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I'm building a DC power supply and am stepping down my voltage from 120V (from the outlet) to 24V through a transformer. I will be soldering the 3 wires that plug into a North American electrical outlet to the corresponding wires of the transformer, which are as shown:

(Ordered from Amazon. Link: https://www.amazon.com/gp/product/B00PSMWVM0/)

However, I'm not sure which wires from the cable that plugs into the outlet should be soldered to which wires on the transformer. The datasheet only specifies this:

This doesn't tell me whether the black/green wires should be soldered to hot/neutral/ground. How might I go about figuring this out without killing myself or frying my transformer?

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    \$\begingroup\$ It does tell you that green lead is ground and it is attached to the transformer metal chassis. But if you don't know how to wire it and don't understand the documentation, why did you buy it instead of a transformer that is more easy to understand? Or simply a wall wart that outputs 24VAC safely to whatever you build? They are even cheaper. \$\endgroup\$
    – Justme
    May 3, 2022 at 18:52
  • \$\begingroup\$ OP, you're only looking at half the story. You ignored current (and power). Just because you get the voltage you want with no load, if you attempt to drive anything too large the transformer may overheat. If it has a thermal fuse, then that will just be annoying. If it doesn't, it could be hazardous. What's your LOAD? (i.e. what are you using this for?????) \$\endgroup\$
    – Kyle B
    May 3, 2022 at 19:39

2 Answers 2

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Connect hot (usually black) of your cable to one primary (black) lead of the transformer. Connect neutral (usually white) of your cable to the other primary (black) lead of the transformer. Connect ground (usually green) of your cable to the green lead of the transformer.

The transformer already has a fuse/thermal protection so you don't have to add another fuse. But make sure the connections are secure and fully insulated. All the connections should be in an enclosure; if it's a metal enclosure, you should also bond the green connection to the enclosure with a screw and lug.

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    \$\begingroup\$ How do you know for sure that it has thermal protection? And why would thermal protection make a regular fuse unnecessary? Sounds dangerous having a device without a fuse. \$\endgroup\$
    – Justme
    May 3, 2022 at 18:55
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    \$\begingroup\$ Kinda agree, an external fuse would be a "very prudent idea". Thermal fuses protect the transformer. External fuse protects the user.... \$\endgroup\$
    – Kyle B
    May 3, 2022 at 19:41
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    \$\begingroup\$ @Justme The transformer datasheet (edwards-signals.com/files/…) indicates it has thermal overload protection, which should keep the transformer from catching fire. I'll agree adding a fuse makes it safer, but if you want to protect the entire device including the AC connections inside, you'll have to put the fuse in the line cord plug. \$\endgroup\$
    – Mark Leavitt
    May 3, 2022 at 20:06
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You can use either of the black wires for neutral and hot. You should connect the earth wire. The transformer is Class 2 but is not an impedance protected type, so it contains a thermal fuse or circuit breaker of some kind. The manufacturer’s data says “Non-regenerative thermal overload protection” which is a bit ambiguous to me whether it’s a one-time fuse or whether it resets. I would assume it doesn’t reset, just dies relatively safely if overloaded.

I believe it’s intended to be mounted to the side of an electrical junction box which keeps the low voltage secondary separate from the mains.

Normal bell wiring is AWG 18, and for that gage you don’t need an output fuse, however 40VA is quite capable of causing a fire with the right amount of resistance so you may need a fuse in your particular application.

The important thing for safety is to ensure the earth lead is connected and that it is impossible to contact either mains lead. It would also be a good idea to ground the secondary or one side of your DC supply, so if the transformer fails it cannot cause a shock.

Strongly recommend having someone who is knowledgeable and present in person actually look at your setup before you attempt to power it. Shorting or overloading the transformer can cause it to fail. A fuse would be a good idea, but you will probably want a slo-blo type since the transformer will have to provide a surge of current to charge the capacitors and that can cause nuisance blowing. Also keep in mind your no-load DC voltage might well be more than 40V (due to xfmr regulation and peak of 24VAC RMS) so ensure your circuit can withstand that.

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  • \$\begingroup\$ What is the "right amount of resistance" to cause a fire? Do you mean too low, causing too much current to flow? \$\endgroup\$
    – Mark Hofmeister
    May 5, 2022 at 15:13
  • \$\begingroup\$ @MarkHofmeister If the resistance is too low (say, a direct short) & the wiring is adequate (say AWG 18) then the transformer will simply overheat and shut down/die relatively safely. If the resistance is high then no problem. But say the resistance is more like 12 ohms, so 2A flows. If that 12 ohms is confined to a smallish volume, 48W is more than enough to start a fire & represents only a slight overload on a 40VA transformer. Think of the maximum power transfer theorem (complicated a bit by protection in the transformer). \$\endgroup\$
    – Spehro Pefhany
    May 5, 2022 at 15:40
  • \$\begingroup\$ This is also why bad connections are riskier in home wiring than shorts or opens. Connections being confined to junction boxes helps mitigate the problem (a fire in a metal box has less chance of spreading), but now we've also got arc detection (AFCI) breakers that attempt to detect and shut down a circuit when arcing from loose connections is detected. \$\endgroup\$
    – Spehro Pefhany
    May 5, 2022 at 15:43

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