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Thread: What if power is 4.3 not 5v

  1. #1
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    What if power is 4.3 not 5v

    Hi All,

    Again - I'm here not afraid to 'show off' my lack of knowledge in the world of electronics :-) (but I'm learning every day!). So, probably really dumb question, but here it goes...

    It turns out my 'puter is a bit weird in that the USB bus stays powered even after computer shutdown. This is coming to haunt me in the following way: we (I'm working with someone who does know electronics - thank god) powered the USB hub off of the Opus plugging into the 5v line. As it turns out, though, this was not only powering the hub, but the hub then - with its increased power - was in turn powering the computer making the computer think it wasn't really being turned off - so I remove key from ignition, and the computer keeps running (very weird, I actually don't recall specifically as to what we all tested, sorry, but that was the essence of it).

    Anyway - I'm told putting in a diode would ensure that power would flow just one way, but the price to pay for it is .7v. So, that leads me to the question: what if this hub, which has a requirement of 5v is only getting 4.3v? Will it not work at all? Damage the hub? Supply 'some' power (better than none)? Other?

    Any help is appreciated (as always).

    Thanks,
    Per

  2. #2
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    4.3V is outside the spec for most TTL chips. They tolerate 4.75 - 5.25V. Higher may damage the chips. Lower may cause them to act erratic.

    The good news is that many devices are built from CMOS technology that can work at lower voltages too. Many even transform the 5V into 3.3V to operate. So you can just try and see what happens.

    You can also use a different type of diode, a Scottky. These have a lot lower voltage drop, somewhere in the 0.2 - 0.3V range.

  3. #3
    Variable Bitrate Banderon's Avatar
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    Wait, can you explain this to me a bit better? Your hub, which is powered off the Opus, is not letting the computer shut down because the +5v line is active? And you want to put in a diode into the USB cable between the hub and motherboard? So, basically your computer is staying on because it's receiving voltage on the +5v line?


    Why not just disable "power-on via USB" in the BIOS?

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    That BIOS setting has no effect . I called Shuttle on this, like I mentioned above, and they confirmed it's by design and you can not disable it anywhere.

    The USB hub - which can run off of bus power and/or external power, does need external power to run all of the devices I have properly. So, my friend ran a 5v power line from one of the OPUS power connectors to the hub - thereby powering the hub. We also wired in a reset switch that's mounted between the two front passenger seats that allows me to reset the PC should it get hung or something.

    What we noticed right away (and I'm a bit cloudy on it at this point - we need to retest a couple of scenarios) was that all of a sudden, the front led's were lit even when they shouldn't have been (hard drive access, etc.) - that sounds like the reset switch since its wired into that connector - but as soon as we pulled the power from the hub, they would turn off again.

    While powered via this 5v line, then. the PC wouldn't basically turn off. So, I'm assuming that it was drawing power from the hub somehow..but yeah - it doesn't make too much sense to me at this point. Right now, the 5v power is disconnected, the reset switch is active, and everything works like a charm (minus, of course, me being able to use all of my USB devices).

    I will try to do some retesting tonight and let you know how it goes. But any ideas? I honestly can't believe that theyr'e making mobo's that keep the USB bus powered...and your idea to diable usb power on was exactly my first idea..but no go.

    Truly weird..

    -Per

  5. #5
    Variable Bitrate Banderon's Avatar
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    So yah, it sounds like you need to cut open the USB cable between the hub and PC, and pop a diode onto the power line to keep the power from backing up to the PC. Or, if you took the PCB out of the, ssolder on a diode at the connector.

    Frankly, I'm surprised. I'd think the hub would have a diode as standard. My friend had a similar problem with his phone's car charger. When the car was off, the power would back up from cell into the car, draining the phone. He wired in a diode and fixed it.


    I'm a little confused about how you're thinking of wiring things (and I'm not sure how the OPUS works exactly), but... if putting in a diode would drain too much voltage, you could use a +7v line instead of the +5v, and use a 5v regulator to drop the line down to the 5v that you need, AFTER it passes through the diode.

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