My home, built in the 50s, has a Sierra low-voltage lighting system, where low-voltage 24DC switches control relays that power the normal 120AC lights. In the last week, the switches have failed intermittently, and I'm not sure how to diagnose the problem. Different switches fail at different times, so it seems like it must be a problem with the transformer, since it seems unlikely that multiple switches and/or relays would all suddenly go bad at the same time.

This is a rough schematic of how the system is wired:

low-voltage lighting schematic

Assuming that the problem is with the transformer, I took a picture of the transformer box: (If anything is unclear from the picture, I will gladly take another for you. Just tell me what you need to look at.)

Transformer box

If I understand correctly, the parts in the box are...

  • A. Transformer
  • B. Fuse
  • C. Capacitor?? (If so, that's not in the schematic.)
  • D. Rectifier
  • E. ??? in-line something or other? Maybe just a fancy connector?

I really just have two questions:

First, how should I go about diagnosing my problem? (I have an automotive multimeter that can test voltage, resistance, connectivity, etc.)

Second, is there a simple way to bypass the system to get a relay to turn on and off in the meantime. (I have a few lights in the house that are stuck on at the moment, and I guess I'll just leave them on all night.)

Edit 1: Failures seem to go away after waiting for a while. After writing this question, I went out and both of the switches that were failing before turned off with no problem.

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    \$\begingroup\$ Replace the cap, diode and transformer with a 24V >1A DC supply. $15. A reverse fly back clamp diode is recommended since relays are inductive. \$\endgroup\$ Jan 18, 2021 at 5:28
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 There are different kinds of power supplies. Does it matter whether I get one that looks like a cell phone charger vs an old-school laptop charger, vs the steel cage with lots of vent holes? I would prefer to not require a ground, since I don't currently have ground. Also, your not about ">1A" got me thinking and I looked up the relays, and they are 20 amp. kyleswitchplates.com/… Will a 1A supply work for that? \$\endgroup\$ Jan 20, 2021 at 4:12
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    \$\begingroup\$ It doesn’t matter much, you can use an old 19V laptop charger on 24V relays and those old ones are 80 Ohms so I=V/R is pretty small. \$\endgroup\$ Jan 20, 2021 at 4:22
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75, I can't find any products called "reverse flyback clamp diode". Is that something built in to the supply, or is it something that attaches after the supply? Thanks for your help. :) \$\endgroup\$ Jan 20, 2021 at 6:51
  • \$\begingroup\$ Any diode like 1N4000 series in reverse across coil suppresses the flyback HV action when turned off. It’s std. procedure \$\endgroup\$ Jan 20, 2021 at 15:39

1 Answer 1


Transformers don't generally go bad, and if they do they tend to burn up.

The capacitor would be the suspicious thing to me. Perhaps you can read the markings on it. You may have to rotate it to see the markings. It's probably filtering the DC. Also, it may not be, but from the photo the "diode" appears more to be a full-wave bridge rectifier.

Electrolytic capacitors do tend to dry up and lose capacitance over time, and 60 or 70 years is sufficient time even for a high-quality capacitor to lose its mojo.

If you lack the ability or equipment to measure the capacitor or ripple voltage you could simply procure another of similar (or higher) capacitance and similar (or higher) voltage rating and connect it in parallel with the existing one (carefully observing polarity, to avoid a great deal of unnecessary drama).


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