Astable / scope problem

Peter Bullock

Mar 9, 2015
9
Hi,

I have constructed an astable circuit and I want to use the scope to display the voltage across the capacitors. The problem is, when I use
both channels of the scope connecting one channel to each capacitor the astable stops working and both LEDs stay lit. It's ok
when I use just one channel on each capacitor but not with both. Does anyone have any ideas what this might be ?

FC.

BobK

Jan 5, 2010
7,682
Are you connecting the ground leads of the two probes to different places? If so, you are creating a short between them.

Bob

Peter Bullock

Mar 9, 2015
9
Are you connecting the ground leads of the two probes to different places? If so, you are creating a short between them.

Hi Bob,

I am connecting the +ve probe lead to the +ve leg of the first capacitor and the earth probe lead to the -ve leg of
the same capacitor. Now, with only 1 channel being used the scope (I think) correctly shows the voltage across the capacitor: it rises and falls, but when I connect the second channel leads to the other capacitor - in the same way - the short occurs.

PJ.

BobK

Jan 5, 2010
7,682
You cannot do that. The grounds on the two probes are connected together, so you are altering the circuit when you connect the two grounds to different spots in the circuit.

Bob

Peter Bullock

Mar 9, 2015
9
You cannot do that. The grounds on the two probes are connected together, so you are altering the circuit when you connect the two grounds to different spots in the circuit.

Bob

Hi Bob,

This could probably have been expressed more simply as : Where do I place the probes in the circuit to measure the voltage across each capacitor ?

PJ.

duke37

Jan 9, 2011
5,364
I do not see your circuit.
Many scopes need a common earth so you cannot measure the voltage across the two resistors.
You can measure the voltage relative to common. Each channel can measure the voltage at one end of the resistor and some scopes can subtract the two values to get the voltage across the resistor.

BobK

Jan 5, 2010
7,682
Hi Bob,

This could probably have been expressed more simply as : Where do I place the probes in the circuit to measure the voltage across each capacitor ?

PJ.

Bob

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