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Author Topic: noise with laptop  (Read 2399 times)

Posts: 183

« on: October 03, 2006, 07:06:09 AM »

i have a toshiba laptop not connected to any of my radios.when i use it with battery power,no problem but when i connect the ac adapter it blank the reception on my any one now at least how to minimise the noise level? filter or something?
thanks  kp2bh  jimmy

Posts: 18

« Reply #1 on: October 03, 2006, 05:16:44 PM »

ac adapters are notorious sources of rf noise. My first thought would be to put an RF choke on the adapter cable.

Posts: 81


« Reply #2 on: October 10, 2006, 09:58:38 AM »

I also have the same problem and would like suggestions for a solution. I have an old disused Dell laptop that I'd like to use as my station computer. The battery is dead so I have to use the AC adapter. The AC adaptor doesn't radiate much RF noise, but it puts a lot of noise into the AC line. If it's plugged into the same AC circuit as my receiver, there's lots of electrical noise in the receiver, but if I run an extension cord across the room and plug it into a different circuit, there's very little noise. For various reasons, this isn't the solution I want to use for the long term. I'd like a fix that would allow me to plug the laptop into the same AC circuit as my rig.

I tried using RF chokes, but they didn't help. Not a surprise, since the problem doesn't seem to be with radiated RF.

Posts: 625

« Reply #3 on: October 11, 2006, 09:26:17 AM »

There are a number of things you can do.

First check the voltage ratings of the adapter you are using.  Usually on the safety agency label (UL/CSA etc) it tells you so many volts out at so many amps or watts.  Most laptops now days are DC in the range of 14V to 21V.

1)You can buy or build a linear power supply with the right voltage and current rating.  Look at digikey or mouser for units that might work. cut the cord off of the exiting AC adapter and graft it on to the linear power supply.  If the interference is from the adapter, this is fool proof.

2) If you are lucky and have an old laptop with a 12V battery (NIMH etc), you can cut open the old battery, pull out the bad cells, and drill a hole in it and run some wires out.  Then you can connect this new "battery adapter" to a 12V linear power supply.  I have rigged up an old 486 laptop to do this.  This is also fool proof.

3) You can get some big hi perm. toroids and wrap both the AC power cord thru them and the DC cord thru them.  This forms a common mode choke.  It might work to lower the interference but I have never seen it to completely eliminate it.

4) Take the AC adapter apart and mount it in a metal box with the common mode chokes as described in 3) plus add .1uf or .01uf caps between the DC outputs and the case (both plus and minus).  Be careful of adding any caps to the AC line!  You can cause a shock hazard.  I have bought AC rated canned emi filters for use on the AC line part and again look at the digikey and mouser sites for them.  This approach works really well, but it is very drastic.

I have done all of these things and know they can work as long as the power adapter is really the source of the EMI.  There will still be some residual emi from the LCD display and the processor.  If you hear it on your radio, you can tell where it comes from by holding your hand near the LCD and then moving it away.  If the noise changes amplitude, it probably comes from the display.  

My experience has been that if you reduce the emi from the AC adapter, the residual noise is very low and acceptable.

Good luck.
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