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I am working on a scale model electric project and am in the process of setting up the power supply arrangement as shown in the diagram. My plan involves shorting the neutral of the step-down transformer to a dedicated earth line.

My questions are:

Is it safe and advisable to proceed with this setup? If not, what are the alternative options to ensure safe and reliable operation? I would greatly appreciate your advice and insights on this matter.

enter image description here

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    \$\begingroup\$ What is the purpose of that circuit? Why does it have two transformers instead of one that takes in 120VAC and outputs isolated 25VAC? How did you intend to provide two different earths to the cirvuit, where would they come from? What is the difference between "House Earth" and "Dedicated Earth"? \$\endgroup\$ Commented Jan 24 at 22:45

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If one side of the DC load is to be connected to the Dedicated Ground, then neither side of the 25 V AC feeding the bridge rectifier can be connected to the Dedicated Ground - doing so would short out two of the diodes in the bridge rectifier.

If neither side of the DC load is connected to the Dedicated Ground, then you may connect one side of the 25 V AC to that Ground.

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  • \$\begingroup\$ I can share more about my project. The DC circuitry will be installed in an enclosure that also contains the DC motors. This enclosure will sit on a metal surface, which will act as the return path for the DC voltage. Since the electronics and motors are attached to the metal, the return current should not affect them. Literally, the DC load is connected across the 25V AC line and the dedicated earth. Therefore, I prefer to connect the neutral and the dedicated earth to ensure both safety for touch and protection for the electronics. \$\endgroup\$ Commented Jan 25 at 0:24
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You don't actually need two transformers to achieve isolation. The one step-down is enough; its secondary is already isolated from the primary.

Is there a safety issue with having a separate ('dedicated') GND distinct from safety GND? No, that's perfectly ok. That's typically the case with electronics. Phone chargers for example have a separated secondary ground.

One drawback to having the separate GND is that some leakage noise can develop between it and safety GND. This leakage is limited in current, but you can still measure it as a voltage between safety GND, sometimes as much as 60V or so. This can cause problems sometimes, such as hum or noise pickup (example: IR receivers) or other sensitive circuitry. Adding a tie to earth GND can mitigate this.

By the way, if you're using a full-wave (4 diode) bridge, you can't connect anything from the transformer secondary to GND. Connect GND only at the DC side of the rectifier. If you're using a half-wave (2 diode) rectifier with a center-tap secondary, tie the transformer center tap to GND. As below (simulate it here):

enter image description here

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  • \$\begingroup\$ I added a reply comment to Peter Bennett regarding my project. Please refer to that as well. Thank you. \$\endgroup\$ Commented Jan 25 at 0:27
  • \$\begingroup\$ Thank you for the circuit simulations. That 1k Resistor is the load I guess. I am using Full-wave bridge rectifier for my DC Load. And the 115V/25V ac step down transformer has a center tap. FYI. \$\endgroup\$ Commented Jan 25 at 0:44
  • \$\begingroup\$ @SridharanPadmanabhan: If you are using a bridge rectifier (four diodes, as hacktastical's top drawing), you must NOT connect either side of the AC supply to the DC negative side of your DC load. If you are using a full-wave rectifier (two diodes, as the lower drawing) the center tap of the transformer and the negative side of the DC load must be connected together. Whether the DC negative is connected to any Ground, or not, is up to you. \$\endgroup\$ Commented Jan 25 at 1:10
  • \$\begingroup\$ The point Peter Bennett and I are trying to make is that your 'neutral' (which I take to mean, secondary center tap) cannot be tied to your 'dedicated earth' if you use a full wave rectifier: it will blow up the rectifier. You only tie the center tap to ground if you use the half-wave style shown. \$\endgroup\$ Commented Jan 25 at 3:19
  • \$\begingroup\$ Thank you both for your explanations. I'm confident that I'm gaining valuable insights from our discussions. However, I am intrigued by the workings of electric locomotives. These locomotives are connected to a high voltage AC line through a pantograph and the tracks, which serve as the earth. Despite this, their bodies remain safe to touch, and their electrical systems function efficiently. I am attempting to implement this concept and would like to understand any critical aspects I might be overlooking. \$\endgroup\$ Commented Jan 25 at 3:36
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Should be no problem connecting the output to earth, since 25Vac cannot electrocute anyone.

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