Probing transistors - Page 1
Forum section: electronics / beginners

author: shadowless
author_rank: Regular Contributor
timestamp: August 03, 2011, 02:56:19 am
content: 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.

author: ivan747
author_rank: Super Contributor
timestamp: August 03, 2011, 02:59:38 am
content: What kind of power supply are you using? Does it has an Earth pin (the third, round pin)? Are you connected to a computer?

author: Psi
author_rank: Super Contributor
timestamp: August 03, 2011, 04:03:46 am
content: 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.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 03, 2011, 08:42:20 am
content: Quote from: Psi on August 03, 2011, 04:03:46 amAs 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

author: tekfan
author_rank: Frequent Contributor
timestamp: August 03, 2011, 11:43:21 am
content: Quote from: shadowless on August 03, 2011, 02:56:19 amDoes 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.

author: shadowless
author_rank: Regular Contributor
timestamp: August 03, 2011, 05:20:15 pm
content: I am running differential voltage in the circuit so it is not possible to ground it... as the 'ground' is floating.Wanted to see the waveform in the transistors. Does it mean that i can probe the transistor emitter but connect the ground to say a -ve of a battery in the circuit instead of the collector of the transistor, would that work?

author: Unknown
author_rank: Unknown
timestamp: August 03, 2011, 06:17:58 pm
content: If the ground of the DUT is floating, connecting shouldn't make much difference (apart from parasitics). If it is not, you can power the DUT from a floating power supply (battery, lab power supply). If it is at the same potential as earth ground, you can connect the ground clip to your DUT ground. Expect to see some sparks flying if you get it wrong.The ground clip of 99% of the oscilloscopes will and should always be grounded, learn to live with it or shell out the $$$ for a differential probe (expect to pay more than for the scope). The CH1-CH2 method is a poor man's differential probe with lousy common-mode rejection ratio (don't be surprised by 50/60Hz interference).

author: ejeffrey
author_rank: Super Contributor
timestamp: August 03, 2011, 08:42:38 pm
content: Quote from: shadowless on August 03, 2011, 05:20:15 pmNo, you only connect the ground clip to ground. Thats why it is called the ground clipIf you have a floating scope or a floating device you can clip it anywhere you want, but even so if you use two probes they must clip to the same point.If you really can't probe it single ended, (i.e., you have a large or high frequency common mode signal) a differential probe is the proper tool. Maybe look on ebay for a used one?

author: shadowless
author_rank: Regular Contributor
timestamp: August 05, 2011, 01:24:47 am
content: Thanks for the numerous solution, think i will try the 2 probe setup first