Fried scope?

Thread Starter

k1ng 1337

Joined Sep 11, 2020
1,012
Hi I have a pocket scope similar to the one shown below. I went to test the scope with the onboard test signal and instead of leaving the negative lead floating, it was connected to ground on the breadboard. As soon as I touched the test terminal with the positive lead the screen went black. Why did this happen? And is there an easy fix? I thought scopes were isolated for the most part.. the good news is I only paid 50 bux for the unit so my tears have already dried.

Regards,
Marke2bf19ac-cdbf-4d5a-bc48-a97c2bc0a363_1.d7ce9aa5184cc7169014e7d53d2e9526.jpeg
 

BobTPH

Joined Jun 5, 2013
9,256
Assuming your pocket scope is battery powered, I don’t see how that could happen. The probe will have a 1MΩ resistor in series, followed by protection diodes. You would need 1000s of volts to fry it that way.

Bob
 

Ya’akov

Joined Jan 27, 2019
9,264

BobTPH

Joined Jun 5, 2013
9,256
Hmm. I always assumed there a battery and the power supply was for charging, but the exploded view does not show a battery, so you are right.

Still, I would expect the supply to be isolated.

Bob
 

BobTPH

Joined Jun 5, 2013
9,256
But TS may have been using direct wiring not a peobe. Also with a X1 probe the 1M is in the scopr not the probe.
So in either case, there is a 1M resistor between whatever he touched and the rest of the scope. I still don’t see how this can fry the scope without a very high voltage.

The danger when you connect the ground clip that you are creating a short circuit, but then the fault occurs when the ground clip is connected. Connecting the probe cannot create a short circuit.

Bob
 

Thread Starter

k1ng 1337

Joined Sep 11, 2020
1,012
Was the ground for either supply connected to earth ground?

I have one of those scopes and I power it from a 9V battery.
No and nothing was on the breadboard and I was using the included probes. The only thing out of the ordinary was both supplies shared an indoor extension cord that made three turns around a lamp as to stay on the table. I tested the supplies and it's the unit. I'm going to bust it open later and I'll take a photo.
 

MisterBill2

Joined Jan 23, 2018
19,374
Even a totally isolated scope can be damaged by an excessive input voltage, and certainly NOT all scopes have an adequate input protection scheme, nor a 1 meg series input resistor.
And since we are given no hint about the input voltage, it is entirely possible that part of it is fried, or burned, or just plain zapped. Since we can only guess at what voltage was applied.
 

Thread Starter

k1ng 1337

Joined Sep 11, 2020
1,012
Even a totally isolated scope can be damaged by an excessive input voltage, and certainly NOT all scopes have an adequate input protection scheme, nor a 1 meg series input resistor.
And since we are given no hint about the input voltage, it is entirely possible that part of it is fried, or burned, or just plain zapped. Since we can only guess at what voltage was applied.
Is damage from a negative voltage possible in this scenario? I see many ICs having a min voltage of -0.3 but I'm not sure what that means, I assumed it means the chip can tolerate a tiny amount of complete current reversal through the chip, opposite to it's intended current direction. I've only dealt with negative voltages in op amp circuits that have a corresponding positive value such as a triangle wave.
 

dl324

Joined Mar 30, 2015
17,106
Is damage from a negative voltage possible in this scenario?
I was so confident that what you did couldn't be problematic that I tried with my DSO150 and it is now non-functional. My scope was operating from a 9V battery.

The test signal comes from the microcontroller and goes to J8.

1638570747307.png
I don't have time or incentive to troubleshoot now.
 

Thread Starter

k1ng 1337

Joined Sep 11, 2020
1,012
I was so confident that what you did couldn't be problematic that I tried with my DSO150 and it is now non-functional. My scope was operating from a 9V battery.

The test signal comes from the microcontroller and goes to J8.

View attachment 254158
I don't have time or incentive to troubleshoot now.
Now I feel like I'm only out $25 instead of $50 haha

Can someone explain the parameter vcc having a maximum rating of -0.5v?
 

Attachments

SamR

Joined Mar 19, 2019
5,078
Thought I remembered that it has a 50V input max. It does! Here's the PDF...
How to Use (circuitspecialists.com)
I'm on my second one due to some physical internal mismatches between PCBs and case problem that at first was intermittent and then it quit. It is not very well constructed and at best delicately built and not at all physically shock proof. Don't ever drop it! I use a small 9V battery box with switch Velcroed to the back of it for powering it.
 
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