Filter for Laptop Dock

Hello All,

I have some understanding of basic Electronics theory but not enough to design circuits, what I need is to filter the "noise coming from the power supplies that feed or laptop docking stations at work, it introduces audio noise to the speakers and some display issues on the VGA output. The supplies are 19 V 4 amp dc, can I just put a capacitor in parallel with the docking station or do I need something more complex? I do not have an oscilloscope so I am not sure what the frequency of the noise is, I expect it varies from supply to supply as the interference is worse on some than others. I just want a general purpose filter but have no idea on the capacitance value to use or the type of cap, electrolytic vs ceramic or polyester. I will end up having to make about 30 of these things so I'm hoping it is not too complex.

Any help or pointers would be great.

I am honestly not sure if this will work for your problem. Im sure someone will be able to provide more detail.

Have you considered using simple Ferrite Beads? my understanding is that they suppress noise, normally found on most cables(including laptop charges) there is a large bump towards the end, this contains the ferrite bead which has high magnetic permeability that allows it to reduce EMI.

Just a thought

I did try those originally but it never made any improvements, I looped the cord through a couple times. I figured that maybe those only help with induced RF signals? Thanks for the reply though.

It really sounds like your power supply is failing. You might want to replace it. When switch mode power supplies start to fail they put a lot of noise on the DC output.

Actually it is a known issue with this model, we have over 50 of them and they all do it from Day 1 new out of the box, Toshiba discontinued this line so they are not releasing an updated one as they claim it not to be a "functional" issue. I colleague of mine works at a large company with over 200 of these units and same deal.

Seems like it must be radiated noise. Where did you put the ferrite. It could be getting out of the supply on either the power cord or the DC line - or if its plastic from thru case itself. I'm nor familiar with the station so maybe a couple of dumb questions:
If you power one laptop from the battery and a second from the station on the same desk do you get the noise on the laptop running on the battery or only on the one running from the station?

Agree with ronv, you have to determine the noise source and entry. A fix could be as simple as "lead dress" or the routing of the leads.

If you had a similar power supply, you could eliminate that.

I'v tried ferrite on the AC line,the DC line,multiple adapters, under the desk on topof the desk, what have you. When the units are running on battery there is no interference. As far as I can tell the PSU's are just crappy and don't have a very clean output, I would like to try the capacitor in parallel but I am absolutely clueless as to what type and value to use as a test. Obviously I need one rated for the voltage of the output so at least 24V I was thinking. It is just a case of the right farad rating and the type (electrolytic, ceramic, etc.)

Worth a try. I would go with a 0.1 Ufd. ceramic in parallel with say a 6.8 Ufd. tantalum cap. Careful with the tantalum it is polarized and sometimes not marked well.
If you can't find the tantalum use a good low esr electrolytic say 47 Ufd.

SOunds good to me, may I ask why two caps in parallel? Is it to catch different frequencies of interference?

I recall a great explanation was given to me by a friend of why we combine electrolytic caps with ceramic. Not sure if it is a similar reason with tantalium caps. He said that electrolytic caps are manufactured as parallel plate caps but to get them nice and small they roll the length of the plate (imagine a rolled up carpet) . This configuration (i.e the rolling of the plate) gives the capacitor internal inductance (recall inductors are loops of wire). The problem is that this little inductance causes a slight delay in the discharge, and to filter out that reaction delay we use a ceramic(or similar non rolled) capacitor that is able to react faster and smooth our signal better.

Please could someone confirm this theory?

Ceramics have good RF performance.

Tantalums have low ESR.

Another thought to at least eliminate the power supply as the problem, try an www.igo.com for an adapter based on a device. The "Tip" determines the polarity, voltage and current limit. They are great adapters.

Yes, the ceramic are very good at high frequency as is the tantalum. Since his problem seems to be switching noise this seems to be the best bet.