Author Topic: EEVBlog #285 - Isolated USB Lab Supply  (Read 48558 times)

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Offline EEVblog

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #75 on: May 31, 2012, 08:23:52 am »
Server motherboards and workstation grade motherboards. There are several ones.

So you claim.
Even if that's the case, my supply will still work with virtually 100% of intended machines out there that don't.
Seriously that's all I care about, I'm trying to be practical and design the cheapest/simplest device possible.
If anyone has any real evidence that says my device will not work on the vast majority of USB ports, then please present it. And I mean real evidence, not just speculation and hearsay and guesses.

According to the USB standard, I am 100% ok with:
- Powered hubs.
- Any USB port with the charging specification, that means every wall wart charger, car charger etc, and any modern computer that implements it.
- I'm also ok with any standard USB port that does not implement 100mA current limiting (which the standard does NOT say to implement, it's only a software power budget thing). That is practically almost every USB port in existence in notebooks and desktop computers. If anyone has any real evidence to back up that is not so, please present it. If my PSU worked on 95% of USB ports I'd still be happy I think. As it stands I think it'll work on practically 100%, and that's a fact. A very inconvenient fact for those arguing that I'm making a bad choice.

I am not ok with any power specifically configured as "low power". But nothing you can do about that of course, the PSU will only be capable of 0.5W

Yes, I'm not designing a proper compliant USB device, I know that, I accept that, and am doing that to keep it simple and low cost. Sue me.

Quote
Now, the enumeration isnt hard.. Slap on an ftdi232 chip and set the current limit in the eeprom to 500 mA . Done. And you can now control the supply through a serial port , or log the current consumption ! Instant chart recorder ! Now, that opens possibilities. For people that dont need that feature and are cost sensitive: dont solder the ftdi on ...

No, you don't get to control the device for free. Adding a FTDI chip requires comms isolation. You've just double the cost and complexity of the project, not to mention additional space. It is a no-go.

Dave.
 

Offline EEVblog

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #76 on: May 31, 2012, 08:26:25 am »
I'll elaborate. The reason I thought binding posts are overkill here is simply because they are large, bulky, expensive, protruding, and generally capable of 15-30 Amps.  Overkill.  It's ultimately your design, and others may not think the same as me, but I'd be happy with a thinner device, not a bulkier device. I was thinking thin, not bulky, and mentioned captive or pluggable red/black cables with croc clips.  As you said, it should fit inside a laptop bag. I'd like to see it just 1cm thick.  Capacitive touch sliders and buttons, and a 2 line LCD, like free_electron mentions.  If there's no USB data capabilities, then for sure it needs some control and display on board. Just like the binding posts, I wouldn't want to have a large rotary encoder protruding. 

In my opinion, something as large as the uCurrent is too large for a laptop bag. (your prototype uSupply shows the same packaging, but I acknowledge that  you might have different ideas for the final product)

I do. Like I've said, don't take the earlier prototypes as the final form factor. The binding posts are going horizontal.

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If you put 4mm binding posts, I'd like to see them side mounted, not top mounted, in order to make the package ultra thin. Not much larger than the base of the binding post itself.

Bingo.

Dave.
 

Offline Stephen Hill

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #77 on: May 31, 2012, 08:29:53 am »
Dave,

I have to agree with others that the USB 2.0 Specification states that a device must not draw more than 100mA until it has negotiated 500mA with the host.

The following are from section 7.2.1 of the Universal Serial Bus Specification Revision 2.0. I have emphasised the key points.

Quote
The power source and sink requirements of different device classes can be simplified with the introduction of the
concept of a unit load. A unit load is defined to be 100 mA. The number of unit loads a device can draw is an
absolute maximum, not an average over time. A device may be either low-power at one unit load or highpower,
consuming up to five unit loads. All devices default to low-power. The transition to high-power is under
software control.
It is the responsibility of software to ensure adequate power is available before allowing
devices to consume high-power.

Quote
High-power bus-powered functions: All power to these devices comes from VBUS. They must draw no
more than one unit load upon power-up and may draw up to five unit loads after being configured.

However, you may be correct that in general most usb hosts do not enforce this specification.

Maybe you could do a quick video which tests the current capability of a few USB ports to either prove or disprove this myth?

