Oscilloscope maximum input voltage when scope is off ? - Page 1
Forum section: electronics / testgear

author: Jester
author_rank: Frequent Contributor
timestamp: October 14, 2017, 08:17:50 pm
content: I'm sure I'm not the only one that turns off my scope with probes still attached to a live circuit now and then.I would imagine this can't be good for the input circuit, however I would think with low enough voltages and x10 probes it should be okay.Does anyone have any comments as to when damage will/might occur?

author: Jwalling
author_rank: Unknown
timestamp: October 14, 2017, 09:37:20 pm
content: Model # and manufacturer?Measure the input resistance when off. What is it? Maybe a relay disconnects the input.

author: Keysight DanielBogdanoff
author_rank: Unknown
timestamp: October 14, 2017, 09:45:48 pm
content: I certainly wouldn't recommend exceeding the maximum for when the scope is on. Other than that, I'll have to dig a little. Interesting question!

author: nctnico
author_rank: Super Contributor
timestamp: October 14, 2017, 10:32:10 pm
content: Every oscilloscope front-end I've come across switches to the highest possible input voltage range. So if you stay within the maximum voltage for the oscilloscope input you'll be fine even when it is off.

author: Jester
author_rank: Frequent Contributor
timestamp: October 14, 2017, 11:07:18 pm
content: Quote from: Jwalling on October 14, 2017, 09:37:20 pmTek DPO 202450 M\$\Omega\$ @ BNC when off, I guess that's pretty safe.1M\$\Omega\$ @ BNC when on

author: dl1640
author_rank: Regular Contributor
timestamp: October 17, 2017, 01:15:06 am
content: my Yokogawa DL1640 measures a open-circuit when off

author: timb
author_rank: Super Contributor
timestamp: October 17, 2017, 01:49:50 am
content: Quote from: Jester on October 14, 2017, 11:07:18 pmI believe the MSO/DPO2000 Series is entirely relay free (I’ve never heard one and don’t recall seeing any on the teardown pictures I’ve seen), so the front end isn’t entirely disconnected on shutdown. So, at 50 MOhm and, with a 10x probe, you should be good to the maximum input ratings of the scope, off or on.That said, you should get in the habit of disconnecting the probes from the circuit under test when you’re done for the day. Leaving things connected overnight can cause accidents and frustration the next day (accidentally cross connecting the grounds of two loads that need to be isolated, having the probe slip off and spending 30 minutes chasing it down as a scope issue, etc.)I’ve learned it’s almost always better to start fresh each day. (This rule only applies to actual scope and multimeter probes; obviously something like a logic pod with minigrabbers hooked to the tiny pins of a SMD IC would stay attached overnight, though I would do a wiggle check and make sure they’re all still connected the next day.)

author: David Hess
author_rank: Super Contributor
timestamp: October 17, 2017, 03:51:11 am
content: For typical well designed oscilloscopes, it should not matter. The high impedance buffer is protected by passive series and shunt networks. The series network is something like a half megohm resistor so 400 volts produces 0.80 milliamps through the shunt input protection diodes and that is just not going to do anything. The series resistor has a bypass capacitor of hundreds of picofarads to preserve high frequency performance so the same voltage derating with frequency applies. Digital voltmeters have a similar protection scheme.If the 50 ohm termination occurs before a high impedance buffer, then the same analysis applies. A dedicated 50 ohm input which would be found on a high bandwidth oscilloscope could be a different matter; without bias, the base-emitter junction of the input transistor could be driven into reverse breakdown damaging it.