I started to use scope to check and take measuements on the circuit and components this years. I used to do it by multimeter (DMT) which don't harm any components at all, however, scope does.

Lately, I check and noticed that the scope probes' negative are connected with scope's a.c power cord plug ground pin, this means that there is no floating voltage between the earth ground and the scope circuit's common ground.

My made machine consists of two boxes, one produce high voltage power of 90VDC and another one is the logic control which comprises mainly MCU and is of low voltage, they are linked by optocoupler and their corresponding common grounds don't go to their a.c power plug's earth ground. By measuring their common ground to their earth ground voltage (i.e the floating voltage) when the machines are on, I found that they are respectively 19VAC and 1.7VAC.

Can anybody advise if there are something wrong that make MCU or IC are sometime burnt during taking measurement by the scope?

Thanks

 

First, almost any line-operated scope will have the signal (probe) ground connected to earth via the line cord ground. This means that connecting probe ground to the circuit will ground that node of the circuit. Whether this is a problem depends on the circuit. Many logic circuits have their ground connected to earth i.e. 5Vcc is referred to earth ground. Most AC line-voltage stuff are likewise referred to earth via the neutral-earth connection. Connecting a probe ground to Vcc in the logic circuit will short it. Connecting probe ground to the non-neutral side of the AC circuit will have you shopping for a new probe after blowing the end off the old one.

If you can refer your circuit to earth, do it.

To scope around a truly floating circuit (or one whose reference is at an inconvenient potential) you can use two channels in the A - B mode. Channel B becomes your reference. Other approaches include isolating the scope from ground using an isolation transformer.

In the case of a ground-referred AC circuit that you want to probe around in, the circuit can likewise be powered through an isolation transformer.

ALL of this involves some safety issues. If you don't understand so far, its time to consult a good book or get some specific help on the subject of grounding and measurement. Keep in mind that floating an AC powered scope allows its frame to become fully energized with whatever voltage you have the probe 'ground' connected to. Very hazardous if you don't know what you are doing. The same goes for floating the circuit itself.

Both of these cases can be mitigated somewhat if you use a battery powered portable scope but you still have to mind the reference potential i.e. if you connect probe ground to AC hot, the BNCs of the other channels can be hot as well.

BTW: the reason you are measuring those stray voltages is because your scope is referred to Earth and your circuit is not. Stray charges are building up the voltage on the chassis. That's one of the reasons for grounding your chassis and referring your circuit ground to Earth. As for burning circuits, if the scope and circuit have different reference potentials, connecting them will force them to the SAME reference potential. Current must flow to make this happen. If the current happens to flow through a sensitive device, zap!

 

No, as John points out, it is not the scope that is burning out your electronics. It is you.

You cannot connect the ground clip of the scope to a part of the circuit that is not ground.

You cannot measure the voltage across a resistor, for example.

The scope is designed to measure voltages with respect to ground.

 

If you want to make measurements with a scope between two points in a circuit where neither are at ground potential use a differential scope. It uses two probes. To find out more google "differential scope".

 

You can do a differential measurement with most dual-channel scopes. Select the input to add the two channels with both set at the same sensitivity, and reverse the polarity on one of the channels. The scope will then display the difference between the two probe voltages.

The common-mode rejection is not high for such a connection but it should work ok if the common-mode voltage is not too high compared to the measured differential voltage.

 

Understand the using differential measurement method in scope, thanks.

In view of which grounding configuration is better, I further investigated:

1) a miling machine's stepper controller box, its chassis connected with power cord ground but not with the circuit common ground, although the floating voltage measured is much lower around; 1VAC.
2) an ordinary desktop computer, its circuit common ground goes with power cord ground and its housing.

By consistering the immunity of interference due to the circuit exposing in an environment of electrical arcing, which configuration is better?
And, for an external signal cable connecting to the circuit, does its shielding need to connect with circuit common ground or chassis ground (power cord's ground)?

Thanks,

 

am a new student here,please can you all put me through?

 

am a new student here,please can you all put me through?

No -- only you can do that. We are here to help if you have a question.

 

Understand the using differential measurement method in scope, thanks.

Hello, just want to understand:

In view of which grounding configuration is better, I further investigated:

1) a miling machine's stepper controller box, its chassis connected with power cord ground but not with the circuit common ground, although the floating voltage measured is much lower around; 1VAC.
2) an ordinary desktop computer, its circuit common ground goes with power cord ground and its housing.

By consistering the immunity of interference due to the circuit exposing in an environment of electrical arcing, which configuration is better?
And, for an external signal cable connecting to the circuit, does its shielding need to connect with circuit common ground or chassis ground (power cord's ground)?

Thanks,