Cheers
Stephen
 

Offline EEVblog

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #78 on: May 31, 2012, 08:34:19 am »
What I mean with hubs is the following.  An unpowered USB hub draws up to 500 mA from the upstream host and can provide up to 100 mA each to up to 4 downstream devices.  However, they don't implement 100 milliamp current limit on the downstream port because most of them also support operating in powered mode with a 5V 2A power supply.  If the external power adapter is used the ports operate as genuine high power ports.  If you plug your supply into such an unpowered hub, you will potentially draw more power than it is capable of supplying, and there will be no current limiting to stop it.  Instead, the hub and any other devices connected to it may lose power because the upstream host is being asked to supply more than 500 milliamp.

Unpowered USB hubs will be an issue, and one that isn't going to be solved with negotiation. The simple fact is you can't have a USB PSU capable of drawing potentially 500mA, and have other devices on the hub as well taking power. It won't work.

Dave.
 

Offline EEVblog

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #79 on: May 31, 2012, 08:46:30 am »
Quote
High-power bus-powered functions: All power to these devices comes from VBUS. They must draw no
more than one unit load upon power-up and may draw up to five unit loads after being configured.

However, you may be correct that in general most usb hosts do not enforce this specification.

For the last time, what people don't understand here is whilst this is telling you what a compliant hardware device should do, it is effectively a  software negotiation specification. It is not a hardware specification. Ports do not limit the current to 100mA first and then open that up to 500mA when requested.
That clause is in there to ensure that multiple devices on the same bus that may require more than 100mA, do so by asking nicely first "please sir, can I take more than 100mA?". The PC knows how many devices are already connected to that bus, so can them tell the device that's it's ok (or not) to use the 500mA available (at all times anyway) on the port.

If you know you are the only device connected to the port, then it certainly possible to cheat and simply draw your 500mA.
It's not nice, and it's not a USB compliant device, but there is virtually no real practical issue with cheating like this.
There are several good reasons why I'm taking this shortcut, and I accept any limitations it may have. I'm a bad, bad, person!, get over it people, really.

There are ports called "low power ports" that only have 100mA at all times, but these are relatively rare. Nothing you can do about that.

Quote
Maybe you could do a quick video which tests the current capability of a few USB ports to either prove or disprove this myth?

I've done it countless times over many many years. General USB ports do not limit the current 100mA. 500mA is available at all times.
Even if I measured 100 USB ports in a video, people would still crap on that what I'm doing is wrong.
Those who don't believe me can go around themselves and stick a 10 ohm resistor on USB ports and see for themselves.

Dave.
« Last Edit: May 31, 2012, 08:53:31 am by EEVblog »
 

Online ejeffrey

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #80 on: May 31, 2012, 08:51:58 am »
Unpowered USB hubs will be an issue, and one that isn't going to be solved with negotiation. The simple fact is you can't have a USB PSU capable of drawing potentially 500mA, and have other devices on the hub as well taking power. It won't work.

I don't care that the power supply doesn't work in this case.  Obviously it can't work, and negotiation won't fix that.  My problem is potentially crashing the hub and other devices on the hub by drawing down their power supply.  This is not good.  Especially if those devices are USB flash drives.

I don't doubt that your power supply will work with 99% of host ports and many hub ports, including unpowered hub ports.  However, it absolutely has the potential to cause malfunctions in compliant devices including the potential for data loss.  I don't consider this acceptable behavior.  It is clear that you do, and that you expect the user to have some sense to use it as designed (i.e., only with high power capable ports).  That is a fair position, but denying there is a problem is simply incorrect.
 

Offline hlavac

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #81 on: May 31, 2012, 09:00:22 am »
That is a myth. USB ports do not require negotiation to supply 500mA, they must supply 500mA at all times regardless.
Approved USB devices are required to "play nice" and negotiate first before drawing 500mA, but it makes no actual difference to the USB hosts ability to supply it.

There are situations where a port can not provide 500mA - specifically bus powered hubs. These need one power unit for themselves and provide up to four ports one unit each (100mA) for a total upstream load of 500mA. If you connect a high power device to such port it will not be configured by the host and error message about insufficient power will be issued.

From USB 2.0 specification, section 7.2.1.4 High-power Bus-powered Functions:
Quote
A high-power function requires staged switching of power. It must first come up in a reduced power state of less than one unit load. At bus enumeration time, its total power requirements are obtained and compared against the available power budget. If sufficient power exists, the remainder of the function may be powered on.

Also note hubs are allowed to introduce additional funny things like voltage drops way below the 5.00V.
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Offline samgab

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #82 on: May 31, 2012, 09:00:58 am »
Many modern microcontrollers have USB peripherals on them; presumably you are using a microcontroller on the board to drive the displays, so why not pick one with USB and do the enumeration to get 500mA?

