High Voltage Oscilloscope Probe...

Out of curiousity:

--My scope has a max voltage rating of 300V at the inputs... I was at the local electronics store yesterday, and they had some probes 100mhz probes rated at 600V... If I were to use this probe, can I effectively test circuits up to this voltage (really looking to test tube circuits up to say 500V) on my scope without harm, or is that simply the max failure rating for the probe itself?

--Can I use a 100mhz probe with a 20mhz scope? I'm guessing yes, the probe can be used on all scopes up to 100mhz...

--The current probe for my scope (unfortunately I don't have the model of the scope, though I could check tonight) does not have a ground clip, rather a terminal on the scope itself... If the probe I'm lookin at purchasing has the ground clip, must I tie that to the ground reference on the scope, and carry that out to the circuit as ground?

For looking at voltages higher than the scope rating, you want a probe to have attentuation when loaded by the scope input. A division ratio of 10:1 is typical; 100:1 exists. But beware! If you use a.c. coupling, most scopes will lift the nominal 1Mohm termination, and you will now be charging the input cap up to the full d.c. in your circuit. It may not like that.

The voltage rating of the probe probably pertains more to what you can probe without getting a shock or excess voltage across the internal components of the probe. It's the user's responsibility to figure out what he/she is hitting the scope with.

Just to be sure I understand correctly -- if I use a properly compensated 10X probe, the scope input would actually see 35V worth of a 350V source under test?

I'm not sure I understand your warning regarding AC coupling though... Does this involve testing a DC voltage for AC ripple; the full DC voltage reaches the input regardless of probe attenuation?

Though it must be frustrating to constantly answer rookie questions, I really appreciate the first hand responses on this board... I've been trying to source some good info for learning the in's and out's of scope work... Just seems some of these basic things aren't covered, or are done so in such a way that I'm not connecting the dots mentally...

Not to worry about newb questions.

Yes, IF you are using d.c. coupling at the scope, which is typically switch-selectable, then your 10Mohm scope probe loaded by the 1Mohm typical scope input resistance winds up presenting the input amp of the scope with 1/10 of the voltage at the probe tip. The probe has a 9Mohm R plus some frequency compensation networks in it to make this work.

But if you select a.c. coupling, as might be handy to look for ripple riding on the d.c. as you mention, then there is no 1Mohm there anymore, and one side of the cap charges to the full voltage. If the cap is rated for that you should be o.k. If not, in a world of hurt as the cap breaks down and likely blows out the scope front end.