BNC EARTH/GND question - Page 1
Forum section: electronics / beginners

author: Zielsziek
author_rank: Contributor
timestamp: September 03, 2025, 07:00:17 am
content: Hello, I have built a curve tracer and placed it in a metal box. As is required, I connected the metal case to earth ( I have sanded the paint where the crimped lug is and I have used a star washer, it's not visible in the pic). My project worked fine on the bench (without the case) but now, in the case, my transformer got boiling hot and I got no oscillation.The only thing that changed is the project's home so I suspect that I might have overlooked something. I found no problem with having the BNC GROUND and mains EARTH connected together since the oscilloscope has the same setup. My supply is a linear one, with +15 and -15. The earthing system where I live is TN-S.How should I proceed? When I get home today I will isolate the BNCs but I want to get a better understanding of what happened.Thanks.

author: tooki
author_rank: Super Contributor
timestamp: September 03, 2025, 07:33:41 am
content: Any chance there’s a simple short to the case from one of the PCBs, heatsinks, or standoffs?

author: Ian.M
author_rank: Super Contributor
timestamp: September 03, 2025, 07:37:54 am
content: The overheating transformer implies a short-circuit in the PSU or on one of the power rails. Its impossible to be certain as you haven't given us the curve tracer schematic, but it is possible it does not have the BNC outer shells connected to 0V and relies on the PSU having a floating output so the scope can ground them via its BNC inputs, letting 0V swing to whatever potential it needs to be. If you used metal standoffs to mount the PCBs you may have inadvertently grounded its 0V via the case, or as Tooki suggests, one of the standoffs may be touching another power rail.Post the schematic, and start your diagnosis by unbolting both board and temporarily slipping a insulating sheets under them, so they are effectively 'back on the bench'. Whether or not it then works will tell you whether or not the short-circuit has done permanent damage, e.g. blown a diode in the bridge rectifier. If it does work, you then need to closely inspect the underside of the boards for proximity of traces to mounting holes and component lead length vs standoff height to figure out what's shorting.

author: PGPG
author_rank: Frequent Contributor
timestamp: September 03, 2025, 12:05:03 pm
content: For PSU alone (not connected to main PCB) when observing its GND output pin relative to PE (oscilloscope grounding crocodile connected to PE) I would be not surprised seeing AC 115V or even more.Trafo relatively high in-out capacitance makes output coil common mode voltage being somewhere between L and N (depends for example on which coil end is connected to L and which to N). As your PSU is not connected to PE it has no way to make its output GND having no voltage relative to PE.This is a source of small current going from PSU GND through main PCB to BNC - case - PE.If your PSU is overloaded than it is something different than I describe, but I believe it will be helpful to understand also this.

author: radiolistener
author_rank: Super Contributor
timestamp: September 04, 2025, 05:29:50 am
content: It’s very likely that the issue originates within the circuit itself, where some part may be shorting through the chassis ground.In practice, connecting the BNC shields directly to earth is not always optimal, especially when signal dynamics and noise performance are important. A good approach is to use an output BNC whose ground is not connected to the circuit GND, keeping the connector isolated from the chassis. This avoids unintended ground loops and prevents excessive currents through the transformer or other sensitive parts of the circuit.

author: Zielsziek
author_rank: Contributor
timestamp: September 04, 2025, 12:31:42 pm
content: Hello, yesterday when I arrived home I checked for any mechanical damage to the circuit. All is fine.Even with the PSU hot the voltages were still right. No drops. The heatsinks are isolated and the standoffs are isolated since they are plastic.I unplugged the PSU, let it run for 3 hours under supervision and it was cold as ice. The voltages are ok. I had a hunch that it might be a ground loop and ordered isolated BNCs.I forgot to mention that I have 230V AC.

author: Ian.M
author_rank: Super Contributor
timestamp: September 04, 2025, 01:14:40 pm
content: Quote from: Ian.M on September 03, 2025, 07:37:54 amThe BNC shells are connected to VG not Gnd, however VG is just some capacitors going to the +15V rail, so that's effectively just an AC local ground and bonding it to PE ground shouldn't cause any issues.One thing to be aware of is heatsink grounding and device substrate connections of non-isolated power semiconductors. e.g. the tab of a 79xx regulator connects to its -VIN but the tab of a 78xx regulator is COM (0V) so if you mount both on grounded heatsinks without a TO-220 isolation kit under the 79xx, that's your short-circuit right there!The TDA2030 on the main board is also problematic this way, its tab connects to -VS.

author: Zielsziek
author_rank: Contributor
timestamp: September 04, 2025, 01:21:37 pm
content: I have made sure to use plastic screws under the heatsinks, so nothing touching them. I will try another test today, I will simply remove the BNCs and connect to the cables directly. Thanks!