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Psu usb

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gtr_stitch

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Hey all,
So I'm currently working on turning a PSU into a benchtop power supply. Not having any problems with it. It's kinda straightforward. But I was reading another post on here about a USB kinda built into it, and thought that would be a novel idea. Knowing that USBs supply 5v helps as well, and if I am right, the cord to the floppy drive has a 5v supply. Pretty convenient. My question is, could I just solder a USB connection to the floppy drive wires and be fine, or would I need some kind of current limitation, as I know that there could be a fairly high current flowing through those wires?
An example to further illustrate - if I had soldered a USB to the floppy drive supply and connected, say, my phone to charge, would there be any kind of negative effects?

Thanks guys!
 
Hey all,
So I'm currently working on turning a PSU into a benchtop power supply. Not having any problems with it. It's kinda straightforward. But I was reading another post on here about a USB kinda built into it, and thought that would be a novel idea. Knowing that USBs supply 5v helps as well, and if I am right, the cord to the floppy drive has a 5v supply. Pretty convenient. My question is, could I just solder a USB connection to the floppy drive wires and be fine, or would I need some kind of current limitation, as I know that there could be a fairly high current flowing through those wires?
An example to further illustrate - if I had soldered a USB to the floppy drive supply and connected, say, my phone to charge, would there be any kind of negative effects?

Thanks guys!

If converting a standard ATX type PSU to a bench PSU the first thing I suggest is fusing the outputs. That before all else.

Next, as to using it to supply 5 volt USP power? Sure that will work. Option one being to just hack and remove a USB connector and wire it. Option two (which I would do) is find a USB extender cable (Male one end Female on the other) and hack that. Makes for a nice USB socket on a wire extending from the PSU.

Now if you just plan to do things like charge a cell phone or other gizmo designed to be charged by USB power I would think about using the PSU 5 volt SB (Stand By) output. Newer PSUs offer 2 amp SB power and older ones an amp or so. I would fuse it at 1 amp. The merit here is the PSU need not be ON and running full bore to charge a cell phone.

Just My Take
Ron
 
Awesome. Thanks Ron! I went back and looked at the specs of my ATX, and the SB offers up 5v at .1A max. Personally, I wouldn't have much other use for it other than charging my phone while working. My bench is a tad out of reach of any USB, which is why I thought the idea would be somewhat handy. I appreciate the info! I will definitely have a little more insight, should I go that route.
 
Hmm...one more quick question. I don't know if you would quite know this or not, but I just noticed there are two purple wires. One leads back to the board and is clearly labeled +5vSB. No doubt about that one. The other, however, leads to a secondary board and is labeled CTRL. I hate assuming, so do you think that theyre both SB?
 
Purple would be the normal color scheme for 5 volt SB. I don't know what to tell you as to the labeling CTRL. The only times I have seen that term used on PSUs was on some HP units and other non standard OEM systems. Those also being older PSUs also. Since you only have 100 mA SB power I would not worry about using it. You don't want to overload that rail.

Ron
 
Okay so...things were going fine until I power it up. Following this diagram (attatched below). When I supply power, the standby light comes on...no problem...but there is a high pitched hissing sound coming from the unit. Any thoughts?
 
How old is this PSU? Never had one do that to me. You could run up to any Radio Shack and get a few 10 Ohm 10 Watt resistors and place one between the 3.3 volt bus and ground and or the 5 volt bus and ground. That will afford some loading. New units should not require it. Sounds like you are hearing the smps switching frequency.

Never mind, I see the resistor. You could try it on the 3.3 volt rail. :(

Ron
 
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Its so weird. Everything works just fine. Even plugged in a cd drive and all seems well. I took off the cover to see if I could 'see' where the sound is coming from, and it 'looks' like its coming from one of the transformers, so you may be right. It starts off a little erratic, but after about a second it seems to settle down, but still there. Kinda like its revving up. It's very weird and a little unsettling :/
 
I wouldn't worry about it. Long as it settles down.

Ron
 
Well, after much toiling and a little paint, my power unit is up and running. still get that hissing sound. left it on for a while and i havent noticed any adverse effects, so i guess that sound is just something i'll have to live with. thanks much for your help!
 
From my limited experience with SMPS. I have found that the Hissing is generated from an under-loaded power-supply. A lot of time when you purchase SMPS they specify the minimum current draw that the power supply should operate at.

Experiment and see if adding load to the 3.3V, 5V and 12V lines help in lowering the Hiss.

Regards,
 
You do NOT want to put a fuse in the ground line (BLACK wire) You need individual fuses in each power wire.

I understand the temptation to only use one fuse, but it will not provide the protection you want. Blowing a ground line fuse will not stop the flow of current in the other three wires. What it will do is change the voltages that your load circuit sees. The three voltages will stay the same relative to each other, but the ground point would then be floating. Depending on what the individual current loads are will determine what the apparent voltages end up being. The 5V line could go up or down. And could possibly even go negative.
 
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