Need to get 7.5volt from a PSU, what is the best way?

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Deimos

[H]ard|Gawd
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I have a 16 port hub that needs 7.5Volts DC to run, I also have other equipment that I want to power off a Computer PSU, but these items are within the range (12Volt for my ADSL Router)

I know you can get 7.5 by hokking up 12 to + and 5 to negative, but have only seen this done with DC Fans, not much risk when the only thing at stake is a PSU and a fan, but this sounds dodgy to me, and I know you can build a better solution, just wondering what is the best.

My idea was to wire up a few diodes in series with the hub to get the desired 7.5 Volts, but with a 1.7Volt drop per diode, I can't get the desired voltage (6.9 is the closest safe voltage I can get with this solution).

I could research, but if someone knew the solution off the top of thier head it would save me a few hours.

Thanks
 
Just one other thing I thought of, the PSU can output up to 16 amps on the 12 volt rail, yet the device I have only needs around 1 amp, will I need to put a resistor acroos the 12 Volt and negative rail to protect the device from over current?
 
I'd look for another solution, perhaps a simple 3 leg regulator on the 12V line.

1st thing that comes to mind is that the virtual ground on the hub will be different from the common ground on all the other parts in the system, guaranteed to cause problems. A fan doesn't have to interface with anything else, you can get away with that.
 
A hub I had simply had a 7805 inside for a regulator and had a 7.5V supply so that it had enough headroom to regulate it down. Might want to pop it open and see how complicated the supply part of the board is.

Also, how much current does it need?
 
The switch needs 1 amp at 7.5 volts, and the router is 1 amp at 12 volts.

I won't be running a computer off the PSU, but if this project works out, I will be doing something similar for my server as I have 2 of these switches.

One of my work mates told me I could get a voltage regulator so I will head down to the electronic store sometime this week to check pricing and size.
 
I would actually like to build the circut, I enjoy doing small electronic projects, I have spare fero board lying around too, components are cheap...
 
since it wasn't mentioned, you don't have to worry about over current as you called it, all you need is the supplied voltage, and have sufficient amperage. (1 amp in this case)

Also since it wasn't mentioned the 12V-5V trick gives you 7 Volts not 7.5, as to the differences in ground I would say that's probably good advice to come up with a way to regulate the voltage instead. If you aren't familar with electronics a voltage regulator may not be easy to you. If you look on the back of the package (of say an LM317) it should give you a basic idea of the circuit required, and a formula for output voltage. All you have to do is choose your resistors (they determine the output voltage), and build the circuit. The capacitors are optional, and I forget what they are for, but i believe it will say on the package, or you can post a prototype schematic for these guru's to look at. Make sure the regulator you pick says it can handle the current for your router (I don't know much about it but ask here perhaps). And of course CHECK your output before you hook it up to a router with a digital multimeter to make sure you have something close to your desired output of 7.5 volts, if not simply adjust your resistors values (you probably want to buy a pack of various ones)

To other readers maybe you can elaborate on how much current regulators can handle, and a few model numbers he should look at as the only one I know of is the LM317
 
I have done an electronics course, but that was 9 years ago, I dropped it to go in to IT, but if someone can give me a circut diagram, I can build it, I have forgotten how to read the color coding on resistors, but I stilll have my reference material around somewhere...
 
A 7.5V regulator coming off your 12V line is probably your best bet. You may have to get a small heatsink for it. Wasted power = P(w) = wasted V) * I = (12V - 7.5V)*1A = 4.5W.

You can find such a regulator here . There is also a link on that page for the manufacturer's datasheet , which contains all sorts of applications of the regulator, and tells you how to hook it up. You want to use the one on the bottom of page 3.

The capacitors are usually there to make sure that the power being supplied is nice and clean. Even if the hub has an internal regulator, you will probably need to use an external one, since the internal regulator is designed to have 7.5V coming in instead of 12V. (lots less power to dissipate)

I would not use the "7V mod" on this for a couple reasons:
1) Modern PSUs don't like having current come into the 5V line, which is what happens with the 7V mod. In a full computer, other components pull way more out of the 5V line than anything running on the 7V mod puts into it, so your PSU's happy. But it doesn't look like you will be running anything off that 5V line.
2) You'll be getting 7V, not 7.5V. This probably won't make a difference, but until we know about the internals of your router, we won't know for sure.
 
