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Beware powering via USB

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#1 astrokeith

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Posted 27 February 2021 - 10:36 AM

I've recently need doing some projects with Raspberry Pi's - a feature of which is a built-in low voltage warning.

 

I've noticed the warning coming on quite a lot, which led me down a path of checking supplies, cables and connectors.

 

The upshot is, some of my 'cheaper', longer USB cables had a resistance approaching an ohm for the round trip. At 1 amp thats a whole volt drop. Whats worse, as the processor ramps up or a ccd exposure starts, the current changes - and so does the voltage at the device.

 

I sacrificed a couple of cables to an autopsy. There was almost no copper in them! 5 strands of copper thinner than a human hair.

 

Half of my box of assorted USB cables have now gone to recycling (not that there is much copper to be salvaged!). 

 


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#2 jerahian

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Posted 27 February 2021 - 10:43 AM

Is there a way to check the quality of a USB cable without an autopsy?



#3 BlueMoon

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Posted 27 February 2021 - 10:49 AM

Is there a way to check the quality of a USB cable without an autopsy?

I had the same issue as astrokeith but with an inline on/off push button switch. Full one volt drop across the contacts was giving me low voltage warnings.  You can check for resistance with an ohmmeter but it's a bit tedious on the micro-usb connectors.



#4 doole

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Posted 27 February 2021 - 11:02 AM

Where do we go the get the adult ones?



#5 astrokeith

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Posted 27 February 2021 - 11:17 AM

Where do we go the get the adult ones?

Good question!

It seems USB C can be certified. But previous generations are hit and miss.

A recognised brand name such as Anker is probably OK. Not from the multitude of weird brands on Amazon!



#6 BlueMoon

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Posted 27 February 2021 - 11:25 AM

 

A recognised brand name such as Anker is probably OK.

From my experience Anker and Belkin are about the best quality. Here's a short article on distance and a few other considerations: https://www.yourcabl...em_ep_42-1.html


Edited by BlueMoon, 27 February 2021 - 11:29 AM.


#7 PartlyCloudy

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Posted 27 February 2021 - 11:31 AM

Is there a way to check the quality of a USB cable without an autopsy?

Well... if you search (I just did) for USB3 AWG 22 or 24 cable, that has some hope of giving you that actual gauge when you purchase one advertised as such.  Not all of the wires need to be that heavy, depending on the USB specification (see the Wikipedia USB page for an idea of that can of worms) but the power lines do.  You still need to be careful, since I have bought cables online that said they were one gauge and turned out to be another (those were standard 12 volt power cables, not USB).  Watch the buyer feedback on those.

 

As for what to search for (you may know all this.. if so, sorry!):

 

So, voltage drop = current * resistance of wire.  Stranded wire resistance per foot varies some, but probably not enough to worry about in this case.  One chart I found listed resistance of 24 gauge wire at 21-26 (approx) ohms per thousand feet.  Assuming the connectors are properly made, I discount that resistance.  For 1 amp of power (12 watts at 12 volts, as it happens, or for usb,5 watts at 5 volts), and 10 foot cable:

 

voltage drop  = 1*(10*26/1000) or about .26 volts.

 

(forgot to add:  If they just tell you the power required, not the current, you can figure that out as Current = Power/Voltage. 12 watts/12 volts = 1 amp, 1 watt/5 volts would be .2 amps, etc.)

 

Continuing with the 1 amp example:

 

For 34 gauge, the resistance jumps to 237 ohms/1000 feet or a 2.3v drop in 10 feet.  That is a drop from whatever the starting voltage is (for USB, that would be 5v of course).  This would be bad.  AWG 28 stranded is about 65 ohms/1000 feet (.65v drop).

 

I think they only use one power wire in USB2.x and 3.x (USB C is another kettle of worms as well... Interesting and with some new features, but not really important here).

 

I also have a USB extender cable that has a power jack on it, I believe, near the "far" end.  I expect that is to either bump up the capacity or something else.  Any longer run I always terminate into a powered hub, then make the last hop shorter but with hopefully good (AWG 24 or similar) cables.

 

Hope I got this right...  Got my EE in 1981, so am a bit rusty..

 

-Mike


Edited by PartlyCloudy, 27 February 2021 - 11:36 AM.

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#8 BlueMoon

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Posted 27 February 2021 - 12:30 PM

Another article to supplement what PartlyCloudy posted: http://jon.netdork.n...-notifications/ with some Raspberry Pi specifics included.


Edited by BlueMoon, 27 February 2021 - 12:31 PM.


#9 alphatripleplus

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Posted 27 February 2021 - 01:32 PM

Hmmm... I've been a little suspicious of some of my USB cables. Thanks for the tip.



#10 RogerM

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Posted 27 February 2021 - 05:22 PM

Voltage drop and another parameter called 'insertion loss' (signal loss attributed to connectors) are issues we dealt with in the labs on a frequent basis.  In the instrumentation field we dealt with voltage drops on cabling runs in excess of a hundred feet by using twisted pair wiring for each +/- signal, power and ground returns.1/Rt = 1/Ra+1/Rb....using 22AWG wires at a minimum, sometimes going to 18AWG.  The only time we went below 22AWG was for interconnect wiring such as jumpers on a PCB that were less than an inch or two in length.

 

Most of the time the main parameter driving commercial-off-the-shelf (COTS) consumer brands revolves around meeting a data rate transmission spec but that spec uses parameters based in the digital realm.  As long as a receiving device can detect level shifts that represent a logic '0' or '1' that's all that matters.  Sometimes the deviation between what a device calls '0' can fluctuate more than 1/8 to 1/4 volt...same goes for a logic '1'.  With that in mind digital systems can tolerate voltage level shifts between 0-5V to 0.25V-4.75V (almost a full volt difference) for standard 5V TTL based logic circuits.  Low power systems can run logic levels around 3V-3.3V so its to be expected when one uses an off market cable constructed with 28AWG to 36AWG wires to have either data comms and /or general systems issues (such as powering another device off a powered or passive USB network.)

