NI 6009 LED switches OFF when switching the power supply of the connected circuit

[9831238195]

Well, I really cannot uncerstand how that much pot difference is coming !! there may be some issue with the quality of transformer ( I am not sure). The power supply design is a very standard one.

Surely i need to use an earthed ground . I will also try to clip the transients by adding back to back zeners between power supply and ground. Any help expected from good quality shielding? if so how can that be done? any additional precautions that can be taken?

[K C]

If this is a bad transformer there will flow a high current through the 0V of your USB connection because this is connected to earth if your computer is connected to an outlet with earth !?!    

If your computer is not connected to earth there will be half the mains voltage at your computer 0V connection !! Connected together with the leakage of the transformer !!

Using zeners might help but I would use ceramic capacitors for this job.

Kees

[SnowMule]

Just curious, but why not just use some cheap wall-wart switch-mode power supplies instead of building your own?

[9831238195]

Actually an SMPS may introduce an undesirable RF noise.

[JÞB]

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@9831238195 wrote:

Well, I really cannot uncerstand how that much pot difference is coming !! there may be some issue with the quality of transformer ( I am not sure). The power supply design is a very standard one.

Surely i need to use an earthed ground . I will also try to clip the transients by adding back to back zeners between power supply and ground. Any help expected from good quality shielding? if so how can that be done? any additional precautions that can be taken?

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Read the datasheets of those regulators!  Especially look at the "Line Transient Responses"

Now you know where the design flaw is. 

[9831238195 · Accepted solution]

The problelm stands solved. I found that, this happens even when the NI-6009 is not connected to the circuit sometimes. ( bit spooky)!!! but may be possible due to strong electromagnetic interference from transformer.

However, some measures were taken to prevent this from happening:

1. Used over-voltage and surge protection circuits like , zener diodes, Line filters.

2. Used the earthed ground.

3. Enclosed the circuit in a metal box and shielded the NI DAQ card in a separated groove inside the box, away from the transformer.

4. Used a capacitors of 10uF across adjusting resistors for voltage regulator ICs as pointed out by Jeff.

Thanks for lots of help and support. 🙂