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Referencing This excellent answer

I have a two wire furnace controller, 24VAC, no common. I measured the current draw of the relay (when the thermostat wires are shorted), it draws 0.2A

Wish to add an Ecobee Lite 3, which draws about 1.9 watts (just 80mA).

Am reasonably comfortable with general electrical/electronics. Have opened the control box, and located the "second" terminal of the 24VAC transformer, where one would tap a common wire.

Question: Is there any issue if I just directly solder a new common wire straight to the transformer and use that (+"R") to power the smart thermostat? That would mean a draw of 280mA on a system rated for 200mA. Is that likely to be an issue? Potential consequences? is the worst case scenario just the ecobee lite turning off / relay not having enough power to hold closed, or could it be something more problematic, like making the transformer fail?

I would tap the common wire at the location shown by the blue arrow in this image: enter image description here

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  • Did you mean 0.2A AC draw? Because otherwise you have listed loads of 80.2mA, not 280 mA. And what is the actual VA rating of the transformer? You don't appear to have documented that, and are then claiming it's limited to 200mA without documentation. If it is, then 280mA would be a bad idea (it will overheat and fail) but there are many details unclear here.
    – Ecnerwal
    Oct 19, 2020 at 17:25
  • solder?? We don't use solder in mains or thermostat wiring. Leave the soldering iron in the electronics lab. You would use splices like - once you get in there, look - you'll see. Nobody solders in this area of craft... they did 80 years ago, but it was freakishly thorough by any electronics person's standards - apprentices weren't even allowed to do it, took a master with 10+ years experience. Simplest answer is no soldering.
    – Harper - Reinstate Ukraine
    Oct 19, 2020 at 18:09
  • I have to agree with Harper on the soldering. To continue using knob and tube in a historical region that it was required I had to take a class and pass certification and I had been certified for aircraft electronics in the service and the irons are not simple 60w electronics types. Number of turns and type or splice or tap is also critical so stick with wire nuts or other approved methods. I also agree your numbers are not clear with the VA of your transformer. Vs total load.
    – Ed Beal
    Oct 19, 2020 at 18:29
  • @Ecnerwal Thanks, typo, should read 0.2A. I couldn't find any part numbers, or any info, on the transformer itself. Perhaps if I unsoldered and looked at the bottom, or found a schematic... However, the chassis of the [Honeywell R8184 G 1427] states the unit is 24VAC/0.2A - but whether thats the transformer max rating or just the draw of the relay, or both, I dont know. (I checked the draw of the relay with an ammeter - it's indeed 200mA)
    – Mtl Dev
    Oct 19, 2020 at 19:32
  • @Harper-ReinstateMonica I was referring to soldering to the bottom connector of the transformer - where it is already soldered into the PCB. I.e I would be soldering a wire to an already soldered pad... If we are talking 70mA 24v or even 300mA, why is soldering to a circuit board an issue? Appreciate clarification. And how would one splice to a pad in the middle of a PCB? The connection point I have access to is just between the transformer and the relay...PCB trace only..
    – Mtl Dev
    Oct 19, 2020 at 19:37

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If you cannot find a rating on the transformer I would not add the extra load. With small signal and control transformers it is best to have a “buffer” so you do not over heat it. Small transformers when overloaded will break down much faster than large power transformers as they get hot at first the voltage rolls off and in some cases if they get hot enough and break down the varnish on the wires they can do all kinds of bad things.

When I size control transformers when using relays I usually target a 20+ increase in size so the “pulse” from pulling in the relay doesn’t cause an excessive voltage dip (under sizing with relays can cause chatter and buzzing) chatter is hard on the relay and even worse for the electronics in the circuits think inductive kickback. With the loads you mention I would want a 350ma transformer. A transformer that is oversized will usually last “forever” but fully load or overload the transformer and the life curves roll off very quickly.

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  • Actually yes should be ma , I am so used to va and much larger va was automatic. +
    – Ed Beal
    Oct 20, 2020 at 1:51

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