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I am developing my own PLC that is powered off a 24VAC transformer that is shared with a thermostat and some other controller devices. I wanted to isolate the 24VAC going into my PLC to protect it and I've been trying to find some small isolation transformers to attach to my PCB. I found a link off aliexpress for a cheap audio isolation transformer:

https://www.aliexpress.com/item/10pcs-600-ohm-Audio-Transformer-Isolator-Transformateur-1-1-Transformador-De-Transformers-Ring-600-600-Transformation/32692812269.html

Can I use this to isolate the 24VAC? How can I derive the max amps and power loss from the values given by the seller?

Images attached:

  1. I am currently using a half wave rectifier
  2. I was going to configure the isolation transformer
  3. Here is what the audio isolation transformer on my PLC will look

image 1 2 3

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    \$\begingroup\$ No, because the core and primary winding of a transformer must be selected and wound according to the primary voltage. It's called volt-turns. In addition, the insulation materials used in audio transformers may not have the required thermal class and thickness. \$\endgroup\$ – Janka Sep 27 '17 at 17:59
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    \$\begingroup\$ The 100 ohm DCR of pri/sec coils isn't favorable either for power transfer. \$\endgroup\$ – sstobbe Sep 27 '17 at 18:38
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Most likely not, signal transformers aren't designed to carry much current, the copper wires on them are pretty thin, hence they will burn if you pass even small amounts of current, on the other hand they may be able to carry higher currents but will have very low inductance (example: ethernet transformers) which will saturate the core if used with ac frequencies. In this case it would be the first situation, note that in aliexpress they don't specify the maximum operating current which is a key parameter for this application, so my recommendation would be to search for an adequate power transformer

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Some audio isolation transformers are designed to carry signals between parts of a system that use dangerous voltages and parts that are not designed to handle such voltages safely. Many others are not, since transformers that don't need to provide a guaranteed-safe isolation barrier can be made more cheaply than those that do. Unless the transformer's documentation specifies a 1000V or more breakdown voltage and also specifies what safety standards it satisfies, I would not expect it to be suitable for any such purpose unless it is constructed in a way clearly intended for that purpose.

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    \$\begingroup\$ The sellers page says "Withstand Voltage: 1000V above" and specifies "It can be used on Audio or video signal circuit, power amplifier circuit, set-top boxes and other equipment to isolate noise." \$\endgroup\$ – bradosia Sep 27 '17 at 18:09
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The answer is no.

I just tested two audio 600:600 transformers from aliexpress. The first, as linked above, measured 3.6VAC on the secondary windings when 24VAC was applied to the primary. Another model from aliexpress measured 4.5VAC. Additionally after 1 minute after the test with my multimeter, both transformers were extremely hot and one had the smell of melting plastic.

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  • \$\begingroup\$ brilliant work bradosia and an upvote for the research, but why the 4:1 step down on 600:600 winding? \$\endgroup\$ – onepound Apr 3 at 19:49

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