That requires isolation of the data too. Adding large cost and complexity, for almost certainly no benefit.

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If it fails you could make something like the current display flash to indicate that the USB current is being exceeded. Just like I don't like using a 3A diode above 3A, or a 40 MHz micro above 40 MHz, it's just bad engineering practice. Sod's law will mean that some really popular series of laptops or desktops have current limiting.

I've yet to see one or hear of a confirmed case of one that does 100mA current limiting.
To increase the cost and complexity on such an otherwise simple design just to do proper enumeration that is not needed in practice would just be silly.
Sorry to all the perfectionists out there. Reality engineering wins out.
No further correspondence will be entered into.

Dave.

Yep, I like your idea best. A nice simple device that functions well for it's intended task. I don't see any reason for extra complexity to make it talk to the PC. Basically just use the USB connector as a source of the power for the device, and the device being small, relatively simple, and self-contained sounds brilliant to me. This way, it will be a nice simple kit that doesn't need any complex programming, just soldering a chip or two and some passives, a screen and a connector onto a PCB, and away you go!
 

Offline EEVblog

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #83 on: May 31, 2012, 09:02:39 am »
I don't doubt that your power supply will work with 99% of host ports and many hub ports, including unpowered hub ports.  However, it absolutely has the potential to cause malfunctions in compliant devices including the potential for data loss.  I don't consider this acceptable behavior.  It is clear that you do, and that you expect the user to have some sense to use it as designed (i.e., only with high power capable ports).  That is a fair position, but denying there is a problem is simply incorrect.

I am not denying there is a potential problem.
Thank you for thinking that I'm taking a fair position, I think it's a fair compromise too.
Any well designed USB port (low or high power) should have current limiting on it (the standard says they must), so damage to low power ports is unlikely.
I agree it would be nice to have the unit check to see if the port is low or high power first and then software limit the available output power, but it's just not worth it for the few cases where this might be an issue. It increases the cost, complexity, and size of the design considerably.

Dave.
 

Offline EEVblog

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #84 on: May 31, 2012, 09:08:29 am »
There are situations where a port can not provide 500mA - specifically bus powered hubs. These need one power unit for themselves and provide up to four ports one unit each (100mA) for a total upstream load of 500mA. If you connect a high power device to such port it will not be configured by the host and error message about insufficient power will be issued.

In that case, too bad. You either limit the PSU to 0.5W, or you dispense with the hub. it's not like you can have the PSU taking 500mA and the other devices on the hub too sucking current current. So the hub is useless, you may as well disconnect it.

Negotiation does not fix this problem on self powered hubs.

Dave.
 

Offline samgab

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #85 on: May 31, 2012, 09:13:07 am »
Which type of USB connector do you think it would be best to use on the device (Micro/Mini/Standard)?
Are they all rated for the same power/current capabilities?
How do the rated connect/disconnect numbers compare?
I read on wikipedia that standard USB A is only rated for 1500 connect/disconnects, whereas the newer micro are 10,000 and mini is 5000... Strange, because the smaller connectors don't seem as robust?
 

Offline DarkPrince

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #86 on: May 31, 2012, 09:13:44 am »
I could, potentially, test my laptop's USB ports then. :D I believe (speculate) that the BIOS throws a fault when I over-load my USB ports. I was using a quite large development board, a logic analyzer, wireless USB mouse, and a USB flash drive... System then throws some IRQ error and reboots. Not worried about it with the power supply, I thought it was just an interesting thought. It is up to the user to make sure such conditions are met when pulling larger loads, not you.  8)

About the hub, always used one with an external power brick, capable of supply 2.5A @ 5V (7 Port HUB).
 

Offline IanB

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #87 on: May 31, 2012, 09:14:10 am »
A curious situation happens with my iPhone. The proper iPhone power cube is marked 5 V 1 A and doubtless the iPhone takes this much current from it when charging.

If I plug the iPhone into a regular USB port on a computer it gets recognized as a USB device and also charges, but it doesn't shut down the USB port. So I presume it limits itself to 500 mA in this case.

If I plug the iPhone into a USB charging port on a particular Duracell battery charger, it overloads the port and shuts it down. At this point I do not know if the phone is trying to pull 1 A and causing an overload, or if it is trying to pull 500 mA and the USB port can't supply it.

