circuit oscillation while laptop AC is connected

[aamit4]

I have a circuit that would run on voltage from serial cable. While my laptop is connected to AC power; the circuit would output garbage data into hyperterminal; but it would work perfectly when laptop is not connected AC adapter and running on battery. Can anybody help me with this? I would really appriciate. Thank you.

 

[goldsmith]

Dear aamit4
Hi
It seems because your adapter has not any EMI filter in it's input ! . use an external EMI filter in series with inpu of your laptop adapter .
Best Wishes
Goldsmith

 

[aamit4]

Thanks Goldsmith for your input. So, i have tried two different laptop and adapter but same results. i tried build EMI filter with inductor and Cap, but didn't work. I guess i need to built better EMI filter. Any idea on EMI filter. Thank You.

 

[albbg]

Use the PC battery operated and be sure everything is working well, then try to switch the AC adapter on without to connect it to the PC, to see if the noise is radiated (need to improve shielding) or conducted (need to improve supply filtering somwhere). It is better if during this test you load the AC adapter with a resistor that rougly simulate the PC load, or even better supply the other PC while using the battery operated one.

However could you detail how you circuit is supplied ?

 

[tpetar]

Is this laptop Dell ?

What exact model number ?

 

[dick_freebird]

Some laptops don't ground the chassis and can put a lot of AC
onto the "ground". I recently had two laptops, a Dell (2-prong)
and a HP (3-prong) on at once, and touching them both at once
I could feel the bite of some small amount of AC current. Which
is probably a pretty decent voltage into an open circuit. Could
be enough to corrupt the RS-232 data especially if the receiver
is not very good at rejecting common mode hum.

 

[tpetar]

Yes because I ask is it Dell. I notice that on few Dells laptops. Even Dell said that there is problem with DC connector and grounding.

 

[BradtheRad]

I have a laptop that produces hum into my stereo system when I have certain things hooked up simultaneously.

Same as with you, the hum went away when I was on battery power only.

I got rid of it by disconnecting the incoming cable tv. Then I could plug in the charger without hearing hum.

I believe the problem was a ground loop.

See what else is hooked up to your system, and see if it helps to disconnect it.

 

[aamit4]

The circuits works fine without AC adapter connect to Dell laptop. The problem is when AC adapter is plugged into the laptop. So, i far i have tried two Dell laptop. One is Dell Latitude E6410 and second is Dell Latitude D620. It will also works fine when it is connected to desktop and getting powr from it. the circuit works on 3.3V.

 

[tpetar]

Yes my friend welcome to the club of Dells grounding problem members.

What you can and tell me result, disable ground on AC adapter plug and try to run laptop over AC adapter. Isolate ground with scotch tape or something else.

 

[goldsmith]

Hi again
I believe that this problem will be solved with a simple differential filter . but at first we need to know the BW of those noises to design a right filter .
Best Wishes
Goldsmith

 

[aamit4]

GoldSmith,

I can't see those noises on oscilloscope becasuse as soon as i connect my ground lead it will stop oscillating.

 

[goldsmith]

I didn't say that you use oscilloscope directly . use a high impedance buffer and then use oscilloscope to measure it .
Best Wishes
Goldsmith