On a new build I'm using a server PSU (because they're far cheaper than rip-off SFF PSU's) but have a 12VSB instead of the needed 5VSB. I added a small DC-DC step down circuit to convert the 12V to 5V and it's reading 5V on my multimeter.

So imagine my surprise when I hit the switch and something frys instantly, on closer inspection its the immediate component coming from the 5VSB ATX pin.

Why would this happen?

 

Looks like a diode, maybe a zener or you put the 5V the wrong way round.

 

Yeh I think its a zener diode for voltage regulation. The polarity is correct so it's not that. Maybe a voltage spike?

 

perhaps your dc-dc converter was not spec'd correctly. A motherboard might draw a significant amount of current at boot up. Sounds like you saved a few bucks and caused more problems.

 

The specs say it can output 6A which is more than you'd usually get from a 5VSB rail. I'm struggling with the electronics theory regarding the conditions which cause a zener to fail and how that relates to my situation.

I agree with your second point

This is the DC-DC converter I purchased

 

what does your system need?

 

For the 5VSB? I don't know, the standard amount for a budget ITX motherboard, it's not power hungry. Most PSU's don't provide more than 3A on the 5VSB rail so presumably it doesn't vary all that much.

If it is due to limited current, why would that cause the zener to fail?

I have noticed in the description of the converter I bought it mentions fast charging identification (through the USB ports), so perhaps without actually connecting a USB device it does limit the current? I soldered the 5V wire to the inductor output which was the easiest solder point before it goes directly to the 5V USB terminals.

Edit: So I just did a quick test. I plugged in a USB device (power bank) in-line with a USB meter and saw it was providing 1.3A. I then switched the USB cable for one without the data wires and it provided 0.5A.

So maybe that is the problem? Without sensing it maxes out at 0.5A? I still want to understand why that caused the zener to fail though, from my limited knowledge don't components usually fail from too much current or voltage?

 

Hi
The standby voltage is powering the return signal to the PSU via the PS_ON connection on the ATX 24-Pin. Does this exist on server PSUs (if so I'm guessing it expects 12V)? If it doesn't connect back to the PSU where does this pin terminate? Something else that stands out is that on an ATX PSU the 5Vsb would have OVP and OCP limiting voltage and current. If you are stepping down from 12V to 5V then you could potentially be doubling the current available to the mobo standby pin as the PSU is limiting it at 12V not the 5V.

 

Yeh the PS_ON connection is present.

Regarding the OVP and OCP, the 12VSB is rated at 1.2A max, which is approx 5V 2A when stepped down, that's within what you'd expect for that rail so if it were to draw more the PSU's OCP should have kicked in anyway.

For reference the PSU is from a Dell PowerEdge R210, it has an ATX 24 pin connector but not exactly to ATX spec (in addition to the 12VSB it has 12V going to the -12V and -5V pins which I disconnected before doing anything).

I've read about people doing the opposite of what I'm trying, using an ATX spec PSU to power a Dell server, and for that they used a separate 12V power supply for the 12VSB which worked. So in their case wouldn't the return signal have caused problems too?