 

As a general observation, I always pass up on USB cables that appear to be less than 1/4" thick in diameter (considering insulation thickness) as it's an approximate guide as to the thickness (gauge) of the internal wires.  Thinner cables are more flexible but there's another thing, in our hunt for flexible cabling (because we want the least interaction between a stiff cable and how it can affect a mounts performance) we may naturally select a cable based upon that one parameter...flexibility.  There are ways to make 'cable runs' using thick-stiff cables that will have the least 'physical' impact on system performance. Large-ugly loops or cable droops may be unpleasing to the eye but there is a purpose to that that's not cosmetically driven.

 

There's a reason why we used wire/cable from industrial suppliers like Belden, Anixter... you knew the materials used, gauge size, etc were certified to be exactly what you specified. The same reason applies for connectors from industrial suppliers such as Amphenol, Lemo...nowadays, I totally pass up on the 'bargain' items listed on such places like Amazon and I even pass up on the majority of COTS at retail box stores such as Best Buy, etc.  I stick to what I've learned even though it may cost a premium (or I hand fabricate my own cabling), satisfied that I've never had issues arising from having saved a few dollars looking for the 'best deal'.


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#11 t-ara-fan

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Posted 27 February 2021 - 08:13 PM

 

A recognised brand name such as Anker is probably OK. Not from the multitude of weird brands on Amazon!

I buy my Anker cables from Amazon.

 

I have a variety of USB cables lengths in my setup. 12", 18", 24", etc. So I can use a cable as short as possible, and avoid having a 6' cable running 18" with it coiled and zip-tied neatly.
 



#12 dx_ron

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Posted 16 March 2021 - 12:46 PM

While I would love super-duper flexible cables, I'm ok with "reasonable" usb cables and mostly go with Anker. But one particular type drives me nuts - the usb-A to usb-B "printer" cables. I need one running from the pi on the tripod to a powered hub on the telescope. All the ones I have tried are bulky and stiff (do they think people swing printers around by the cable, or something?).

 

I did just buy some silicon-jacketed 18-gauge stranded for 12v runs https://smile.amazon...2?ie=UTF8&psc=1

First impression is that it is wonderful stuff, but I haven't used it yet.

 

Roger, as we have the same mount, - do you have a photo of how you arrange your "large-ugly loops"? I think I finally have a decent solution, but I'm always on the lookout for a better way.

IMG_20210303_175108368_HDR.jpg

 

All the usb cables bundled up on top are from the usb dew straps. Need to do something about those. The excess usb from the camera will be replaced soon when the dslr is replaced.



#13 davidmalanick

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Posted 16 March 2021 - 02:28 PM

While I would love super-duper flexible cables, I'm ok with "reasonable" usb cables and mostly go with Anker. But one particular type drives me nuts - the usb-A to usb-B "printer" cables. I need one running from the pi on the tripod to a powered hub on the telescope. All the ones I have tried are bulky and stiff (do they think people swing printers around by the cable, or something?).

 

I did just buy some silicon-jacketed 18-gauge stranded for 12v runs https://smile.amazon...2?ie=UTF8&psc=1

First impression is that it is wonderful stuff, but I haven't used it yet.

 

Roger, as we have the same mount, - do you have a photo of how you arrange your "large-ugly loops"? I think I finally have a decent solution, but I'm always on the lookout for a better way.

attachicon.gifIMG_20210303_175108368_HDR.jpg

 

All the usb cables bundled up on top are from the usb dew straps. Need to do something about those. The excess usb from the camera will be replaced soon when the dslr is replaced
I used Amazon Silicone for my 12v power also.  6 below zero farinhight

I used the Silicone wires for my 12v power also. Tested them in -6 Deg. weather and they were totally flexible.  Only 2 wires hanging from my mount, the 12v Silicone I made hangs like a piece of string.  

I wish someone made silicone USB cables!  No one does.



#14 RogerM

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Posted 16 March 2021 - 03:27 PM

While I would love super-duper flexible cables, I'm ok with "reasonable" usb cables and mostly go with Anker. But one particular type drives me nuts - the usb-A to usb-B "printer" cables. I need one running from the pi on the tripod to a powered hub on the telescope. All the ones I have tried are bulky and stiff (do they think people swing printers around by the cable, or something?).

 

I did just buy some silicon-jacketed 18-gauge stranded for 12v runs https://smile.amazon...2?ie=UTF8&psc=1

First impression is that it is wonderful stuff, but I haven't used it yet.

 

Roger, as we have the same mount, - do you have a photo of how you arrange your "large-ugly loops"? I think I finally have a decent solution, but I'm always on the lookout for a better way.

attachicon.gifIMG_20210303_175108368_HDR.jpg

 

All the usb cables bundled up on top are from the usb dew straps. Need to do something about those. The excess usb from the camera will be replaced soon when the dslr is replaced.

When I set-up my rig I looked for ways to consolidate like components based upon connectivity.  The majority of my wiring are very short spans, using the supplied ASIAir Pro's supplied DC cables (stiffness doesn't matter because the cables reside on the imaging train itself) and some custom fab'd signal/power cables that (for me) did not take that long to fabricate.

 

As a result, only three cables make the span across the RA axis, one 12V cable for the ASIAir to the mount, mount signal to hand controller and the main 12V feed (the only cable that runs all the way  down from the imaging train to my battery source.)

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