help picking a scope - Page 1
Forum section: electronics / beginners

author: lordulrich
author_rank: Newbie
timestamp: January 14, 2013, 11:09:30 pm
content: Hi, I hate asking these sorts of questions online, but I am a bit over my head. I am looking to make my first scope purchase, and 2 options appeared on cragslist the last few days. A tek 2235 for $125 and a tek 5111A Mainframe Oscilloscope,with Two 5A15N 2MHz Single Channel Amplifiers and One 5B10N Time/Base Amplifier for $50. I did some searching but can't find alot that means much to me (I am just getting started). It would be for hoby use. I seem to be looking at switch mode power supplies alot latey, and microcontrolers. Both scopes are local, a anything i should check before picking one up?Thanks

author: PaulAm
author_rank: Frequent Contributor
timestamp: January 14, 2013, 11:56:58 pm
content: The 5111a is an analog storage scope, but the bandwidth is only 2MHz. Most of the 5000 series were fairly low bandwidth, but one or two got to 50 MHz.The 2235 is an analog scope good to 100+MHzIf you're looking at smps, the 5000 won't cut it. If it's 100% functional, that's an Ok price, not great on the 2235. They are sometimes prone to failed caps, but are repairable and service manuals are available on the net.If you can dicker down to $100 and it doesn't have any problems, I'd go for it.Somebody will probably say you should get a DSO. They're nice; I have a couple, but you can do a lot of good work at hobby level with an analog scope.

author: AndyC_772
author_rank: Super Contributor
timestamp: January 15, 2013, 09:42:51 pm
content: If you're going to be probing switched mode power supplies, consider that they'll be switching at (say) 500 kHz. Within each cycle there's plenty to look at - duty cycle, the shape of the gate drive waveform, leading/trailing edge spikes, ripple, feedback and error compensation... the list goes on. To get an accurate picture of all these signals, a 2 MHz scope definitely won't do - you'll be much better off with a 50 MHz.Fortunately, a (working!) power supply is a fairly repetitive, predictable beast, so you don't need the single shot capture capability of a DSO. If you have the choice of a 100 MHz analogue scope (with compatible 10:1 probes!) then I'd go with that option.

author: EEVblog
author_rank: Unknown
timestamp: January 15, 2013, 09:52:12 pm
content: When working with switchmodes, spending a few hundred bucks on a high voltage differential probe is a wise investment.Even an ebay cheapie if you have to:http://www.ebay.com.au/itm/New-Professional-Differential-probes-DC-25Mhz-ADP25-Max-Voltage-1300V-1000V-RMS-/380411931457?pt=LH_DefaultDomain_0&hash=item5892516b41&_uhb=1#ht_2207wt_1347Dave.

author: lordulrich
author_rank: Newbie
timestamp: January 15, 2013, 11:24:42 pm
content: I suspected the 2235same was the better option. I I will see if i can negotiate the price a bit.Dave, what does the differential probe do for me (or gain me), I appoligize if there is a video about the topic, but I am using a phone foe internet access until the weekend, and your videos are too much for my phone.Thank you all for the advice.

author: Unknown
author_rank: Unknown
timestamp: January 15, 2013, 11:37:31 pm
content: A scope is a grounded instrument. The ground clip is always connected to the earth lead in the mains plug. Disconnecting this earth connection is a safety hazard. Neither the live nor the neutral wire from the mains socket should be shorted to ground, so you can't clip the ground clip anywhere on the primary side of a power supply (the secondary side is isolated by the transformer). The primary side is not very interesting for a linear power supply, but it's where the switching takes place in most switching (eg. flyback) power supplies. There are some tricks around this like isolation transformers or pseudo-differential mode (which uses two channels and has poor noise rejection), but a differential probe is by far the best solution.I think Dave's "how not to blow up your scope" video covers this topic.

author: EEVblog
author_rank: Unknown
timestamp: January 16, 2013, 01:38:56 am
content: Quote from: lordulrich on January 15, 2013, 11:24:42 pmIt helps prevent you from killing yourself!Standard scope probes are mains earthed referenced, and are not insulated for high voltage work. Put the ground lead on the wrong part of your switchmode supply and you will blow up your scope and/or yourself. A diff probe means you can probe anything in a switchmode safely, some things you can't do a standard scope probe.Dave.

author: mariush
author_rank: Super Contributor
timestamp: January 16, 2013, 02:24:12 am
content: I think it would be cheaper to just get a 12v car battery and a small inverter (or a small UPS disconnected from the mains) and have the whole system floating, thereby solving the issue.Differential probes are usually more expensive than such solution .

author: marshallh
author_rank: Unknown
timestamp: January 16, 2013, 03:30:27 am
content: You can also use two probes both referenced to actual ground, and use your scope's math function to display the difference.But you must be sure your grounds stay that way.

author: w2aew
author_rank: Super Contributor
timestamp: January 16, 2013, 04:07:53 am
content: In addition to Dave's excellent video, I did a video on differential probes a few days ago:

author: AndyC_772
author_rank: Super Contributor
timestamp: January 16, 2013, 07:44:02 am
content: Quote from: mariush on January 16, 2013, 02:24:12 amYikes! Bad idea, doing that means the whole scope could be live. Cheap, but dangerous.

author: grumpydoc
author_rank: Super Contributor
timestamp: January 16, 2013, 08:45:53 pm
content: QuoteAs long as the 'scope remains grounded this (running the SMPS from anisolatedUPS) should be safe - it's the same as having an isolation transformer. As the SMPS is now floating you can ground any arbitrary point without problems (though you should probably stick to "low impedance" paths within the power supply). You can also touch anysinglepoint in the supply without getting a shock.Note that a UPS or isolation transformer willnotprotect you if you bridge two points in the circuit - in fact it will increase the danger in that scenario because there's no RCD/GFI to protect you.I must admit, if I have to deal with this sort of thing I a) use an isolation transformer and b) set things up with the power off (and any large or high-voltage caps discharged) - even if you've isolated the thing from ground waving a 'scope probe at it is asking for the probe to slide off the test point and short something.If you absolutely must prod at the thing with the power on then it's also not bad advice to a) keep one hand in you pocket, normally your left, which reduces the chance of current passing across your chest and b) don't work alone.

author: lordulrich
author_rank: Newbie
timestamp: January 17, 2013, 12:12:20 am
content: Ok, picked up the 2235 today got it down to $90 although it needs a new power plug (the ground pin is missing). Will have to pickup a differential probe, as not killing myself is something i get behind.I completely agree about being careful with mains power. Checking an outlet is about all i will do without the breaker triped, outside my home lock-ouy-tag-out always. It took me a while to get used to pokeing at live low voltage circuits.Thanks for the advice.