# How do lab bench power supplies have a separate earth pin and a ground pin?

I understand the use cases for having a separate earth pin and ground pin, but what I am wondering is how exactly this is achieved. Specifically, I am wondering at what part of the circuit this occurs at.

• A bench power supply normally has a positive terminal a negative terminal (not a Ground terminal) and an Earth terminal. The supply itself is isolated from Earth, but either output terminal may be connected to the Earth terminal if desired. Commented Mar 20, 2023 at 5:40

The simplest example: Consider a simple AC step-down transformer "power supply" with nothing but a transformer inside. AC transformers have the property of galvanic isolation, there's no direct connection between its primary winding and its secondary winding. The output is "floating", this is, the output voltage only exists only between its secondary + and - output. Connecting the + terminal to other circuits or the Earth ground has no effect (ignoring parasitic inductance and capacitance). Ideally, the circuit behaves as a battery that exists in isolation.

simulate this circuit – Schematic created using CircuitLab

Thus, this power supply can be freely connected in series with other power supplies without worrying about short circuits. When properly designed to meet relevant safety standards, it's also safe to touch the positive output and Earth ground simultaneously without receiving an electric shock.

Nevertheless, sometimes it's more convenient to use a power supply referenced to the Earth ground of the AC mains. In this case, we can intentionally defeat the transformer's inherent isolation by shorting the - and EARTH output using a piece of wire. This is usually done simply by screwing a jumper wire or metal tab between the "earth" and "DC-" output jacks of the supply. But a mechanical switch or a relay can also do the job.

After making this connection, the power supply is no longer floating, and the voltage exists between +/-, and also +/EARTH.

simulate this circuit

If you still don't see how it works, this is the equivalent circuit after closing the switch SW1 - a short-circuit across the transformer:

simulate this circuit

To create a DC version, simply adding a rectifier and a capacitor.

simulate this circuit

## Practical Power Supply

For a practical DC lab power supply, if it's a linear power supply, it uses additional linear voltage regulators at the secondary output. If it's a switched-mode power supply, it rectifies the voltage from mains AC to high-voltage DC first with diodes and capacitors, then creates a square wave of perhaps 100 kHz using switching transistors, step-down this voltage with smaller high-frequency transformer, before rectifies this voltage to DC again.

But the principle of transformer-based galvanic isolation remains the same.

Furthermore, in practical power supplies, the isolation is not perfect due to parasitic capacitance between the transformer windings, and between the power supply's internal wiring and chassis. Thus, there's always a parasitic capacitance on the scale of 10-100 pF between the DC- and Earth terminals. High-frequency noise will continue to flow despite the transformer. One may even deliberately connect a tiny capacitor around 100 pF to 1 nF across the transformer to give the noise current a predictable conduction path and suppress electromagnetic interference - the leakage current is small enough to be safe for its intended applications, and the capacitor must be certified to relevant safety standard to ensure a short circuit failure is very unlikely.

## AC wiring

The mains AC power distribution system is another source of confusion. For simplicity, in the previous schematics, I showed all AC power sources with two wires, and the chassis ground is attached to the Neutral side of the AC source. But the AC outlet at homes has three wires: Live, Neutral, and Earth (ground). The chassis or "Earth" ground is always connected to the "Earth" conductor, never Neutral.

Its purpose is extra safety and easier fault detection. For AC, polarity doesn't matter, Live and Neutral can be swapped. We can't safely assume the Neutral wire is always at the Earth ground potential and uses it as the chassis ground. If Live and Neutral were later accidentally swapped, dangerous voltage would exist on the chassis. Thus, it's safer to have a separate Earth conductor as a "always safe" conductor. There are other reasons that are beyond the scope of this answer.

But electrically speaking, Earth and Neutral are (ideally, ignoring resistance) electrically the same in most electrical installations. In a TN-C-S system, the Earth conductor is short-circuited to the Neutral conductor at the electrical distribution box.

• I suspected an isolation transformer might be the solution, but didn't quite understand how you could "toggle" earth. From the setup you have described it seems there would need to be a switch or some manual input to short the floating ground to earth. This may be outside the scope of the original question a little bit, but could you possibly explain more about how floating ground can be shorted to earth? Commented Mar 20, 2023 at 5:05
• @mm3psych0sis In most power supplies, the earth is "toggled" on simply by screwing a metal tab onto the its output jacks. Do you have a power supply that can control its Earth connection electronically (or in software)? It must be pretty good power supply. If it's the case, a simple mechanical relay should do the job. Commented Mar 20, 2023 at 5:07
• I sadly do not have a pretty good power supply, or any power supply for that matter, which is partially why I am asking these questions lol. Seems like some sort of switch would be more convenient, but I can understand why safety reasons would dictate a more deliberate approach. Marking as solved, thank you! Commented Mar 20, 2023 at 5:24
• @mm3psych0sis I'm glad that it solved your problem. Also, please check my edited answer with an explanation of the "Earth" conductor in the AC outlet, which may be another source of confusion. Commented Mar 20, 2023 at 5:39