Ineffective switch or artificial ground?  - Page 1
Forum section: electronics / beginners

author: bitman
author_rank: Unknown
timestamp: September 11, 2017, 04:56:16 am
content: Long story short - I have a PicoPSU (see picture) I use in a very small enclosure to power a motherboard/full system. I actually have a few of them, and I like the low power consumption among other things. THIS time I thought I wanted to add a simple mechanical switch. I have a DTDP as pictured on the image too. A simple DIN for the power plug (comes with the PICO). It's pretty simple - or so I thought.The problem is the switch doesn't matter, or is ignored. It's always on. The switch is on the 0v (neutral or whatever that line is called) which may be the issue. But I would like to understand WHY. Regardless of the switch position - any of the 3 - the mobo receives power, and I can use the normal "power on" pin 14 with +5v and presto the board turns on. Even with nothing else connected this happens!Color me confused. Hopefully someone can make this make sense so I can learn from my wrong ways.

author: Bratster
author_rank: Frequent Contributor
timestamp: September 11, 2017, 06:12:14 am
content: Your power supply must be grounded, and some other piece of equipment also connected to the computer is grounded.Or if you have this mounted in a metal case the connector is grounded to the metal case and the motherboard is also grounded to the metal case.That is where the current is Flowing completely bypassing your switch.worst-case that can completely fry some parts.Always put the switch in the positive line.Unless you have some very specific reason for switching a negative.Also your switch looks like is a SPDT, single pole double throw.Sent from my Nexus 6 using Tapatalk

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: September 11, 2017, 10:23:15 pm
content: The faults described above are most likely but should not occur in a ungrounded, detached test setup.Are you sure the switch is ok?

author: bitman
author_rank: Unknown
timestamp: September 12, 2017, 12:23:57 am
content: Quote from: Neomys Sapiens on September 11, 2017, 10:23:15 pmYes - voltmeter shows the switch is doing what it should do. I've actually tried with two different switches, same result.Btw. I realized after reading this, that I did have the monitor attached (nothing else) - meaning a simple VGA connection.

author: Monkeh
author_rank: Super Contributor
timestamp: September 12, 2017, 03:32:02 am
content: Do you have that DC jack screwed into the chassis? Does the monitor have a ground pin?One way or another, you've got another ground. Simple solution: Put the switch on the other conductor. Why did you put it on the low side in the firstplace?!

author: bitman
author_rank: Unknown
timestamp: September 12, 2017, 03:04:39 pm
content: Quote from: Monkeh on September 12, 2017, 03:32:02 amYes - both the switch and DC jack are screwed onto the small case - but the case is NOT grounded. I've never had a computer case (outside of server racks) that were. The monitor definitely has a ground pin, so does the PSU that plugs into the DC jack. Only two wires are on the output of that PSU so I doubt ground is provided on the output?One way or another, you've got another ground. Simple solution: Put the switch on the other conductor. Why did you put it on the low side in the firstplace?!To be honest, I didn't think it mattered and it was easier to get to when start put the switch in. I'll make this another lesson learned and correct the switch.

author: Monkeh
author_rank: Super Contributor
timestamp: September 12, 2017, 03:09:37 pm
content: Quote from: bitman on September 12, 2017, 03:04:39 pmYes, it is - it's metal, it connects to ground on the motherboard via the mounting screws, and ground on the DC jack via.. well, the body of the jack. It is grounded, and it totally, completely, and utterly bypasses your switch. This is in regard to system ground, not mains earth - that's another matter and may or may not be the same.QuoteEvery single metal case you've ever owned with an AT or ATX PSU is grounded. By the mounting screws.QuoteIn this context the negative wireisground. Don't confuse it with the supply earthing conductor, which it may or may not be connected to (in the case of AT and ATX PSUs, many if not all monitors, some laptop supplies, ... and all sorts of other things, this is the case: system ground and supply earth are connected together).