First oscilloscope - connection and probe questions - Page 1
Forum section: electronics / beginners

author: danderson
author_rank: Contributor
timestamp: January 09, 2014, 04:26:40 am
content: Hi all,I finally followed Dave's advice, and nabbed a $60 analog scope on Ebay. It's a B&K Precision 1335A: dual-channel, 35MHz (usable to 50MHz with little distortion, so says the manual), input impedance 1M shunted by 22pF.However, the scope comes without probes, so I need to procure some. And I'm not sure what I need to consider in order to get ones that'll work. Notably, what frequency and impedance should I look for in the probes? Should they match the scope's input specs? Exceed them? Does it even matter?Amazon has these:http://www.amazon.com/Hantek-100MHz-Oscilloscope-Probes-Accessory/dp/B0030KWM30/that I can get delivered just in time for weekend fun, but the probe's bandwidth far exceeds my scope, and the impedances don't seem to match. Does it matter? Really I want to know if those specific probes would work properly, but I'd also like to understand why these things do/don't matter.The other question I have is related to grounding. My house currently has 1950s vintage US wiring, i.e. an ungrounded system. I only have phase and neutral running around. I'm talking to an electrician about how much it'd hurt to get everything grounded anyway, but in the meantime, how much will it suck if I use the scope without an earth connection? Am I to expect additional noise, damage to the scope, certain death?Thanks in advance!

author: Maxlor
author_rank: Frequent Contributor
timestamp: January 09, 2014, 06:22:02 am
content: Those probes should work. Higher bandwidth in probes than the scope isn't a problem, indeed it's usually a good thing.As for the impedance: you're talking about the 10M/1M and capacitance figures? The scope will handle it just fine. The probes are adjustable, that's what's really important (and almost always a given,) you'll have to adjust them before first use to fit your scope. Should only take a second.

author: danderson
author_rank: Contributor
timestamp: January 09, 2014, 07:37:15 am
content: Yes, I was talking about the capacitance figures associated with the 10M/1M values. I've been watching the "Scopes for dopes" talk, and I've only just reached the part about calibrating the probe compensation on 10x probes... And indeed, the scope's internal capacitance falls in the compensation range quoted for the probes.Fortunately, I also have access to an electronics lab which has calibrated scopes and function generators, so I'll have a bunch of "independent sources" to double-check what my scope says about various inputs.Thanks!

author: vk6zgo
author_rank: Super Contributor
timestamp: January 09, 2014, 08:01:18 am
content: Quote from: danderson on January 09, 2014, 04:26:40 amThe probe questions have already been adequately covered,so I'll confine my comments to this paet of your question.Although it's a bummer that you don't have an Earth connection on your power points,it isn't the end of the world.People have used Oscilloscopes in such situations in the past without injury.If the 120Vac wiring inside your 'scopedoes nothave a fault condition that connects the Active (HOT)conductor to the metalwork of the 'scope you are safe under normal circumstances.If you had such a fault,& touched both the Oscilloscope & a nearby Earthed object,such as a tap,sink,etc,you would get a dangerous shock.Note:Even if there is no connection between Neutral & Earth in your house cabling,that doesn't mean that there is not one somewhere else---there almost certainly will be.MEGA-NOTE!!!!:-If you decided to measure the 120vac out of your power socket,with your 'scope,you have a 50:50 chance that you will create the fault condition referred to above.Dont do it!!!In both of the situations referred to above,where there is a connection between Active & the 'scope metalwork,the result in the more normal situation where the metalworkisearthed via the power point,would be a short circuit between Active & Neutral,large "bang" & the associated house fuse or circuit breaker will blow or trip.This is the reasoning behind the concept of providing a "protective earth".Any fault condition connecting the metalwork of an appliance & the Mains Active line will result in an overload on that line,& not an electric shock to the user.There is no fuse in the Earth cabling,so the metalwork of protected device cannot rise above Earth potential.

author: danderson
author_rank: Contributor
timestamp: January 09, 2014, 08:32:11 am
content: Quote from: vk6zgo on January 09, 2014, 08:01:18 amGotcha, thanks for the detailed explanation. My plans for this scope only go up to about 12V circuits at most, and more commonly 5V and below, so barring faults in the scope itself, I'm not expecting to be deliberately wiring the scope to mains via the probes.Agreed on the connection from neutral to earth... When we had the main electrical panel replaced, a solid earth connection was installed via multiple ground rods. So, neutral return is definitely grounded at the panel. All the outlets without equipment ground also have ground- and arc-fault breakers in the circuit, as required by the local electrical regs: if you want a three-prong outlet where the third prong isn't grounded, it has to be a GFCI outlet.Given that I do have access to an EE lab with proper grounding, it sounds like I'd be wise to check for gross conduction faults there, just in case, before bringing this scope home.Thanks again!

author: w2aew
author_rank: Super Contributor
timestamp: January 09, 2014, 03:39:54 pm
content: Quote from: danderson on January 09, 2014, 07:37:15 amI'm glad that you're watching the "Scopes for Dopes" class that I taught. If you want more detailed info about the probes, compensation, etc., check out this video:

author: ConKbot
author_rank: Super Contributor
timestamp: January 09, 2014, 05:29:42 pm
content: Quote from: vk6zgo on January 09, 2014, 08:01:18 amThe ground and neutral have to be bonded at the load center/panel in the US. That's the only place they can be bonded together too. If they aren't bonded, then you're relying on the impedance of the ground between your panels ground and the ground on the pole transformer to be low enough to sink enough fault current to trip the breaker. I'm not sure if the UK does, but I know a lot of EU countries use a whole house RCD/GFI, so something like that may be acceptable there. Though it would suck having to trouble shoot a fault in the dark because it just keeps taking out the main breaker. If they are bonded in multiple places, then neutral current can flow over the grounding conductor and cause the voltage to rise.In older wiring, the conduit can be used as a grounding conductor. But this relies on all those joints being sound.

author: IvoS
author_rank: Frequent Contributor
timestamp: January 09, 2014, 08:37:02 pm
content: Quote from: ConKbot on January 09, 2014, 05:29:42 pmIn US, neutral and grounds can be bonded in: main distribution panel, meter can, main disconnect or service drop. Talking about residential.