DIY Power Supply causing PC monitor to switch off (see video)

[bitshift]

Hey Guys,

So I've assigned myself the classic task of building a basic bench power supply. My setup as follows:

• 50Hz 2A Center tapped transformer 240VAC to 15-0-15VAC
• Full wave rectifier (1N4002) + smoothing caps (4700uF)
• Some op amps and adc's driving a LM317 and a LM337 (controlled by an Arduino)

The supply is working great but what I've noticed is that when I turn it off, the Samsung LED monitor on my desk seems to brown out or something. Not every time but at least every second time. They're both connected to the same wall socket.

I cant seem to understand why this happening. As far as I know, transformers have a current spike when switching on but not switching off.

I've attached a link to a video of this happening. Any ideas as to whats causing this?

TL;DR

What is happening in this (short, 47 second) video?

Link: http://youtu.be/SAHdi3oygYs

Note: The popping sound in the video seems to be coming from my PC speakers. It's a lot less dramatic in real life I think.

[JohnnyBerg]

What happens if you disconnect the USB?

[bitshift]

With the Arduino powered from a "wall wart" the same thing happens. I don't think it has anything to do with the Arduino though. It was happening before I added the digital control.

Sorry I should have mentioned that before.

[picandmix]

Long and loose wires like that will cause all sorts of problems.

More than likely that switch is the problem,  which looks like as small lamp switch, rather than something able to handle an inductive load like the transformer.

Try replacing the switch, shortening the cables and tidy the layout and see if that helps.
Also a switch contact suppressor might work, ensure its a proper mains rated devices.

If thats not the cause then try unplugging the arduino USB lead and see if that makes a difference.

[bitshift]

I've removed the Arduino and the switch but the same thing happens. I haven't tried shortening the cable though.

How could this be affecting the monitor on my desk though? And only one of them at that?

[JohnnyBerg]

Try connecting the mains the other way, e.g. rotate it 180 degrees. What happens?

[bitshift]

Before I do that I just want to confirm my assumptions:

• Swapping the live and neutral wires will have no affect on the operation of my circuit
• The transformer is center tapped so 0V will still be 0V
• Since this is AC it doesn't actually matter which is which
• The full wave bridge rectifier will still ensure that V+ and V- are both coming out the same regardless

Safety first you know

Just out of curiosity what do you expect to be the outcome of this?

[JohnnyBerg]

Before I do that I just want to confirm my assumptions:

• Swapping the live and neutral wires will have no affect on the operation of my circuit
• The transformer is center tapped so 0V will still be 0V
• Since this is AC it doesn't actually matter which is which
• The full wave bridge rectifier will still ensure that V+ and V- are both coming out the same regardless

Safety first you know

 

Just out of curiosity what do you expect to be the outcome of this?

Perhaps a common mode problem, life that lifts neutral, something like that. Perhaps nothing at all.

Deduction, my dear Whatson, deduction

[bitshift]

Ok, I've swapped the live and neutral wires around but this hasn't changed anything.

Could the switching on and off of the monitor be related to a sudden current draw elsewhere efffectively depriving the monitor of enough energy?

[Ericho]

Use a double pole switch before the transformer.

 

[bitshift]

Use a double pole switch before the transformer.

 

Tried it without the switch at all and still the same result.

[bitshift]

Ok it seems as if the mystery is solved

I moved the transformer much further away from my computer and there is no effect on the monitor and speakers anymore.

My best guess is that the transformer was creating a large enough magnetic field to influence the functioning of my PC monitor.

So on to the next question....

What can I do about this? How do I shield my electronics from the transformer?

[JohnnyBerg]

Million dollar question: is it the magnetic field of the transformer or the wires to the transformer that causes the pain?
Or perhaps the outlet?

[bitshift]

I need to take my previous post back. JohnnyBerg it appears that the suggestions of unplugging the USB to the Arduino is the actual solution.

When remove the USB I still get a crack coming out of the speakers but no effect on the monitor.

I connected my scope to one of the wires coming out the arduino and into the breadboard. When I switch the transformer on the signal is nice and clean. When I turn the transformer off, I get a large transient on the wire.

Could it be that the changing magnetic field in the transformer induces a current on the signal wires from the arduino? This then somehow makes its way to my PC?

[JohnnyBerg]

I need to take my previous post back. JohnnyBerg it appears that the suggestions of unplugging the USB to the Arduino is the actual solution.

It was my first though 

There are some smart people out there, who invented a isolated USB ..

Most common problem is current flowing though ground and earth. I am not a native English speaker, but in my language we call it a "current ground loop".

Solution is to connect all grounds to 1 common point, avoiding loops. That sounds simpler than it is, hence the isolated USB