Advice for Beginner New to Test Equipment (Bench Supply & Digital Oscilloscope)

HojKin

Joined Apr 19, 2018
2
Hi All,

This is my first post, sincere apologies in advance if this is a common question. However, I really didn't know what to search for.

I have dabbled in electronics for few years, but recently wanted to take it more seriously. Being on a budget I purchased a cheap bench supply (A typical Chinese ebay unit, with regulated voltage an current supply), and a cheap Digital Oscilloscope (Hantek DSO5102P).

My concern is.. I am aware of the dangers of using an Oscilloscope with circuits which are ground referenced, and my understanding is that I should only use my scope with circuits which are floating. However, with my bench supply being quite cheap, or if I am ever analysing a circuit supplied by a wall plug transformer or USB etc, is there anyway I can check a circuit is appropriate before using my scope?

Also, if I am using a x10 probe, what risks would this rule out, and what should I be looking for to work safely.

I appreciate this may be a stupid question, but I'm not even sure what terms or keywords I should be looking for.

James

DickCappels

Joined Aug 21, 2008
7,565
Hi All,

This is my first post, sincere apologies in advance if this is a common question. However, I really didn't know what to search for.
(Some text removed for clarity)

My concern is.. I am aware of the dangers of using an Oscilloscope with circuits which are ground referenced, and my understanding is that I should only use my scope with circuits which are floating. However, with my bench supply being quite cheap, or if I am ever analysing a circuit supplied by a wall plug transformer or USB etc, is there anyway I can check a circuit is appropriate before using my scope?
(some more text removed for clarity)
(Emphasis in quote above added by me)

Welcome to allaboutcircuits.com, or simply "AAC".

You and your scope are more at risk if you work on floating circuits. When grounding the thing you are looking at is possible, do so unless there is a strong compelling reason to not do so.It is a matter of knowing that what you are working on is at ground potential.

Except in very rare and dangerous situation you should be measuring voltages with respect to ground, which preferably is earth ground. For measuring, for example, a small signal riding on the AC line the preferred method is the differential method demonstrated in post #2.

By the way, since you mentioned the AC line, a X10 probe might not be safe to use on your power line. I checked the maximum input for my scope and the voltage ratings of my switchable (X1-X10) probes and determined that I had to move to X100 probes to safely look at the 240 VAC mains power. Your situation may be different.

dl324

Joined Mar 30, 2015
12,929
Welcome to AAC!
I have dabbled in electronics for few years, but recently wanted to take it more seriously. Being on a budget I purchased a cheap bench supply (A typical Chinese ebay unit, with regulated voltage an current supply), and a cheap Digital Oscilloscope (Hantek DSO5102P).
IMO, in the long run, purchasing "cheap" equipment isn't worth the money. If you save up and buy a good quality piece of equipment, you less likely to be in the market for a better one when your requirements grow.

BTW, $400 isn't a cheap scope. There's a lot of used test equipment on the market. I didn't buy a scope until I had been doing electronics as a hobby for 30 years. When I did, I bought a used Tektronix 7704 analog scope with 4 plug-ins for around$100 and it's still by my bench; on a cart because it's quite large by today's standards. It would have cost around 100 times that new. It's also user serviceable. The manual includes theory of operation, schematics, and troubleshooting tips.

I am aware of the dangers of using an Oscilloscope with circuits which are ground referenced, and my understanding is that I should only use my scope with circuits which are floating. However, with my bench supply being quite cheap, or if I am ever analysing a circuit supplied by a wall plug transformer or USB etc, is there anyway I can check a circuit is appropriate before using my scope?
As a general rule, scopes should always be grounded; meaning its ground is connected to earth ground. This is done for safety and means that the probe ground clip can only be connected to circuit ground.

When you connect your scope to a circuit, you connect circuit ground to earth ground.

If you need to make measurements between two points when one isn't circuit ground, you need to use the difference function on the scope. If it doesn't do A-B directly, it will have A+B with an invert option for the B channel. If it doesn't, you need to buy a better scope.

Digital scopes have their strengths, but they also have weaknesses. Specifically, blind time (for glitch detection) and aliasing; analog scopes don't have these. I have a couple different digital scopes, but I almost always use an analog scope.

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HojKin

Joined Apr 19, 2018
2
Thanks everyone for the helpful replies. Its really appreciated.

To summarise what I have learned then...

1 - Most oscilloscopes are grounded via the mains earth, and therefore the ground lead of the probe is also referenced to ground.

2 - In cases where the circuit does not exceed the max. potential difference rating of the scope, it seems appropriate to attach the ground lead of the probe, to the ground side of the circuit? And use the probe to analyse the rest of the circuit. This applies to referenced and floating circuits.

3 - The scenario to avoid, is attaching the ground lead of the probe a live side of a ground referenced circuit!

Thanks again for all your help.

DickCappels

Joined Aug 21, 2008
7,565
You seem to understand completely.

KeepItSimpleStupid

Joined Mar 4, 2014
4,937
2 - In cases where the circuit does not exceed the max. potential difference rating of the scope, it seems appropriate to attach the ground lead of the probe, to the ground side of the circuit? And use the probe to analyse the rest of the circuit. This applies to referenced and floating circuits.
Grounded circuits generally don't need the ground clip connected. It matters SOMEWHAT with high speed signals.

If your circuit does not have a ground prong, then it's not grounded unless there is some other path.

