Author Topic: EEVblog #279 - How NOT To Blow Up Your Oscilloscope!  (Read 7825 times)

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Offline tazelikechickn

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EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
« on: October 03, 2013, 10:59:58 am »
Heys Guys,

I know this topic has been discussed before, but after reading the comments, I couldn't find the answer I was looking for.

I am repairing a SMPS for a battery jump pack for vehicles. I have found a Mosfet blown & so replaced that and hooked up my Rigol oscilloscope to check the duty cycle. Before I did that, I watched Dave's video on how to hook up the scope. The power supply does not have an earth pin and therefore is isolated. So I thought, "not probs - I'll just hook the earth clip on the scope on the negative on the main filter cap and then just check for duty cycle on the PCM IC. But, all that did was blow the fuse and 2 of the rectifier diodes! Am I doing something wrong? I have used the scope several times (I haven't had it too long) on several amplifiers that I repaired (they had earth pins & so I earthed thru the chassis).

After the video, I walked into the shed with more confidence, but after blowing up the fuse and diodes, I have more questions than answers.

Can anyone help?
 

Offline Rerouter

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Re: EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
« Reply #1 on: October 03, 2013, 11:05:39 am »
unless it has a transformer in it it may not be isolated,

most house holds have neutral and ground bonded at the main breaker box, so you applying ground after a bridge rectifier would be the same as 2 diodes in the bridge rectifier failing shorted, and any positive peak would mean massive sods of current coming in from the active, through one diode drop then back to mains ground, then at the bonding point back to mains neutral.
 

Offline tazelikechickn

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Re: EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
« Reply #2 on: October 03, 2013, 11:14:28 am »
Thanks for your reply.

I originally hooked up the earth clip to the diode of the bridge rectifier and blew the fuse (be patient with me - I am learning) I realised that I was hooking an earth to the AC input - Idiot!

It has been suggested that I should have an isolating transformer to sort the problem, but was hoping that I was just doing something wrong. The SMPS has a transformer (is that what you meant?) Is there any way that I can hook up my Rigol and check for Duty Cycle or am I barking up the wrong tree?

Thanks for any help you can provide.
 

Offline Rerouter

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Re: EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
« Reply #3 on: October 03, 2013, 11:26:41 am »
there are a few different types of switchers, it could be a big inductor on a bobbin, a flyback transformer, or a switchmode transformer with a feedback coil / other possibilities,

if you do want to measure the duty cycle, grab a second scope probe and do the poor mans differential by subtracting one channel from the other, this is the safest way, an isolation transformer has its own hassles and can be to a similar amount dangerous to your health,
 

Offline tazelikechickn

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Re: EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
« Reply #4 on: October 03, 2013, 12:16:51 pm »
I have checked for frequency with a digital multimeter.

I don't have a schematic for the power supply, but from what I can see it is a SM transformer with an opticoupler to provide feedback for the PWM IC.

I have attached a photo that might make it clearer.

Not sure what you mean in the second comment about  the second probe. Where would you place the probes? Are you suggesting that you just use the probes themselves without the earth clip.

Sorry to sound like an amateur, I just want it clear in my head before I do any further testing.

Once again, thanks for your help.
 

Offline Rerouter

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Re: EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
« Reply #5 on: October 03, 2013, 01:34:15 pm »
yes, you leave the earth clips off, and hold one probe to your circuit ground and the other to your circuit signal, you then use the subtract function on the channel switch which will give you A-B which is your voltage signal, with no ground loop risk or shorts to mains, this is said to be a differential probe as your only measuring the difference between 2 points,
 

Offline tazelikechickn

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Re: EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
« Reply #6 on: October 04, 2013, 09:09:39 am »

Thanks Rerouter,

You have made it nice and clear for a newbie like myself.

I use a Pico oscilloscope in the Auto industry all the time, but I have had to rethink a few things dealing with the Rigol.

Out to the shed with me!

This should save me a few extra fuses!

Once again, thanks for your patience and help.
 

Offline tazelikechickn

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Re: EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
« Reply #7 on: October 04, 2013, 09:36:14 am »
Just one other question, can you use this method to read a Duty Cycle or Frequency signal? It appears to only read voltage difference.
 

Offline Rerouter

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Re: EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
« Reply #8 on: October 05, 2013, 05:56:41 am »
it comes down to the software if its a DSO, in reality it should work just like a single scope probe, frequency, duty cycle, standard deviations of those if it has some math, etc,
 

Offline lemon

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Re: EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
« Reply #9 on: October 05, 2013, 08:50:13 am »
Just one other question, can you use this method to read a Duty Cycle or Frequency signal? It appears to only read voltage difference.

