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I had a bad plug-in transformer with a 3 wire output AC GRD AC, with following specifications PRI 120V 60HZ 0.24A , SEC 12VAC 20VA, I could not find a 3 pin transformer with same specifications instead find one on Ebay with little different specifications of PRI 120V 60HZ 0.31A, SEC 12VAC 25VA, My question is can I use this one instead of original specifications of the damaged transformer ? I would like to tell that the 3 wires goes to a Digital TIME TRACKER by FAST COMPANY. The timer gives a beeping sound at the end of the time set cycle

Digital Time Tracker

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  • \$\begingroup\$ If it suits the PCB then go ahead. \$\endgroup\$ – Marko Buršič Aug 21 '16 at 6:33
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Original:    PRI 120V 60HZ 0.24A, SEC 12VAC 20VA - 3 wire output AC GRD AC.
Replacement: PRI 120V 60HZ 0.31A, SEC 12VAC 25VA.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1 (a) 12 V AC transformer. (b) Center-tapped, 12-0-12 V AC transformer. (c) Centre-tapped 6-0-6 V AC transformer.

You state that the original is three wire and that it is 12 V. What isn't clear is whether that's a 12-0-12 (Fig. 1b) or 6-0-6 (Fig. 1c). You need to double-check.

You don't state that the replacement is centre-tapped. Again, you need to double check.

Power rating is fine. You may find that the output voltage is slightly higher as the 25 VA transformer will be running at a lower percentage of its rated output.

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  • \$\begingroup\$ While I don't doubt it as being something worth investigating, I don't recall ever seeing a plugpack with a centre tap, typically most are 2 wire Class II or 3 wire with an earth. OP does also mention ground connection so I would imagine that this is the case. \$\endgroup\$ – D-on Aug 21 '16 at 11:24
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    \$\begingroup\$ I think a photo of the PSU would have been more useful than a picture of the load. \$\endgroup\$ – Transistor Aug 21 '16 at 11:37
  • \$\begingroup\$ Without a doubt. Think you've wasted some time with a good answer that doesn't apply. \$\endgroup\$ – D-on Aug 21 '16 at 11:42
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The voltage is the same, which is what you want and the VA is slightly higher so if anything it has a little overhead. Will work fine.

Edit: Sorry misread the original question. You should try and source one with an earth as without knowing the purpose it would be difficult to make the call, may be performance related such as shielding, could be for circuit protection such as surge devices or may be for safety.

This can vary though. In Australia for example alarm panels have a provision for earth via the AC plugpack however direct bonding to communications earth (which is bonded to electrical earth also) is preferred.

So you may be able to also earth it seperately if you cannot source the exact plugpack.

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