Discussion of grounded scope probes and how often they are damaged.
Have you ever seen an oscilloscope probe with a melted ground lead or clip? If not, you will eventually as you dig deeper into electronics design and troubleshooting.
As it turns out, in my 15 years of electrical engineering melting a scope probe is one of the more common mistakes I’ve seen among both junior as well as seasoned engineers and technicians. So why does it happen?
To understand why this happens requires a pretty good understanding of COMMON MODE and DIFFERENTIAL MODE voltage as well as a solid understanding of GROUND RETURN PATHS. For the purposes of this article however, we’ll try to keep it simple.
Let’s take a multimeter as a first example. Multimeters are ISOLATED from ground and from you circuit. This means that if you attach the negative lead of the multimeter to ANY part of your circuit, that circuit node becomes the reference for your measurement. That’s why if you measure an AA battery with a multimeter, depending how you hold the leads you could read 1.5V or -1.5V. If you connect the negative reference to the positive terminal a measurement of the negative terminal will be 1.5V LESS than the positive terminal.
Imagine for a minute that the multimeter negative lead was permanently attached to the negative side of the battery (perhaps internally or through a second ground lead). Now if we attach the negative lead of the meter to the positive side of the battery (remember we have also a permanent connection between the negative lead and the negative side of the battery) we instantly get a short circuit of the battery through the ground lead (I know this seems like a stupid thing to do but keep with me for a minute!)
The main problem is that an oscilloscope is always grounded to earth ground through the AC mains plug (unless you use some isolation techniques that we won’t get into here). This earth ground is then shared with the oscilloscope ground clips. The mistake many people make is thinking of the oscilloscope ground clip the same way we think of a multimeter negative lead. If you connect the ground lead of a scope to a circuit node that has a voltage on it with respect to earth ground you will create a short. Additionally if you connect the CH1 ground to a node on an isolated or battery powered circuit and then connect the CH2 ground to another point in the same circuit, those two points will be shorted together.
Dr. Patrick Marcus is the President of Marcus Engineering in Tucson Arizona. Marcus Engineering is a full service electronics product development company specialized in high end technologies for the Medical Device, Military, and Industrials markets. Dr. Marcus is an adjunct professor at the University of Arizona in Entrepreneurship and Engineering, and frequently mentors startup technology companies from a technical and business perspective
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