Electronics > Projects, Designs, and Technical Stuff

CRO "floating" inputs?, fail?

(1/8) > >>

Eliminateur:
I just come from almost settings my eyebrows on fire due to shoddy design(or grave mistake on my part too)..., let's begin...

i'm trying to test for oscillations in the main filter of a SMPS, which is 311Vdc out the bridge rectifier, my analog CRO ha a 300vpp peak, but i have a X10 probe...
so i hook it up to 5V/div, X10 div and test the cap with mains unplugged(because it was charged) and surely, shows 292V~, nice and dandy...

i power up the smps and the moment i touch the ground croc to the negative of the cap, sparks and bits of melted copper/tin and solder flew everywhere, i jumped back half a meter(not very scared, i'm used to shit blowing in my face and working with high energy), the main breaker tripped, etc etc, much laughter ensued(not from me)...
So i have a blackened oscilloscope croc with a melted/missing front, a chunk of PCB trace missing, coworkers laughing and i'm totally baffled thinking i accidentally shorted two adjacent PCB traces, but no, it was the negative island all the way....

intrigued, i put the DMM in continuity test and to my horror i find that the input "outer rim" of the BNC connector is part of the chassis which is GROUNDED to mains ground!, WHAAAA, FAIL! which essentially grounded the neg output of the bridge rectifier(-150Vdc~) when i touched it, aren't CRO inputs supposed to be fully floating?.
or i need an esoteric differential probe?
so how do i go about testing this? if i can't ground it....

ofc not stopping there i changed main cap and since smps fuse didn't blew(incredible, a 1.5A fuse didn't blew but the 20A thermomag AC breaker did trip) i plugged it again, this time a huge flash on the component side took out the breaker  >:(, turns out i took things for granted since fuse was ok and cap had high charge but the neg branch of the rectifier fused on the first accident... second one took out the fuse..., i hate this friday and this CRO...)  >:(

DaveW:

--- Quote from: Eliminateur on February 18, 2011, 03:08:08 pm ---intrigued, i put the DMM in continuity test and to my horror i find that the input "outer rim" of the BNC connector is part of the chassis which is GROUNDED to mains ground!, WHAAAA, FAIL! which essentially grounded the neg output of the bridge rectifier(-150Vdc~) when i touched it, aren't CRO inputs supposed to be fully floating?.
or i need an esoteric differential probe?
so how do i go about testing this? if i can't ground it....

--- End quote ---

Almost every scope I've used has had the neg side connected to mains earth. Working on an SMPS I tend to run the DUT through an isolation transformer which gets round this, I assumed this was absolutely common practise. You can buy scopes with isolated grounds, but I haven't come across them since back at uni in the power labs and I'm pretty sure they're a lot more expensive.... Commiserations on the scope though!

saturation:
Ouch, sorry to hear and read, glad you are not hurt.  More comments as I've time to reply.

Alas, all scopes are earth grounded, unless specifically stated they are not, like the portable Fluke 199 series.

Simon:
you could run the scope off an inverter ? others will know more on this. Your other option is use both channels and not the earths making one channel "earth" and the other signal, then use the subtraction feature of the scope to give you the difference and voila'

jahonen:
Many people seem to learn the fact that scope input jacks are galvanically connected to the AC ground the hard way.

Just connect the DUT via isolation transformer and keep the scope grounded. Yes, you could use a differential probe but in this case isolating the DUT is better way, for general safety, as you are working near the DUT anyway. Isolating the scope is not recommended since you might accidentally connect the scope also to something else or someone else might touch the scope while you are measuring.

Regards,
Janne

Navigation

[0] Message Index

[#] Next page