Recommended logic analyzer/scope tools? (was: RL02 write faults,

Tony Duell ard at p850ug1.demon.co.uk
Sat Jan 7 18:38:58 CST 2006


> I would never trust a single reading from a single instrument to tell me it's

Nor would I, actually.

> safe to get "hands on" with mains - If you happened to pop the meter fuse
> last time you were using it and knocked off  before you realized it (I've done
> this) it gives you a nice zero reading  - I want to at least see it reading ON,
> then disappear when I switch it off, and even then I remain cautious.

The 'correct' procedure for high voltages (or so I am told) is to measure 
the voltage with the circuit live (and make sure you get a reading), then 
turn off the power/pull the fuse and check again (make sure you get 
zero), and then to check the meter again on a similar high voltage source 
to make sure you get a reading there (just in case the meter decides to 
fail at the wrong moment). Even then, with mains, I'd touch it with the 
back of my hand first, so if it is still live it'll throw me off (I hope).

Incidetnally, on most DMMs (including my Fluke), the fuse is for the 
current ranges only. The voltage and resistance ranges work fine with the 
fuse blown. But of course there are plenty of other faults that can cause 
a zero reading with a voltage on the input. Heck, even test leads can go 
open-circuit.

> 
> An a similar theme (and I'm surprised how many "engineers" I have had to
> have this discussion with) - It doesn't make a lot of sense to be doing these
> kinds of measurements if you don't know what to expect. Know what is

Something I was taught early on (in a different context, but it applies 
to electronics too). 'It is much easier to make measurements than to know 
what you are measuring'. With most 'socpes, if you touch the probe on a 
point in your circuit and twiddle the knobs you will get _some kind of 
trace_. It may well mot be any use in actually tracking down the fault, 
though. It may not even be a true representation of the signal at that point.

-tony




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