Thanks for your suggestions guys.

I got myself a voltage regulator kitset from Dick Smith electronics (an NZ equivalent to radio shack, except they rip you off, and the sales people are morons), it was $15 so not too bad, and can output up to 1.5 amps when heatsinked.

It has a 100K Ohm Pot, and it can output anything between 1.5 - 9V so is perfect.

The only hurdle I have come across is the resistor values, the guide that comes with it is crap, and the values are not very well explained, does anyone have a link to a resistor value reading guide? I'm not keen on getting attacked by spiders and cobwebs to find my electronics book right now.
 
Deimos said:
Dick Smith electronics (an NZ equivalent to radio shack, except they rip you off, and the sales people are morons)
Click to expand...

Sounds exactly like Radio Shack to me. :D
 
just google for the resistor color code shouldn't take you more than 30 seconds.. however I think I remember it soo here goes.

Follow the rainbow colors mainly.. here's an EASY way to remember it for the rest of your life.. BLACK, BROWN + rainbow + GRAY, WHITE. For some reason it's easy to remember that black is zero and white is last, so just plug in the rest. Here it is.

0 - Black
1 - Brown
2 - Red
3 - Orange
4 - Yellow
5 - Green
6 - Blue
7 - Purple
8 - Gray
9 - White

http://www.elexp.com/t_resist.htm

always a good idea to double check the resistance.. use your ohm meter when it's out of the circuit.
 
Allelectronics part numbers:

CAT# PS-440 (Open it up for better cooling and the amperage won't be problem, the router probably doesn't use 1A except in certain conditions)
CAT# PS-1012
 
Sheldron said:
http://www.allelectronics.com/cgi-bin/category.cgi?category=search&item=DCTX-7512&type=store

Just pull it apart, wire the 120vAC into your PSU's AC input, repackage the inverter any way you like, and you're good to go... or you can build one yourself if you really want to, though it'll probably end up alot larger than a pre-manufactured one.
Click to expand...

:confused: The whole purpose of the mod is to get rid of the transformer and substitute it for a PC power supply.

Anyway, I built my circut today, and all is well, I got the 7.5 Volts almost exactly off the 12 volt rail, then I decided to test the existing power unit for the switch to see what voltage it is out putting (its rated at 7.5 Volts 1A) and surprise surprise, the damn thing is out putting 10.5 Volts, I verified this with my other one, they are both out putting the exact same voltage, so that was a waste of time tweaking that POT to get 7.5 Volts, bugger...

So I rewired the circut to 9V using a fixed resistor since that is the highest I can go, I'll be heading down to Jaycar tomorrow to get a box, heatsink, molex and DC Jack for it.

If everything goes well I will be putting a 16 port switch in my server as well, sweet :D
 
Was it outputting 10.5V with nothing attached (unloaded)? If so, it's an unregulated wall wart--which means the more load you put on it, the lower the voltage gets. I would see what kind of voltage you get out of it when you hook up the 16-port hub.

I suggest you open the hub, because at this point I'm certain there's going to be a regulator inside. See if you can figure out what voltage the regulator is outputting to actually power the hub. There's a slight chance it's 5V, in which case you could wire the computer PSU directly to the output of the hub's regulator (bypassing it).
 
One thing you have to remember with power supplies, is their voltage is typically rated with a load. With a rating of 7.5V/1A, it indicates that when the load pulls 1A the output voltage is 7.5V. When you add a load to a power supply it's voltage changes depending on the load, because of the internal resistance of the PS.
 
I cruised down to Jaycar and got myself a circut box, and another voltage regulator, built it and tested it with my switch, everything seems to be working fine, I'm heading down to the electronics shop tonight to get the rest of the parts to complete my "house mod" I'll show you guys what I have done, so you can get an idea of my intentions.
 
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