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Forum Index : Microcontroller and PC projects : Power from a USB Hub
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James_From_Canb Senior Member  Joined: 19/06/2011 Location: AustraliaPosts: 265 |
I bought a 10 port USB 2.0 hub from Deal Extreme. I didn't bother with a power supply as I have a few old 5V wall warts left over from dead equipment. The MM works well if it is the only item plugged into the hub, and I don't use a power supply. I can even use a flash drive or two. But that's not the point of a 10 port hub. When I plug in the power pack I get a hum from my speakers. It's more apparent when the CPU load goes up, or that's how it seems. I checked out the Deal Extreme forum for the USB hub and there were lots of comments about cheapskate Chinese manufacturers leaving out a diode and replacing it with a 0 ohm resistor. Having cooked the occasional USB port before I am particularly sensitive to any power issues that might damage my laptop. I was in the process of getting an SMD diode to replace the resistor, but I read a posting on TBS yesterday about the voltage drop associated with diodes. I'm using a 5 volt power supply. If I install a diode, is the voltage drop likely to affect the MM? From the look of the posting it will, but it doesn't hurt to get it confirmed. It would also explain why the manufacturers use a resistor rather than a diode. Also, if I can't use a diode can anyone suggest a way of stopping the hum in the speakers? They're powered speakers plugged into the earphone socket on my laptop. I suspect an earth problem, even though they're both run off the same powerboard. The speakers have a 3 pin plug and the USB wall wart has 2 pins if that's relevant. I know that last question isn't strictly MM related, but it's tied in with the first question. Thanks James My mind is aglow with whirling, transient nodes of thought careening through a cosmic vapor of invention. Hedley Lamarr, Blazing Saddles (1974) |
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| wizged Newbie  Joined: 05/02/2013 Location: AustraliaPosts: 13 |
I had a hum loop problem some time ago where I had an earthed amplifier connected to a laptop running off mains power. The laptop power supply was also earthed and the problem disappeared when I unplugged the laptop power and ran it off batteries. If you have more than one earth point in your mix you might have a similar problem. Mine was fixed by using an audio isolation transformer from Jaycar. It was catalogued in the car audio section for some reason so not straightforward to find but it works well. I can probably find a link to it if you get stuck. |
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| hitsware Guru  Joined: 23/11/2012 Location: United StatesPosts: 535 |
3 wire mains plugs are hum prone. Try an 3 to 2 adaptor or just break off the third prong. |
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djuqa Guru Joined: 23/11/2011 Location: AustraliaPosts: 447 |
No Way Jose' That is neither a solution nor SAFE (In Aussieland / NZ it is also Illegal) VK4MU MicroController Units |
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| wizged Newbie Joined: 05/02/2013 Location: AustraliaPosts: 13 |
Jaycar doesn't seem to have the exact doover that I used any more but this seems to be the replacement, which should do the same job - http://www.jaycar.com.au/productView.asp?ID=AA3086 |
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MicroBlocks Guru Joined: 12/05/2012 Location: ThailandPosts: 2209 |
Could the hum come from a switched live/neutral. Is it going away when you plug it in the other way (2 prong plug of course, a 3 prong should be wired correctly in the first place, but you never know). Check with a live indicator (screw driver with a small neon light). Microblocks. Build with logic. |
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bigmik Guru Joined: 20/06/2011 Location: AustraliaPosts: 2870 |
James, The Powered HUB most likely has no `protection' diodes as just about all the cheapos I have lying around dont.. These are a problem if you wish to use a power adapter on them. As the +5V from the USB port and the +5v from your supply will connect together and effectively `fight' each other. One solution if you are ALWAYS going to use the power adapter on the hum is to open it up and cut the 5V wire (most likely the red one) where it comes from the USB input (or sacrifice a short USB M-F extension cable and cut the 5V line. This will mean the HUB and all peripherals will be powered ONLY via the external plug pack. That may or may not stop your hum though as I feel that is caused by earth loops. Regards Mick PS. This is an interesting Blog by Dave Jones on Grounded Power supplies that has a bearing on things here.. EEVBLOG Also I amplify Davids warning ... DO NOT break off the earth pin or use an illegal earthed to non-earthed adapter lead. Regards, Mick Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<< |
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| hitsware Guru Joined: 23/11/2012 Location: United StatesPosts: 535 |
Then don't use your toaster ...... :) |
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| JohnS Guru Joined: 18/11/2011 Location: United KingdomPosts: 3659 |
20+ minutes!! Do you by chance have a URL for something in simple text / bullet points to save listening at such length? John |
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| paceman Guru Joined: 07/10/2011 Location: AustraliaPosts: 1326 |
He's certainly a bit long-winded and the delivery needs polish, but he makes some important points and explains why with simple diagrams. The part about hooking up peripherals (e.g. USB gear etc) is something I hadn't thought about. I reckon a fair bit of the guff at the end about the differential probes could be given a miss though. |
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| bigmik Guru Joined: 20/06/2011 Location: AustraliaPosts: 2870 |
Yeah Sorry John, I generally watch all of his blogs and remembered this one so referrenced to it.. Basically it was about being extremely careful when using a CRO on earthed appliances.. As Mains earth is almost always on the GND leg of the Probe so if you are probing a device plugged into the MAINS that has Mains earth connected to singal GND you MUST only connect the probes GND to Signal GND to avoid a short circuit. He went on to show that even powered off a PC's USB port, the Signal GND is almost always the same as MAINS Gnd (via the PC). He showed that using a battery powered or not earthed power supply you could connect the probe gnd anywhere you liked without risking damage. OK.. It isnt directly related but is a bit of an eye opener about mains GND. I would still recommend watching the entire blog vid. anyway. Regards, Mick Mick's uMite Stuff can be found >>> HERE (Kindly hosted by Dontronics) <<< |
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| JohnS Guru Joined: 18/11/2011 Location: United KingdomPosts: 3659 |
Thanks. 20+mins for something I think I already know is not gonna happen. John |
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