# Mains earth and op amp ground

I was trying to power a transimpedance amplifier built with an op amp on a protoboard.

I only had a power supply that looked like the first image below (no COM, which I would have connected to GND of the op amp).

The answers I got for my initial question posted on this website were really good and perfectly explained what I was supposed to do. (I initially asked questions about different kinds of power supplies)

I made a mistake of connecting the mains earth of the power supply (circled in the first image below) to the + pin of the op-amp, where GND (COM of a power supply) was supposed to go. I understand why I was not supposed to do it. The mains earth is ill-defined since it is disconnected from both positive and negative voltage outputs of the power supply.

It seems I broke my op amp when I connected the mains earth to + pin of the op amp where GND (so COM) was supposed to go. I see that I was not supposed to do it.

My question is how did I break the op amp by connecting the mains earth to the + pin of the op amp?

• And your question is? Aug 25, 2018 at 18:24
• What was IN- of the op-amp connected to? Aug 25, 2018 at 18:25
• That is a single supply. you will have to create a "virtual ground" midway between the supply terminals to act as a circuit ground for your op-amp circuit. You should not have any connection to the Mains Ground (the green post on the supply). Aug 25, 2018 at 18:41
• @laptop2d Thank you for the response. My question is how I broke the op-amp by connecting the mains earth to the + pin of the op amp. Aug 25, 2018 at 18:48
• @ThePhoton it was connected to some current from my other circuit, which I confirmed works and when I properly hooked up the op-amp with its + pin connected to COM of the power supply unit that has COM, everything worked well. Aug 25, 2018 at 18:49

simulate this circuit – Schematic created using CircuitLab

Figure 1. What you've done.

Figure 2. What you're not accounting for.

The power supply has capacitive coupling between the high-voltage side of the transformer and the low-voltage side. This occurs through the transformer capacitance and the deliberate capacitance circled in Figure 2. The effect of this is that the PSU output has a weak alternating voltage on it. You might be able to read this with a digital multimeter between one of the terminals and the mains earth.

simulate this circuit

Your op-amp has a very high input impedance and is very sensitive to over-voltage. Most likely is that the capacitively coupled voltage exceeded the maximum allowable input to the op-amp.