You can always measure the potential difference between the grounds before connecting, but be aware that parasitic capcitances can upset that measurement. You can also measure resistance without the second device plugged in and not connected to anything.

Typically, you isolate the device your working on with an isolation transformer, but sometimes you can't.

Some tube TV;s had series filaments that added up to the line voltage. To avoid a shock hazzard, an isolation transformer was used,
The method is usually an isolation transformer followed by a Variac. This setup is very useful for audio amplifiers. It's almost essential.

[/quote]

3 - The scenario to avoid, is attaching the ground lead of the probe a live side of a ground referenced circuit![/QUOTE]

That's difficult to do, but you need to avoid that.

One MAJOR difficulty is repairing of switchmode power supplies. What happens is the mains are either full (120 Vac) or half wave rectified (240 Vac) so that you start out with a DC voltage that's sqrt(2)*120. This voltage is 2 diode drops above ground and that's a problem.

The problem with A-B on the scope or (A+B and invert B) is that the measurement is Pseudo-differential, not differential. You have to me able to measure A-G and B-G for this to work where G is ground. A battery operated voltmeter is truly differential.

Bench meters, however, have a maximum voltage that they are allowed to float.

The only way to safely measure differential voltages is with a differential probe with isolation. These are really expensive.

I won't get into high voltage measurements.

==

I'd like to mention that 1x probes don;t have "compensation", so a square wave won't look square. The cable and the scope both have capacitances. The trimmer adjustment on the probe uses the calibrate signal on the scope to get a square wave output. It makes an voltage divider using impedances.

The 1M input of your scope turns into 10M with a 10X probe.

Wuerstchenhund

Joined Aug 31, 2017
189
BTW, $400 isn't a cheap scope. There's a lot of used test equipment on the market. I didn't buy a scope until I had been doing electronics as a hobby for 30 years. When I did, I bought a used Tektronix 7704 analog scope with 4 plug-ins for around$100 and it's still by my bench; on a cart because it's quite large by today's standards. It would have cost around 100 times that new. It's also user serviceable. The manual includes theory of operation, schematics, and troubleshooting tips.
And its more than 30 years old and has most of its service life already behind it. I remember the Tek 7000 Series well because we had lots of them when they were new, and they weren't exactly the most reliable scopes even back then. A 7000 Series is pretty much the last thing you'd want for a beginner.

The Hantek scope the OP bought isn't a particular great scope (for example, it has only 40k sample memory which is poor), and even in that price range he probably could have got a better scope (like the Rigol DS1054z or the Siglent SDS1102X-E), however it's still a much better option for a beginner than a 30+ year old analog boat anchor which can't even offer true persistent storage.

Digital scopes have their strengths, but they also have weaknesses. Specifically, blind time (for glitch detection) and aliasing; analog scopes don't have these. I have a couple different digital scopes, but I almost always use an analog scope.
Of course analog scopes do have blind time, which is caused by the trigger re-arm.They even have been overtaken in terms of update rate (the best analog scopes get near 600k waveforms/s, DSOs can exceed 1M waveforms/s).

Aliasing is really only a problem if your digital scope is very old/crp (i.e. insufficent BW to sample rate) or if you don't know how to operate a DSO properly.

Also, your analog scope will also lie to you, i.e. like not showing signal noise that is visible on a DSO.

Plus the analog boat anchor has no storage capability (unless you have one of the rare storage scopes which come with problems of their own), no measurements, no math, no FFT, nothing. What's on the screen (which isn't much) is all you get. And it will easily miss small/rare glitches that a good DSO has no problem to catch, unless you already know they are there.

Reality is that analog scopes are dead, and they won't come back. And there's a reason why they are dead, because digital scopes are so much better in pretty much every aspect - if you can operate them, that is. Getting an analog boat anchor may be useful if it's as a vintage collectible, or if you're so broke you can't even afford to pay attention (so a free analog scope is all you can afford), but for pretty much everything else it's money flushed down a drain.

In any case, a beginner is much better off even with a cheap DSO than with any analog scope. Even if it's a Hantek.

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MrChips

Joined Oct 2, 2009
23,960
Reality is that analog scopes are dead, and they won't come back. And there's a reason why they are dead, because digital scopes are so much better in pretty much every aspect...

In any case, a beginner is much better off even with a cheap DSO than with any analog scope.
I beg to differ.

If you can afford to drop $400 on a DSO then that's great. However, paying$50 or less for a decent used analog scope is better than no scope at all.

Wuerstchenhund

Joined Aug 31, 2017
189
I beg to differ.

If you can afford to drop $400 on a DSO then that's great. However, paying$50 or less for a decent used analog scope is better than no scope at all.
Of course any scope is better than no scope, but if you can't afford the $300 which you need for a decent new DSO I'd still rather search around a bit to find one for free than spending actual money on one, because analog scopes are often given away anyways (just ask your local radio club, they might have one you can have) and the$50 could go towards saving for a DSO. And always keep in mind that many techniques that are employed with an analog scope can be wasteful or even counterproductive with a digital scope.

Having said that, you can often even get a DSO for little money, like the venerable HP 54600 Series (I think I paid some \$150 for my 54645D which is a 2Ch MSO, i.e. with Logic Analyzer). They are simple but reliable digital scopes.

However, all this doesn't change the fact that it is silly to recommend someone who clearly has some money to spend to do so for an analog scope instead of a DSO.