transformer windings over 100 ohms. I tried today with a different multimeter (and leads) and got the expected readings. I assumed that it was the leads at fault, but perhaps not.

At least that is a lot cheaper to fix than the cost of a new transformer.

-john-

Looks like it. The two windings came in at 66 and 70 ohms respectively with combined reading of 135 ohms. Looks like a partial short in one of the windings. I measured it 3 ways just to be sure - using the multimeter, using a constant voltage source and checking the current and using a constant current and checking the voltage. All

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It is not uncommon for the DC resistance of a center tapped winding to = differ on each half. For the same number of turns, one half could have = more length of wire because that part of the winding is further from the = core piece and one turn uses more wire. A better way to test is to put = an AC signal on each half separately and see if the secondary gets the = same output. David .

It is not uncommon for the DC resistance of a center tapped winding to differ on each half. For the same number of turns, one half could have more length of wire because that part of the winding is further from the core piece and one turn uses more wire. A better way to test is to put an AC signal on each half separately and see if the secondary gets the same output.

--- I disagree. Using EI laminations and winding the entire transformer on the center leg, it makes nore sense, if only for the sake of economy, to wind the CT primary bifilar and avoid using more wire than necessary. After all, an extra ten feet per transformer is about two miles of wire, 1000 transformers downstream. Putting down both halves of the primary at the same time also saves time and tends to make both halves of the primary look more alike than winding one half on top of the other.

-- John Fields

John, That may be true, but this transformer may not have been wound with a = bifilar primary. If bifilar I agree, if not, I stand by my original post = and it should be tested by either measuring inductance or seeing if = there is a difference in a secondary by feeding a signal into each half = separately. I know a short on one side will affect both halves but a = difference should still be seen. David

John, That may be true, but this transformer may not have been wound with a bifilar primary. If bifilar I agree, if not, I stand by my original post and it should be tested by either measuring inductance or seeing if there is a difference in a secondary by feeding a signal into each half separately. I know a short on one side will affect both halves but a difference should still be seen.

I think this is only half right. Winding them bifilar would certainly make them more "balanced" in both turns and impedance, but the total amount of wire used would have to be the same.

Think of it this way: I still need the same number of turns in each winding and the same number of total turns. Winding them bifilar (assuming the same wire gauge) will still occupy the same amount of the winding window.

So instead of "an extra ten feet" of wire in the outer winding, we end up using five extra feet in each winding.

I don't know how a real transformer manufacturer would look at it, but I suspect that the bifilar way has real appeal as long as the voltages are small.

Once the voltages are large, we may want to spread out the ends and wind them separately, which most likely means that we're back to the original method of one on top of (and longer than) the other.

----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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