Yeap, you can.
At the following image the red one is the noise (both tip and gnd clip to chassi = circuit ground), the yellow is the duty cycle of an diode.



The second probe (yellow graph) must to be without any ground clip joined like the following photo (for the difficult areas, you must isolate the probe tip and ground by an isolated tape or shrinkable)
 

Offline tazelikechickn

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Re: EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
« Reply #10 on: October 06, 2013, 04:07:43 am »
Hey Lemon,

Thanks for that. I think one of my main problems is that my optocoupler is not working and maybe I am not getting a duty cycle reading.

I have a Rigol 1052, that I have used on other items and have been able to repair them, but since I blew the fuses and diodes, I lost a bit of confidence. Great news that I can use the scope without the earth lead.

Hopefully when I get the optocoupler, I should be able to see the signal.

Once again, thanks for your interest in helping a newbie.
 

Offline tazelikechickn

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Re: EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
« Reply #11 on: October 06, 2013, 08:27:37 am »
Hey Lemon,

I was thinking (not a good sign!) Do you need to use the second probe and earth clip to an earth point or can you just use the single probe without the earth clip to pick up a duty cycle?

When I used the earth clip anywhere, with nothing else attached, it blew the fuse and a diode.

So just to get it straight when I get the new optocoupler in place - I can hook up just a single probe with out the earth clip to pick up the signal. Where does the oscilloscope pick up the earth to complete the circuit?

PS. Thanks for the photos - It makes it a lot clearer!
 

Offline Rerouter

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Re: EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
« Reply #12 on: October 06, 2013, 10:11:33 am »
if the device uses mains earth for its chassis, you can put the earth clip to that, equally if the power supply was fully isolated on its output, you could hook up the ground clip to any point and measure against it,

however what happens with the poor mans differential is as both probes have a 1Meg ohm impedance it will slowly drag the isolated output down to something close to mains ground that the scope can happily measure without a ground clip, and for anyone not fully understanding of the risks is a safer method,

any benefits of a ground clip are circumvented by the use of the second probe (for speeds under 1MHz) as if there is some spike that raises the isolated supply from mains ground you still have A-B, both channels may increase in voltage but as your only looking at the difference between them you see no additional noise or spikes,
 

Offline CHexclaim

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Re: EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
« Reply #13 on: October 07, 2013, 05:00:58 am »
Hello everyone.

I have understanding on the ground loops that may appear so I would like your opinion on how I sometimes deal with this.

I tend to isolate from earth the instrument and not the device under test. I understand that the scope, for example, may require earth grounding for the filters in it's own power supply, but also does the unit under test.

I know that you must be careful and aware of the safety issues involved, but apart from that, is it possible that the scope can get damaged if it is isolated from ground during testing?

Thanks, Charlie.
 

Offline andyb510

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EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
« Reply #14 on: January 13, 2014, 02:53:36 pm »
Hi all, I am new to this blog and to Electronics. Watched this video numerous times. I was wondering why could we not have an inline fuse on the probe earth lead? If we had a short it would save the probe. Or would this have an effect on your readings?

Thanks

Andy


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Offline c4757p

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Re: EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
« Reply #15 on: January 13, 2014, 03:01:12 pm »
I was wondering why could we not have an inline fuse on the probe earth lead? If we had a short it would save the probe. Or would this have an effect on your readings?

It would affect the readings. At high frequencies, even the earth lead itself has too much inductance, and low-inductance probing techniques are necessary. A fuse would severely limit the frequency capabilities of the probe.

Once you get used to the necessary earthing of the probes, it's really no longer an issue. I can't recall ever shorting something out with the ground lead.
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Offline andyb510

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EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
« Reply #16 on: January 13, 2014, 04:33:28 pm »
Thanks for your reply, much appreciated.


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Offline electronics man

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Re: EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
« Reply #17 on: January 15, 2014, 09:25:25 pm »
Why can't they have a lower bandwidth probe whith an optional fuse for those who are not working on high frequancy and wants to be safe
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Offline c4757p

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Re: EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
« Reply #18 on: January 15, 2014, 09:44:23 pm »
Because ground connections that can suddenly become invisibly disconnected with no notice at all are typically considered a bad idea.
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Offline Rerouter

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Re: EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
« Reply #19 on: January 16, 2014, 06:17:59 am »
if your not working on high frequency and want to be safe, buy a handheld scopemeter, or battery powered DSO, that way the scope can be floated without any grounding issues, or better yet a proper differential probe (for low frequency you could even make one, but that's up to the individual)
 

Offline electronics man

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Re: EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
« Reply #20 on: January 16, 2014, 07:13:02 pm »
Scopemeters often cost more than regular cheep entry level scopes and diferential probes are expensive so are there no other options apart from isolating you devise under test
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