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I'm an enthusiast so please be gentle! I have a transformer question and can't find a similar previously asked question.

I have an ADT/Honeywell security/smoke system. We experienced a utility generated power surge which burned up at least 7 transformers and caps in the house including HVAC, doorbell, freezer, oven AND the ADT security system transformers, Models XF-1620 and 1321. The 1620 outputs 16.5V/20VA and the 1321 outputs 16.5V/25VA/. I want to replace with one transformer outputting 16.5V/45VA. I think it is just that simple. Am I missing something? I am NOW in the hunt for a whole-house surge protector strategy as well.

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    \$\begingroup\$ Welcome to EE SE, your logic on replacing two transformer with one, is right, however, I am quite surprised to hear that the transformer failed, this is a part that is rather rugged, and it is quite unlikely for it to burn. Could it be that you have not confused a power supply for a transformer? Anyway, if that is the case, your logic still works; just replace two power supplies for one. \$\endgroup\$
    – Elmesito
    Commented Feb 26, 2019 at 20:37
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    \$\begingroup\$ Since this is for a security system, I'd ask ADT to make the necessary repairs, rather than modifying thing myself - that way, ADT is responsible if the system doesn't work correctly. There are quire likely other problems besides these transformers. \$\endgroup\$
    – Peter Bennett
    Commented Feb 26, 2019 at 21:09
  • \$\begingroup\$ @Elmesito - Above a certain voltage, the core will saturate and then only winding resistance will limit the primary current. A surge of sufficient magnitude and duration will overheat a transformer. \$\endgroup\$
    – user28910
    Commented Feb 26, 2019 at 21:26
  • \$\begingroup\$ Thanks again.. its a "plug-in transformer". Input and output are both AC. The spike lasted well over a second. This was not a nano or millisecond event. I saw lights cycling. I called ADT and they will not touch it unless I pay for a monitoring contract with them. Fwiw, they will not even look at it for time/material cost. Most of the other repairs involved replacing PCBs with power supply and/or transformer failures. It seems the spike cooked any power transformation component and protected downstream components. All downstream fuses are intact in all appliance that had them, so far. \$\endgroup\$
    – p.conway
    Commented Feb 27, 2019 at 14:11

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Focus on commercial, off-the-shelf

The idea of a single transformer is fine, just get large enough as you plan. However I would dissuade you from acquiring one out of an electronics supply, because they are not UL listed as mains electrical equipment.

Appropriate equipment these days will typically screw into a junction box knockout, or mount on the lid of a junction box. It is sold either at electrical supply houses or heating/cooling supply. Amazon is a fine source as long as you watch out for "sold by Joe Blow and ships from Amazon" types, make sure they are selling reputable UL-listed items and not the usual Chinese garbage off Alibaba. With mains power, don't fool around.

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  • \$\begingroup\$ Excellent point, thanks.. I'm thinking ELK TRG1640 AC Transformer. Their specs say "UL Listed". Just sent them some email to tray and clarify. \$\endgroup\$
    – p.conway
    Commented Feb 27, 2019 at 14:25
  • \$\begingroup\$ @p.conway That's fine, the classic "Wall Wart" is an acceptable way to mount a transformer of that type, if you have an appropriate receptacle handy (i.e. classic dual-oval style with the mounting hole in the middle). I'm not familiar with the vendor. \$\endgroup\$
    – Harper - Reinstate Monica
    Commented Feb 27, 2019 at 14:40

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