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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
My scope probe has a ground lead. I never seem to use it because when I connect the probe to a point for measurement it references it to ground. If I want the voltage between two points I can invert and add a second probe value to the first. Why is this there? If I connect it do I risk shorting the measured point to earth ground?
What happens if I don't ground my oscilloscope and let it float? Can I then use this ground lead? Is there any danger?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Many people (especially 'old-timer' TV repairmen) remove the earth from their 'scope This is one of my pet hates !! Although it does allow 'scoping with ease, you may be risking your life. If you connect your 'scope earth to a high voltage the whole 'scope will be at high voltage.
For MOST hobby circuits the electronics will be safe to connect your 'scope earth because there will be a transformer to produce the low voltages. For SSTC bridges etc. you can NOT connect the 'scope earth as all parts of the bridge will probably be 'live'. Then you need to 'invert & add' OR (if you're rich) use a differential probe (which is what I do at work)
If in doubt just 'scope your intended 'earth' point in the circuit with just the tip of your 'scope probe to check.
Don't forget, the 'scope manufacturer intended it to be used with the earth connected - line filters connect from live to chassis via emi supression capacitors, which, even if they don't fail, will have current flowing through them, causing the chassis to be at elevated voltage.
You could use a battery-powered 'scope if you're careful.
You could also power the circuit under test via an isolating transformer, allowing almost any part of the circuit to be connected to the 'scope earth.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Thanks Sulaiman. Your comments are very helpful.
I unearthed my scope and connected the ground and the probe across the resistor that I have been having issues as I have discussed in previous threads. I checked the voltage levels prior just to see what I would be setting the scope chassis. I made sure that the voltage in the circuit stayed under 50v (it can go to 1500v).
I have to say that there was no noise at all until about 50-100mv. The wiring, which is all exposed, must have been acting like an antennae. My 60hz signal was clear as a bell.
So, how come I saw noise when I had the scope grounded and used channel 1 and 2 to measure the AC across the resistor? Again, I am not talking about a lot of noise, but it was enough to distort the signal I was measuring. Just to be clear, I checked the probes on each channel and there is no noise prior to connecting them to the circuit.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
In "invert & add" mode, connect the two probe tips together and apply a fairly large signal. In theory there should be no signal displayed on the 'scope, in practice there will be UNLESS the gain and frequency response of both channels are IDENTICAL.
There's not much that you can do about the frequency response, you can adjust the VARiable gain of one or other channels to 'null-out' the common mode signal.
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oscilloscope 101