I presume if I want to find out what is going on I need a USB breakout cable and to insert an ammeter in the circuit. Maybe I'll get round to doing that at some time.
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Offline EEVblog

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #88 on: May 31, 2012, 09:17:08 am »
Which type of USB connector do you think it would be best to use on the device (Micro/Mini/Standard)?
Are they all rated for the same power/current capabilities?
How do the rated connect/disconnect numbers compare?
I read on wikipedia that standard USB A is only rated for 1500 connect/disconnects, whereas the newer micro are 10,000 and mini is 5000... Strange, because the smaller connectors don't seem as robust?

IIRC mini B is the worst with only several hundred insertions tops?
Micro USB is the best, and one of the main reason why the switched to it.
I was thinking Micro-B is best, but happy to have a vote on it. Unfortunately on the new design there is not enough room for two, unless I mounted one on the bottom of the board, but then it's a two sided machine assembly which is not desirable.

Dave.
 

Offline mikeselectricstuff

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #89 on: May 31, 2012, 09:27:48 am »
IMO although micro is the 'new' standard, mini is still more common in the avarage pile of random USB leads.
Both would be ideal, to the extent that a slight size increase may be justified. If there is no room, at least put a footprint on the other side so the a) user can fit the other if they want, and b) it gives a repair option if the socket gets ripped off the board.
 
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Offline EEVblog

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #90 on: May 31, 2012, 09:32:38 am »
IMO although micro is the 'new' standard, mini is still more common in the avarage pile of random USB leads.
Both would be ideal, to the extent that a slight size increase may be justified. If there is no room, at least put a footprint on the other side so the a) user can fit the other if they want, and b) it gives a repair option if the socket gets ripped off the board.

Good point, an extra footprint is worthwhile. I'll see if it fits, as there might be an issue with the case fouling.

Dave.
 

Offline samgab

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #91 on: May 31, 2012, 09:34:57 am »
Quote
"What Your Mom Didn't Tell You About USB

With any standard, it's interesting to see how actual practice diverges from the printed spec or how undefined parts of the spec take shape. Though USB is, with little doubt, one of the best thought out, reliable, and useful standards efforts in quite some time, it has not been immune to the impact of the real world. Some observed USB characteristics that may not be obvious, yet can influence power designs, are:

USB ports do NOT limit current. Though the USB spec provides details about how much current a USB port must supply, there are mile-wide limits on how much it might supply. Though the upper limit specifies that the current never exceed 5A, but a wise designer should not rely on that. In any case, a USB port can never be counted on to limit its output current to 500mA, or any amount near that. In fact, output current from a port often exceeds several Amps since multi-port systems (like PCs) frequently have only one protection device for all ports in the system. The protection device is set above the TOTAL power rating of all the ports. So a four-port system may supply over 2A from one port if the other ports are not loaded. Furthermore, while some PCs use 10-20% accurate IC-based protection, other will use much less accurate poly-fuses (fuses that reset themselves) that will not trip until the load is 100% or more above the rating.

USB Ports rarely (never) turn off power: The USB spec is not specific about this, but it is sometimes believed that USB power may be disconnected as a result of failed enumeration, or other software or firmware problems. In actual practice, no USB host shuts off USB power for anything other that an electrical fault (like a short). There may an exception to this statement, but I have yet to see it. Notebook and motherboard makers are barely willing to pay for fault protection, let alone smart power switching. So no matter what dialog takes place (or does not take place) between a USB peripheral and host, 5V (at either 500mA or 100mA, or even maybe 2A or more) will be available. This is born out by the appearance in the market of USB powered reading lights, coffee mug warmers, and other similar items that have no communication capability. They may not be "compliant," but they do function."

Source: http://www.maxim-ic.com/app-notes/index.mvp/id/3241
 

Offline hlavac

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #92 on: May 31, 2012, 09:37:54 am »
In that case, too bad. You either limit the PSU to 0.5W, or you dispense with the hub. it's not like you can have the PSU taking 500mA and the other devices on the hub too sucking current current. So the hub is useless, you may as well disconnect it.

Negotiation does not fix this problem on self powered hubs.

It does not magically make it work, It stops things from blowing up for unfortunate users.
The user may not know what type of port is he plugging into. Some old/cheap laptops have only one USB "root hub" and have an internal hardware hub to multiply the ports. I had my eee PC blown up by a power hungry USB device with what amounted to power rail short.

I agree doing things "properly" will make it complex and expensive.
I would suggest you at least issue a big fat warning that users should only plug this into a high power host ports or powered hubs.
Maybe check for too low input voltage and shut down if the power is not strong enough. DC-DC converters can do nasty things if there is no undervoltage lockout (AVR Dragon blowup reference here).

