The oscilloscope probe is burning red

Nubility

The oscilloscope probe is burning red [Copy link]

 

The single-ended probe of the oscilloscope has been burned red in two situations:

① Use a single-ended probe to measure the differential clock. The ground spring is placed on one end of the P/N pin and the probe is placed on the other end. The ground spring will immediately heat up and turn red.

② Use a single-ended probe to measure a single-ended signal. Because there is no GND contact point around, place the ground spring on the power pin, and then use the probe to touch the signal to be measured. The ground spring will also heat up and burn red.

Result: The surface of the ground spring is burned gray and loses its elasticity. However, the oscilloscope, probe, and the circuit under test are not damaged at all. Why?

maychang

If the grounding spring is red, the temperature is at least 800 degrees Celsius. The material of the spring is usually carbon steel, which cools slowly after high temperature, so it loses its elasticity. The heat treatment is called annealing.

However, to heat the ground spring to red, at least hundreds of watts of power are required (just compare it to the power of a soldering iron). These hundreds of watts of power can only come from your circuit. Look at where this heating power comes from.

Nubility

maychang posted on 2024-6-29 14:00 If the grounding spring is red, the temperature is at least 800 degrees Celsius. The material of the spring is usually carbon steel, which cools slowly after high temperature, so it loses its elasticity. The heat treatment...

The key point is that except for the spring being burned and annealed, the oscilloscope and probe are not broken, the circuit is not broken, and even the software has not been restarted. I can't figure out why this is

wangerxian

How can this happen with an oscilloscope? What circuit?

maychang

Nubility posted on 2024-6-29 14:15 The key point is that except for the spring being burned and annealed, the oscilloscope and probe are not broken, the circuit is not broken, and even the software has not been restarted. I can't figure out why this is the case

[But the oscilloscope, probe, and the circuit under test are not damaged at all. Why is that? ]

The guess is that the "ground" on your circuit board is electrically connected to the mains, and your oscilloscope housing is connected to the protective ground. The current flows from your circuit board through the spring, the outer shielding layer of the probe cable, the oscilloscope housing, and the protective ground wire, but not through the probe.

In fact, ordinary probes can generally withstand mains voltage, and the general voltage resistance is 750V or 1000V.

Nubility

wangerxian posted on 2024-6-29 14:37 How can this happen with an oscilloscope? What circuit is this?

This phenomenon is likely to occur when the oscilloscope spring probe is placed on a place other than GND. As mentioned above, the spring is placed on 3.3V, and then a single-ended signal at 3.3V level is measured, and it burns red. I don't know if it is because of DC coupling.

maychang

Nubility posted on 2024-6-29 14:15 The key point is that except for the spring being burned and annealed, the oscilloscope and probe are not broken, the circuit is not broken, and even the software has not been restarted. I can't figure out why this is the case

Another situation is that your oscilloscope case is already connected to the "ground" of the circuit board. At this time, if the spring touches the power supply terminal on the circuit board, the power supply on the circuit board will supply power to the spring, the probe cable shield, the oscilloscope case, and the circuit board ground wire, heating the spring.

littleshrimp

I have encountered the same problem. The GND of the oscilloscope is connected to the ground of the socket. If your device is grounded, it is equivalent to connecting the GND of the oscilloscope and the GND of the device. Use the spring to contact the power supply of the device, and the current will flow through the spring to the ground terminal of the oscilloscope. Because the oscilloscope will not be damaged if it does not pass through other circuits of the oscilloscope. The spring turns red because the resistance of the spring is much greater than the resistance of the shielding wire of the probe, so the spring will be "lit up" and the probe wire will be fine. If an adjustable power supply is used, the negative pole of the output can be disconnected from the ground terminal if conditions permit, so that the adjustable power supply is isolated from the oscilloscope, and it doesn't matter where you touch it.

littleshrimp

maychang published on 2024-6-29 14:00 If the grounding spring is red, the temperature is at least 800 degrees Celsius. The material of the spring is usually carbon steel, which cools slowly after high temperature, so it loses its elasticity. The heat treatment...

I have tried it, and the resistance of the oscilloscope spring is very large, just like a filament, and it can be lit up with a power of more than ten watts.

Nubility

littleshrimp posted on 2024-6-29 15:10 I have encountered the same problem. The GND of the oscilloscope is connected to the ground of the socket. If your device is grounded, it is equivalent to the GND of the oscilloscope and the GND of the device...

But this does not explain why the probe will burn red when connected to the differential

littleshrimp

Nubility posted on 2024-6-29 15:42 But this doesn't explain why the probe will burn red when connected to the differential

I really can't explain it. If you connect the spring to any of the differential lines, it's fine. Will it heat up if you connect the oscilloscope probe to the positive and negative differential lines at the same time?

Nubility

littleshrimp posted on 2024-6-29 19:08 It really can't be explained. It's okay if you connect the spring to any line of the differential, as long as you connect the oscilloscope probe to the positive and negative lines of the differential at the same time...

As long as the ground spring point is not in the GND position, other signals may appear.

littleshrimp

Nubility published on 2024-6-29 19:41 As long as the grounding spring point is not in the GND place, other signals may appear

It is probably a common ground. The driving capability of other signals is relatively large. Try using an isolated power supply for the device under test.

lanhua

"The single-ended probe of the oscilloscope has burned red in two situations:"

The oscilloscope housing is connected to the probe housing. Generally, the third wire of the power cord should not be connected to the plug.

hahajing

If there is no tripping, it means that one board has a large common mode voltage or there is a voltage difference on both sides.

秦天qintian0303

How much power is this? Is the probe active or passive?

Nubility

Qintianqintian0303 posted on 2024-7-1 10:26 How much power does this have? Is the probe active or passive?

Passive, single-ended probe

H12315

Cut off the ground wire of the oscilloscope and try again. Is the circuit board hot grounded?