Posts Tagged ‘short-start atx power supply’

VSO IT Do It With Everything: The Faulty Switch

June 24, 2011

In what I hope will be a final post in the VSO IT Do It With… series I’ve had a problem which called upon all my bodging newfound skills using paperclips, staples and masking tape.

To explain this I’ll try and be not overly tecchie and then put the tecchie bit in italic underneath for anyone who cares.

Faulty Switch

The switch is the thing that connects all our computers together and onto the internet.

Yesterday I needed to turn it off and back on. Turned off ok but wouldn’t come back on; either the power light flickered a bit or just didn’t light up.

We have a gigabit switch at the core of our direct-internet network (not the GRN WAN one). The server on that network which does routing and DHCP locked up and needed power-cycled.

Normally after its back up I’ll drop the switch for a few seconds causing Windows to see ‘disconnected’ and then ‘reconnected’ and so ask for an IP address (rather then keeping the auto-assigned one while DHCP was down for a random period of time).

Business Continuity

As I stood thinking “well that’s not good” more people kept coming to the office asking about the internet. Facebook – and occasionally some health sites – being critical.

The original fault had lost us internet for 5 minutes or so and now they were getting messages telling them I’d unplugged cables (as the switch was off).

Realising this wasn’t going to be simple to sort out I scrabled around and raided my office for my desk switch and numerous cables and adapters.

Within 45 minutes of the original failure I had most of the offices connected using various means (and being lucky in that a few of the offices are vacant right now).

Facebook access was saved.

Using an 8-port 100mbps switch normally on my desk, the spare LAN ports on a wireless router we use just as an access point and just the right number of long cables I had spare (long enough to reach the router) I managed to restore connectivity for 9 locations (one of which was our secondary wifi access point serving 4 PCs and many laptops).


Using various highly advanced techniques (a box with probes that tells me what’s happening) I found the power was coming into the switch.

Opening it up and probing different bits narrowed the issue down to being the PSU/power board; this is the bit that takes the mains voltage and converts it down to a lower voltage suitable for small electrical use.

To be certain this was the faulty component I bypassed it and connected another power supply directly to the switch.

The power board was giving 12VDC out but when connected to the main switch board the output at the connection terminals dropped to 4.5VDC.

Hoping that the power side was faulty and not the switch board I used a variable supply and a fan connector (to plug into the port) to see if it would power up – it did!

Testing switch with a small variable DC power supply to check circuit

Replacement Power Supply

With the faulty board identified it was just a matter of removing it and finding something else to provide the 12V to the switch.

Unfortunately the variable supply used for testing was nowhere near powerful enough to actually power the switch in operation.

A power supply unit for a computer though did provide 12V and would be. It’s possible to trick one of these into thinking the computer is on and so then use its power for the switch.

Normally you would use a paperclip to ‘trick it’ but after testing it with the paperclip I went one better and decided to put an on/off switch in as well.

The all had the added bonus of clipping lots of wires making me feel like a bomb disposal expert.

I decided to use the 12VDC output from an ATX PSU as the variable supply was rated 500mA and the original internal supply rated to 3A (the ATX supply I used was ok to 14.5A).

Rather than just bridging the ATX connector to short-start I cut a switch off an old AT supply and connected that to some pronged connector I had from an old variable supply which perfectly fitted the pins.

Then it was just a matter of taking the 12V feed from one of the molex connectors to my fan-connector already connected to the DC input on the switch board.

The Fulty Power Supply Unit

Faulty original PSU/transformer-rectifier whatsit

Bridging an ATX PSU to a switch - better than a paperclip longterm

Better than a paperclip - connection to short the ATX supply into working when switch closed

Using an AT switch to control an ATX bridge, held on with masking tape

On/off switch mounted on new external PSU

The Frankenswitch Lives

Closing the on/off switch amazingly now led to the PSU spinning up and the switch itself turning on in a flurry of self-test lights.

I’d disconnected an internal fan that was sticky and may be the cause of the problem so I’ve sat the whole thing on top of some big fans blowing air through it. Though this probably would make a fire worse not better. My on/off switch design through should burn through (and shut off) quicker than a paperclip though.

I’ve started moving people back onto this switch and so far no fires or failures.

Rather boringly though I’m going to get a replacement one as an emergency order instead of leaving the Frankenswitch as my legacy in Opuwo.

Repaired switch on top of rack and working

All plugged in and sitting on top of rack fans for cooling and extra oxygen for inevitable fire (backup switch seen in the background)

Switch working in extended testing

Switch actually working with the first test group of people back on it