Problem when adding the Oscilloscope probe to my circuit output

[NHSA]

Hi,

I was not sure if I should post this poblem or not    but I have not found a solution after 3 days analyzing it   

Attached circuit image with marked oscilloscope probe position.

My positive / negative voltage solution using two zener diodes 1N4732A and a 100 Ohm resitor works fine. Just 15mA current consumption. Sorry the schematics has not been changed to the Zener Diode / Resistor power supply. Still depicts a battery powering or athe resistor voltage divider which is also not valis anymore. Need to change it.

When I do connect the oscilloscope probe which is connected to the oscilloscope I need to crank up the amplifier power nearly to the maximum on trim pot RV3 for Amplifier 1B because the 10LED bar does not show anything anymore. Disconnecting the oscilloscope probe I need to adjust down the trim pot RV3 50K value of Amplifier 1B.

When I just connect the oscilloscope probe (NOT connected to the oscilloscope) it does not happen e.g. the trim pot RV3 50K of amplifier 1B does not need to be cranked up.

I guess something is pulling the voltage potential of the output down to ground ¿?

I tried simulating a probe circuit at the output but no I had no luck getting the same result.

The power brick is a 2 pole main connector. It has NO ground to my mains while the oscilloscope does have it.

Thanks in advance for any tip.

regards Rainer

[David Hess]

If the power supply is not floating, then attaching the oscilloscope probe ground to ground in the circuit likely shorts negative to ground.

[NHSA]

If the power supply is not floating, then attaching the oscilloscope probe ground to ground in the circuit likely shorts negative to ground.

Hi David,

Will check what floating power supply means. Thanks.

Need to test it again with batteries if it behaves same ... Do not remember ...

Thanks Rainer

[tunk]

Is this any help: https://youtu.be/xaELqAo4kkQ

[NHSA]

Is this any help: https://youtu.be/xaELqAo4kkQ

Hi tunk,

Thanks for the link and yes I already saw that video but that is not my problem.

My oscilloscope probe connection has been always correct and on points with nearly no current flow at all.

 

[StillTrying]

Can't you just measure the 0V to -V supply with a DVM while connecting and disconnecting the scope probe, it shouldn't change by even 0.1V, if it does you've got a GND leakage problem.

It's a bit of a pain with the vital bits spread over multiple threads.
https://www.eevblog.com/forum/beginners/positivenegative-power-supply-circuits/

[NHSA]

Can't you just measure the 0V to -V supply with a DVM while connecting and disconnecting the scope probe, it shouldn't change by even 0.1V, if it does you've got a GND leakage problem.

It's a bit of a pain with the vital bits spread over multiple threads.
https://www.eevblog.com/forum/beginners/positivenegative-power-supply-circuits/

Hi StillTrying,

Will test that. I have been measuring the voltage supplied by positive/negative resitor/Zener diode circuit and it is stable when it works but did not look if the voltage changes when connecting(disconnecting the oscilloscope probe. Thanks. 

 Sorry I made a new message. As the two previous posts were solved I thought it makes it clearer starting a new message as this is a new problem.   

regards Rainer

[NHSA]

Can't you just measure the 0V to -V supply with a DVM while connecting and disconnecting the scope probe, it shouldn't change by even 0.1V, if it does you've got a GND leakage problem.

It's a bit of a pain with the vital bits spread over multiple threads.
https://www.eevblog.com/forum/beginners/positivenegative-power-supply-circuits/

Hi StillTrying,

Made some tests. Powered up the circuit with different power supplies.

1. 3 power bricks which connect to main just with two pins L&N 12V output
2. 1 power brick with 3 pins L, N & G 12V output
3. PSU adjusted to 12V output connected to main via 3 pins L, N & G

I forgot to test with batteries 

First observation is that it does not matter which power supply I use, my split Zener diode positive/negative voltage is stable.

Now comes the interesting thing

Test 1. and 2. does need to adjust the amplifier on trim pot RV3 in order to get a signal on the 10 LED bar

Test 3. does not change when connecting/disconnecting the oscilloscope probe e.g. which means no need to adjust the trimpot RV3

I am clueless and give up. 

Thanks all for your help

Rainer

[NHSA]

Can't you just measure the 0V to -V supply with a DVM while connecting and disconnecting the scope probe, it shouldn't change by even 0.1V, if it does you've got a GND leakage problem.

It's a bit of a pain with the vital bits spread over multiple threads.
https://www.eevblog.com/forum/beginners/positivenegative-power-supply-circuits/

Hi StillTrying,

Made some tests. Powered up the circuit with different power supplies.

1. 3 power bricks which connect to main just with two pins L&N 12V output
2. 1 power brick with 3 pins L, N & G 12V output
3. PSU adjusted to 12V output connected to main via 3 pins L, N & G

I forgot to test with batteries 

First observation is that it does not matter which power supply I use, my split Zener diode positive/negative voltage is stable.

Now comes the interesting thing

Test 1. and 2. does need to adjust the amplifier on trim pot RV3 in order to get a signal on the 10 LED bar

Test 3. does not change when connecting/disconnecting the oscilloscope probe e.g. which means no need to adjust the trimpot RV3

I am clueless and give up. 

Thanks all for your help

Rainer

Hi,

Just made a test with batteries and same behaviour as with the power bricks.

Need to increase the amplifying signal with trim pot RV3 ...

Why is the DC voltage potential pulled down when applying the oscilloscope probe ?

How can I achieve the same with some resistors or capacitors on that output stage ?

 

[sperb]

Do you have any other equipment or any other connection to the circuit other than the power supply and oscilloscope?  Any connections with computer, signal generator, audio source, multiple channels of the scope, etc?

Any other connection to the circuit can get you trouble with the scope if you have more than one connection to ground(earth). Your scope probe ground is connected to the ground terminal of the power cord. You just need to find where the other ground connection is.

Regards.

[NHSA]

Do you have any other equipment or any other connection to the circuit other than the power supply and oscilloscope?  Any connections with computer, signal generator, audio source, multiple channels of the scope, etc?

Any other connection to the circuit can get you trouble with the scope if you have more than one connection to ground(earth). Your scope probe ground is connected to the ground terminal of the power cord. You just need to find where the other ground connection is.

Regards.

Hi sperb,

Nothing at all. One probe is connected to the points I show in the first attached image and the other probe just laying around. My oscilloscope has only two channels.

The ground connection is the earthed ground of my main power supply e.g. 128V (I know should be 110V but our electricity company delivers 128V  )

Supplying the positive/negative voltage to my circuit using my PSU, which is connected to the same ground and mains, the problem does not show up. I use channel 1 from my PSU connecting the red and black terminals to my resistor/Zener diode voltage splitter.

Rainer