HV scope probe question - Electronic Circuits Projects Diagrams Free

Oznog · 13th October 2005, 08:59 PM

I have a fluke 87 scope and I need to measure voltage across a 450v cap.

The scope has a 300V rating on the inputs. The probes, however, have a 600V rating.

These are 10:1 probes though. So doesn't this mean that the scope will see only 45V, thus not exceeding the ratings for either the probe or scope?

I just want to know if I'm missing something, that's a nice expensive scope and I'd hate to toast it due to any misunderstandings.

Dean Huster · 14th October 2005, 12:57 AM

You're good to go, Oznog. The probes will handle the voltage and (along with the 1M ohm scope input impedance) will knock the cap voltage down by a factor of 10, plenty safe for the scope. That's one of the reasons for using the attenuator probes. Others are higher input resistance for lower resistive circuit loading; lower capacitance for lower reactive circuit loading; resultant higher bandwidth capabilities for the scope. The only disadvantage of an attenuator probe is the loss of system sensitivity, and that's not usually a big problem in most instances.

Dean

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13th October 2005, 08:59 PM	(permalink)
Oznog	HV scope probe question I have a fluke 87 scope and I need to measure voltage across a 450v cap. The scope has a 300V rating on the inputs. The probes, however, have a 600V rating. These are 10:1 probes though. So doesn't this mean that the scope will see only 45V, thus not exceeding the ratings for either the probe or scope? I just want to know if I'm missing something, that's a nice expensive scope and I'd hate to toast it due to any misunderstandings. __________________ I thought what I'd do was I'd pretend I was one of those deaf-mutes.

14th October 2005, 12:57 AM	(permalink)
Dean Huster	You're good to go, Oznog. The probes will handle the voltage and (along with the 1M ohm scope input impedance) will knock the cap voltage down by a factor of 10, plenty safe for the scope. That's one of the reasons for using the attenuator probes. Others are higher input resistance for lower resistive circuit loading; lower capacitance for lower reactive circuit loading; resultant higher bandwidth capabilities for the scope. The only disadvantage of an attenuator probe is the loss of system sensitivity, and that's not usually a big problem in most instances. Dean __________________ Dean Huster, Electronics Curmudgeon Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines). R.I.P.