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Probing transistors

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shadowless:
I am trying to probe the transistors of the circuit at the collector and emitter with my oscilloscope. Every time I touch the base with the ground clip the transistor turns on, making it impossible to probe.

That didn't happen with my portable handheld scope so not sure what is the cause.

ivan747:
What kind of power supply are you using? Does it has an Earth pin (the third, round pin)? Are you connected to a computer?

Psi:
yeah, sounds like the thing you are probing has a mains earth connection.
Therefor when you connect the scopes ground connection clip to a part of the circuit you are effectively shorting that part to ground (or near it). In the case of a transistor base, this will connect the base to a point that may have voltage with respect to the emitter and switch it on or off. (Device ground and oscilloscope ground, although electrically connected, may have a few volt difference between them).

Since your old scope was handheld it was isolated so didn't have this issue.
The solution is to put the device your testing on an isolating transformer. Then you can ground whatever point you need.

Or you can measure everything with respect to ground and compare readings that way.
Then you wouldn't need to ground the base.

vk6zgo:

--- Quote from: Psi on August 03, 2011, 04:03:46 am ---yeah, sounds like the thing you are probing has a mains earth connection.
Therefor when you connect the scopes ground connection clip to a part of the circuit you are effectively shorting that part to ground (or near it). In the case of a transistor base, this will connect the base to a point that may have voltage with respect to the emitter and switch it on or off. (Device ground and oscilloscope ground, although electrically connected, may have a few volt difference between them).

Since your old scope was handheld it was isolated so didn't have this issue.
The solution is to put the device your testing on an isolating transformer. Then you can ground whatever point you need.

Or you can measure everything with respect to ground and compare readings that way.Then you wouldn't need to ground the base.

--- End quote ---

As this is the standard method of probing within circuits with an Oscilloscope,it might be a good idea to become used to it!
Even with a 'scope with an ostensibly isolated ground connection clip,there may be enough capacitance to the common ground point of a circuit to upset operation in some cases.

VK6ZGO

tekfan:

--- Quote from: shadowless on August 03, 2011, 02:56:19 am ---I am trying to probe the transistors of the circuit at the base and emitter with my oscilloscope. Every time I touch the base with the ground clip the transistor turns on, making it impossible to probe.

That didn't happen with my portable handheld scope so not sure what is the cause.


--- End quote ---

Does your oscilloscope have two inputs? If it does you can add both channels together and invert one of them. You can then think of the probe on the innverted channel as the - lead and the probe on the non inverted channel as the + lead. This way you don't need to hook up the ground clip anywhere since you're doing a differential measurment. It may help to reduce noise if you hook both ground leads of the probes to a common ground on your circuit.

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