FeelTech FY3200s - why bother mains earthing it?

[claytonedgeuk]

I've got a bit of a question on this.  There a bunch of posts around on the FeelTech FY3200s units - that only come with a two pin plug, have no ground - and therefore are free and floating.  The modifications I've seen take a three pin socket, take mains earth from this socket, and connect up the low voltage side of the power supply to the grounds on these properly grounding the probe grounds and USB socket in the process.

If I check out my oscilloscope - all the probes are grounded back to mains earth - so I would expect just in the process of connecting the function generator to the oscilloscope, you're getting a route back to mains earth for the function generator anyhow.

I'm not trying to be clever here - just trying to understand - what extra benefit do we get from connecting mains earth directly on the function generator itself?  Or is this more of a best practice thing - in the event we want to run the function generator away from something like the oscilloscope?

Thanks!


Clayton.

[cdev]

Because the voltage on some of them is high enough to damage projects people are working on, if they don't ground the thing. Very low current, but it is definitely high enough to damage semiconductors.

[claytonedgeuk]

Ah true...I suppose all well and good if you're just hooking it up to an oscilloscope...but generally you're trying to pass the signal you've generated through a circuit first.

[Kleinstein]

To reduce EMI the switched mode supply essentially need a capacitive ground connection. With only a 2 pin cable this are two small class Y caps, usually something like 2- 4.7 nF to both sides. So effectively you have something like 5 - 10 nF to half your mains voltage.  If the supply circuit is bad, this could be more (e.g. larger caps, or only 1). One can measure this voltage with essentially all equipment using 2 wires and a SMPS.

Using a grounded 3 wire cable gives you the option to avoid this. Regulations than can require that the metal parts like the BNCs need to be directly connected to PE and not just via small caps to filter RF emissions.

[jkf1000]

If I remember correctly, there was a youtube video about this device. As part of the review he measured the floating voltages at the BNC output socket against a mains earth reference and concluded that it was substantial enough that some may receive a small shock from there if they were grounded by touching other equipment at the time. I think it was SDG Electronics, but he also chirps in on here occasionally. Check his video for more info and the mods he made. Karl