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I found this transformer and I am not sure if it is working. I am confused with the multimeter readings. For example: when I am using the buzzer option to check the connectivity it buzzes when I connect any of the 3 top ones (16ohms and 32ohms), but when I try to see the connection between the 665ohms it doesn't buzz, but it does show ohms when I use the 2K ohms option. Is this how it's supposed to work?

Transformer

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    \$\begingroup\$ Probably yes. What voltages do you measure in circuit? \$\endgroup\$
    – winny
    Jan 20 '19 at 11:00
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    \$\begingroup\$ Looks like a "center tapped transformer" \$\endgroup\$
    – Linkyyy
    Jan 20 '19 at 11:02
  • \$\begingroup\$ @winny I am scared to put it directly to 230 AC because I don't have a great experience with that. :( I am scared it will explode like my previous ones I did. Is there a different way to test it with lower voltages? I don't have a regulating transformer eather. \$\endgroup\$ Jan 20 '19 at 11:24
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    \$\begingroup\$ @AmirŠaran You can safely check the voltage ratio using a lower voltage; try putting e.g. 12 V across the winding with the high resistance and measure the windings with the lower resistances. \$\endgroup\$
    – zebonaut
    Jan 20 '19 at 11:31
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    \$\begingroup\$ Concerning the "beeper" in your multimeter: Please check the specification or manual of your meter (or try various known resistors). Some meters have a threshold for beeping, often something like 20...30 Ohms. "Continuity" (a beep) therefore means nothing else than "I measured something less than 20 Ohms (or so)". \$\endgroup\$
    – zebonaut
    Jan 20 '19 at 11:36
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What you have here is a center tapped transformer. The primary winding in this case is the 665 ohm one, and the secondary the 32 ohm one which is split into two windings, so each one giving 16 ohms. I don't know what the rating of this transformer is, but if we assume a 230V/12-0-12V transformer you would get voltage measurements between the windings as shown in the picture. You can find more info about these kind of transformers here: https://www.electronics-tutorials.ws/transformer/multiple-winding-transformers.html

But to answer your question, for this type of transformer all of your readings seem normal for it to be in working condition.

Example of a center tapped transformer

(Image source: 12-0-12 Center Tapped Transformer - Components 101)

center-tap voltage calculations

(Image source: The Center-tap Transformer - Electronics Tutorials)

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    \$\begingroup\$ Are you really sure that Np/Ns is proportional to Rp/Rs? The DC (copper) resistance depends on the wire's cross section and length. While the length might be somewhat proportional to the number of turns, the cross section most likely is not, because the secondaries usually use a much heavier gauge than the primary, because Is/Ip=Np/Ns, and the power loss (Pp=Ip²Rp, Ns=Is²Rs) per unit length is usually designed to be somewhat similar. I guess the approximation is therefore very, very vague, at best? Measuring the voltage ratio will yield a better result, and can be done with low (safe) levels. \$\endgroup\$
    – zebonaut
    Jan 20 '19 at 11:29
  • \$\begingroup\$ @MidasGossye Do you know why it's not buzzing when I try to check if there is a connection in the primary winding, but when I try with the 2K option it shows a value. And when I check the secondary windings with the buzzer options it buzzes. \$\endgroup\$ Jan 20 '19 at 11:31
  • \$\begingroup\$ @Zebonaut That's true, I admit that was a very vague approximation. I'll change my answer. \$\endgroup\$ Jan 20 '19 at 11:33
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    \$\begingroup\$ @Amir Saran I guess it doesn't buzz because the resistance is too high to register for the continuity test. If you use the 2k resistance setting then of course your meter will be able to register it. \$\endgroup\$ Jan 20 '19 at 11:35
  • \$\begingroup\$ @MidasGossye Thanks, I though that just checks if there is a connection between the two points, not measuring resistance. \$\endgroup\$ Jan 20 '19 at 11:36
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The readings look reasonable.

You will need to arrange a source of AC to test the transformer further. A known good mains to low voltage transformer is your best source. Many light strings use 12, 16 or 24V AC and transformers are sometimes found for sale at thrift shops.

If you have an old car or motorbike battery charger you should be able to connect to the transformer outputs before the rectifier, solder secure connections and lead them out safely and use with the case closed to keep the mains isolated.

A random bicycle dynamo or standard (permanent magnet) stepper motor chucked to a variable speed electric hand drill will give you some AC at a usually safe voltage and reasonable frequency. Make a little jig to hold it together and then hook it up to the high resistance winding and measure all the voltages and calculate the turns ratio.

If it is a 115V primary it will die if you give it 230V on the primary side.

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