9v from modern ATX PSU for Ethernet Switch

Hi Alask Tegle Sande,

There are a couple of good reasons not to put the switch inside the cabinet. While there seems to be a lot of wasted room you will potentially hamper air flow and cooling of boards and parts. Most of the boards in the computer are exposed and if they touch or bump the outer case of the switch it might short out. You will also loose easy access to the ports of the switch and the LAN wires will need to be routed through the computer case. If space is a premium you might consider mounting the switch on the outside of the case.

As far as drawing power from the ATX this is certainly possible with the appropriate DC to DC converter or regulator. For example in the power supply wiring of the computer (if it is standard) the Yellow wire will have + 12 volts and will be capable of sourcing power to the switch. The ground wire of the power supply is the black one. The next trick is we have to convert the +12 of the Yellow to Black wire voltage down to the 9 volts required for the Switch. We can use a small device like the Recom R-78C9.0-1.0 (Best Choice) or a linear regulator LM7809. I can draw up  the schematic of the circuit if you want to pursue this option. Since you say you lack experience you may want to think it over. You would have to test voltages, the first being the Yellow to Black power wire to verify that it is in fact 12 volts. Next you would have to build the little converter on a circuit board and use the wire from your wall wart to run the power to the switch. This would involve soldering and design so that connections are insulated and not allowed to short out. The connections to the black and yellow wires inside the computer would have to also be done properly and safely. The voltage of the wire to the switch would also have to be checked for proper 9 volts and for proper + / - polarity.  I do not know if this is a consideration but the switch would also turn on and off with the computer.

Let me know if you want a schematic for the converter.

John

Thanks for super quick help John!

I think I've thought the logistics of placing inside the cabinet pretty well, but I'll comment on it anyways:

Airflow will not be a problem. My fans doesn't come close to max speed before keeping it cool enough even during heavy sessions. The switch is mounted on a solid rail quite far from the MoBo, so that's not a concern. Regarding accessing ports: It's placed flush against the back of the case. I've made a cutout for the ports, so this is neither a problem. From my understanding and experience (from an ancient PSU, shorting two pins (standby 5v and ground?) starts PSUs) the MoBo takes care of that.

Sorry about my lack of accuracy: I'm not a total novice, but when it comes to currencies and voltages I find my knowledge surprisingly low (the physics classes had some theory on the subject). I'm familiar with the basics like polarity and measuring voltages. I'm also comfortable with a fair bit of soldering.

I'm a bit tiny familiar with the LM7809. What would the benefits and differences be between a linear regulator and a switching one in this case? I notice that both of them requires a minimum of 12v. Voltages from computer PSUs are supposed to be quite accurate, but if not 100%, are there any tolerances on such components?

The schematic shouldn't be necessary, but I would appreciate it! If you have a preferred or recommended place for buying components like this a recommendation would also be appreciated.

Aslak

Hi Aslak,

Sounds like you have the electronic experience to pull this off. The linear LM7809 will cost about a $1 but will have to be heat sunk. If we assume that your switch will consume 750 mA and there will be a three volt drop across the regulator it will need to dissipate about 2.25 Watts. The 12 volt power supply of the ATX will be fine for your application. If you decide to use the R-78C9.0-10 it will cost you about $9.00. I would normally tell you to get them from Newark but I see that they are out of stock right now. Check your parts suppliers to see who has one in stock. Since the R-78C is a DC / DC switching converter it will not need to be heat sunk. Be sure to follow the data sheet recommendations in either case for adding decoupling electrolytic capacitors on the input and output of the regulator/converter to maximize voltage stability. Most of the time you can find links to the data sheets on the sites of the Vendors like Newark. You probably don't need a schematic drawn by me but if you run into a bump let me know. I don't think I would mess with the on / off wire on the ATX. Turn it on and off with your normal computer switch. Hope this clarifies.

John

Hi Aslak,

Sounds like you have the electronic experience to pull this off. The linear LM7809 will cost about a $1 but will have to be heat sunk. If we assume that your switch will consume 750 mA and there will be a three volt drop across the regulator it will need to dissipate about 2.25 Watts. The 12 volt power supply of the ATX will be fine for your application. If you decide to use the R-78C9.0-10 it will cost you about $9.00. I would normally tell you to get them from Newark but I see that they are out of stock right now. Check your parts suppliers to see who has one in stock. Since the R-78C is a DC / DC switching converter it will not need to be heat sunk. Be sure to follow the data sheet recommendations in either case for adding decoupling electrolytic capacitors on the input and output of the regulator/converter to maximize voltage stability. Most of the time you can find links to the data sheets on the sites of the Vendors like Newark. You probably don't need a schematic drawn by me but if you run into a bump let me know. I don't think I would mess with the on / off wire on the ATX. Turn it on and off with your normal computer switch. Hope this clarifies.

John