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power supply interference

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Wp100

Well-Known Member
Hi,

Have a simple Arduino project, externally powered by a transformer based wall pack, but it can be connected via its usb serial link to the PC for data feedback and it all worked fine until I replaced the external power pack with one of these internal switch mode units, similar to the pic below.

Everything on the Arduino still works ok, but when the usb serial link to the pc is connected, the PCs programs screens display from the wired video security cameras have some horizontal interference bands which slowly moved down the screen.

Any suggestions of what type of filter is needed to overcome the problem and where best to place it, by the psu or more the pc end of the cable ?

thanks

000446.jpg
 

Rich D.

Active Member
It could be a filter issue. But it could also be a ground loop issue. It might be a simple test to disconnect the direct ground from your project to the PC, and insert a resistance to it...maybe 50 or 100 ohms. The hope is that you will still have a good signal ground reference point but you will not get strong ground currents between the two devices with their two power supplies.

If that does nothing to improve it, then it could be noise from the switching supplies, I did an audio project that used those supplies and they were very noisy before I added a good RC filter on the output of the power supply. (Filters are usually best placed at the source of the noise, before it radiates to the rest of the system.)
 

dr pepper

Well-Known Member
Most Helpful Member
Place the supply near the arduino & pc but not connected to the pc, if the noise is still there is radiated or rf noise, if not then its conducted noise.
There are various things you can do for conducted noise, the best & safest would be to use a Usb isolator, there are other tricks like the resistor in the gnd.
 

Wp100

Well-Known Member
Hi,

Thanks for the replies, I have been trying another smps, a black plastic block type from an external usb drive, and that does not give any interference.

The Arduino and the original internal smps are located 10 mtrs away and fed via a usb to RJ45 adapter pair.

If the cable, powered at the arduino end, is next to the display /pc there is no interference on the security cameras videos images, its only when plugged into the pc usb ports that the problem is evident. ( there is no interference obvious on the normal windows screen images)

RichD, wil try the resistor method you mention.
 

Wp100

Well-Known Member
Footnote - did try the resistor to earth and it did remove a fair bit of interference, but adding other filter components made little difference.

After seeing the videos on this link, think it highlights 'you gets what you pay for' is very relevant when choosing one of these types of power supply.
https://www.ukqrm.org.uk/smps.php

Thankfully have the other decent smps that does not produce any interference, the only problem is it will have to remain outside the enclosure as its way too big.

Still a lot to be said for the good old linear transformer supplies!
 

dr pepper

Well-Known Member
Most Helpful Member
Check the noisy smps for a cap from dc bus to the output, often theres a class y cap from dc 320v or whatever the rectified mains is to the Dc output, this removes some capacitively coupled noise from the supply, sounds like it isnt safe but you see this a lot even in things like 'phone chargers.
 
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