Anyway, in a theoretical properly USB negotiating version, I'd consider turning the thing around and keeping the controller on the non-isolated side so that it can easily talk to USB, and figure out how to isolate the feedback/current/voltage readings. May be easier than isolating USB communication because of the speed and bidirectional nature of USB.
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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #93 on: May 31, 2012, 09:42:27 am »
Something of interest is the power tab in device manager; when looking at the root hubs on my Dell it looks rather like registered power rather than real power used, but handy to know.



 

Offline samgab

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #94 on: May 31, 2012, 09:43:11 am »
IIRC mini B is the worst with only several hundred insertions tops?
Micro USB is the best, and one of the main reason why the switched to it.
I was thinking Micro-B is best, but happy to have a vote on it. Unfortunately on the new design there is not enough room for two, unless I mounted one on the bottom of the board, but then it's a two sided machine assembly which is not desirable.

Dave.

I don't think the device needs to have 2 USB ports... But you could look at doing what many of the portable hard drive enclosures do. Due to their current draw, they often use a cable that has one connector that goes into the HDD, and 2 USB plugs at the PC end, to draw more power from 2 ports rather than just 1.
 

Offline EEVblog

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #95 on: May 31, 2012, 09:44:36 am »
I would suggest you at least issue a big fat warning that users should only plug this into a high power host ports or powered hubs.

Of course.

Quote
Anyway, in a theoretical properly USB negotiating version, I'd consider turning the thing around and keeping the controller on the non-isolated side so that it can easily talk to USB, and figure out how to isolate the feedback/current/voltage readings. May be easier than isolating USB communication because of the speed and bidirectional nature of USB.

Yes, in such a case you'd have to weigh up which is the easiest and cheapest method to use. I doubt it would be an easy choice to make, lots of factors involved.

Dave.
 

Offline samgab

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Offline IanB

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #97 on: May 31, 2012, 09:53:11 am »
Something of interest is the power tab in device manager; when looking at the root hubs on my Dell it looks rather like registered power rather than real power used, but handy to know.

Well, you learn something new every day! I did not know about that power tab.

So, apparently it answers my iPhone question:



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Offline EEVblog

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #98 on: May 31, 2012, 09:56:41 am »
Quote
"What Your Mom Didn't Tell You About USB
USB ports do NOT limit current. Though the USB spec provides details about how much current a USB port must supply, there are mile-wide limits on how much it might supply. Though the upper limit specifies that the current never exceed 5A, but a wise designer should not rely on that. In any case, a USB port can never be counted on to limit its output current to 500mA, or any amount near that.

I've found that port protection and current limiting is in fact quite common. In fact it is required by the standard, but the exact value is indeed not specified:

Quote
11.4.1.1.1 Over-current Protection
The host and all self-powered hubs must implement over-current protection for safety reasons, and
the hub must have a way to detect the over-current condition and report it to the USB software.
Should the aggregate current drawn by a gang of downstream facing ports exceed a preset value,
the over-current protection circuit removes or reduces power from all affected downstream facing
ports. The over-current condition is reported through the hub to the Host Controller, as described in
Section 10.11.5. The preset value cannot exceed 5.0 A and must be sufficiently higher than the
maximum allowable port current or time delayed such that transient currents (e.g., during power up
or dynamic attach or reconfiguration) do not trip the over-current protector. If an over-current
condition occurs on any port, subsequent operation of the USB is not guaranteed, and once the
condition is removed, it may be necessary to reinitialize the bus as would be done upon power-up.
The over-current limiting mechanism must be resettable without user mechanical intervention.
Interoperability and Power Delivery
11-5
Polymeric PTCs and solid-state switches are examples of methods that can be used for over-current
limiting.

Good thing my USB supply ensures it does not take more than 500mA by design.
I'm a bad, bad, person for not doing negotiation, but I'm not that bad  ;D

Dave.
 

Offline EEVblog

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #99 on: May 31, 2012, 10:06:05 am »
Here's someone doing a project along these lines:
http://www.kickstarter.com/projects/210251816/usb-based-variable-power-supply-for-small-projects-0?ref=live

Well I'll be damned.
Testing leds with no current limit??
And what's the deal with D2 on the output after the feedback sensing divider?
No isolation, and less than 5V limit (which will drop significantly with the cable) doesn't make it very capable IMO.

Dave.
 


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