Author Topic: EEVBlog #285 - Isolated USB Lab Supply  (Read 49015 times)

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Offline tom66

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #50 on: May 30, 2012, 02:45:46 pm »
It isn't entirely clear to me how isolated the supply is intended to be. Is it intended to be isolated enough to connect the output side to a mains powered device, or simply sufficient to tie them in series to produce +/-12V rails? If it's the latter, isolation isn't so difficult: just make the power supply output floating, and use a differential amp to measure voltage (you already need one for current, so it's not too much additional cost to do it for voltage too) then your USB doesn't need to be isolated. I think it's a recipe for disaster to have a device which is meant to be mains-safety isolated with a µCurrent style construction, but there's no problem with low voltage DC.
 

Online mikeselectricstuff

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #51 on: May 30, 2012, 02:51:14 pm »
Quote
* Power jack
I saw a DC jack on one of your prototypes, this is definitely a good idea.
If you're out-and-about and need more voltage/current then finding a 9-15v wallwart on some nearby device is pretty easy.
Coupling the dc jack with a rectifier deals with the polarity and ac/dc issue quite nicely.
DC jack (2.1mm) would be a useful addition - don't think adding the cap to support AC is necessary nowadays as most wall-warts are switchmode- rectifier for dual polarity probably a good idea though.

Re. the negotiation stuff - may be worth sticking down a footprint for an FTDI chip - maybe the new X series, even if you don't populate it.

In terms of display, if it's LED then 7-seg will probably draw less power than matrix  - you can get some pretty low power ones nowadays. Individual LEDs on the PCB might be able to squeeeze a little more efficiency at a slight reduciton in readability.
« Last Edit: May 30, 2012, 02:53:56 pm by mikeselectricstuff »
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Offline tom66

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #52 on: May 30, 2012, 02:54:21 pm »
For isolated USB you could consider one of these ICs:

http://www.analog.com/en/interface/digital-isolators/adum3160/products/product.html

$3.61 in 100qty might be a killer though - so again, it could be an optional feature. And this of course opens up the avenue for computer control, datalogging etc. It would be very useful to log the power consumption of a widget against time. If you break out some GPIOs on your micro to an unsoldered header (don't need any extra components, just bare GPIOs, let users install divider resistors if they need), you've just created a product which could measure power consumption or efficiency with temperature, humidity, processing load...
 

Offline Psi

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #53 on: May 30, 2012, 03:06:23 pm »
In terms of display, if it's LED then 7-seg will probably draw less power than matrix  - you can get some pretty low power ones nowadays. Individual LEDs on the PCB might be able to squeeeze a little more efficiency at a slight reduciton in readability.

Yeah, from a purely power usage factor, a 7 seg is going to use less power.
But i don't think led power usage is really that much of an issue on a usb powered device.

With a message scrolling on my 28x5 LED matrix ID tags the current draw was ~10mA most of the time. (20mA max if you had lots of pixels in use)
Now, granted, it was designed for a computer gaming event which is a darkened environment so that isn't really enough current for a bright daylight room. But it was pretty damn awesome for 10mA.
The liteon red led matrix arrays i was using only required 0.6mA (DC) to be visible in a semi dark room!
« Last Edit: May 30, 2012, 03:10:44 pm by Psi »
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Online mikeselectricstuff

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #54 on: May 30, 2012, 03:24:04 pm »
For isolated USB you could consider one of these ICs:

http://www.analog.com/en/interface/digital-isolators/adum3160/products/product.html

$3.61 in 100qty might be a killer though - so again, it could be an optional feature. And this of course opens up the avenue for computer control, datalogging etc. It would be very useful to log the power consumption of a widget against time. If you break out some GPIOs on your micro to an unsoldered header (don't need any extra components, just bare GPIOs, let users install divider resistors if they need), you've just created a product which could measure power consumption or efficiency with temperature, humidity, processing load...
If you wanted USB control/readback, it would be much cheaper to have a USB interface on the PC side and isolate the low-speed UART lines with a cheap optoisolator
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Online Monkeh

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #55 on: May 30, 2012, 03:31:03 pm »
Absolutely - must have 4mm sockets and binding posts - a possible compromise if space is short is 4mm sockets plus some sort of spring terminal - like the sort used for speakers on some amps.

I have considered the dual 4mm socket and spring terminal idea, as it does save protrusion space. But ultimately 4mm binding posts are just too irresistible.

Dave.

You can get some which are much lower profile than normal, only around 10-12mm projection, such as these: http://www.digikey.com/product-detail/en/111-2223-001/J587-ND/241133

The downside is the lack of colour coding, but if you can ignore a big plus and minus sign on the device, you can ignore the colour of the jack, too..
 

Offline bxs

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #56 on: May 30, 2012, 04:39:05 pm »
Dave, seeing this project, this comes to mind  :D

A micro for LCD that still had some ADCs free.
 

Offline caroper

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #57 on: May 30, 2012, 04:41:01 pm »
I love the concept, put me down for 2 Kits.
I look forward to watching the series.


Cheers
Chris


Offline timelessbeing

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #58 on: May 30, 2012, 04:42:05 pm »
I think this is a fantastic idea! The only feature I want is a low, low price. I'm looking forward to the kit, and I hope you do a video explaining how it works too! Keep up the great work.
 

Offline T4P

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #59 on: May 30, 2012, 04:53:06 pm »
Dave, seeing this project, this comes to mind  :D

A micro for LCD that still had some ADCs free.

they are a hell lot larger then you know ... it is so big it will take up the entire space that includes the pots
 

Offline RTC

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #60 on: May 30, 2012, 05:45:03 pm »
My Logitech USB hubs limit the current that they supply.
When using a external power supply for the hub, it still limits according to what has been negotiated. If the device tries to draw more, it gets cut off.
My computer is also horrible with this and it cuts off just before reaching the negotiated limit. This is why I use the hubs, they at least give me the full power.
This would make the product unusable unless using a separate mains USB power supply.
 

Offline SeanB

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #61 on: May 30, 2012, 05:54:02 pm »
I will guess older Nokia display with COG, as it has enough onboard intelligence to run the display while being simple to interface to a micro. Plus needs only a few pins, ultra low power, cheap and plentiful. Build a few bitmaps in the software and output them to drive it and away you go.
 

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #62 on: May 30, 2012, 06:00:23 pm »
hi,  just some thoughts.

It's not that mobile if you need to connect it to a laptop to make it work or did i miss something?, why not add a rechargeable battery and make it charge-up via a usb, in-car, solar or even a novel toothbrush type charger.  If it was portable and I could use in the field (literally) now that would be different.

 

Offline T4P

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #63 on: May 30, 2012, 06:13:28 pm »
hi,  just some thoughts.

It's not that mobile if you need to connect it to a laptop to make it work or did i miss something?, why not add a rechargeable battery and make it charge-up via a usb, in-car, solar or even a novel toothbrush type charger.  If it was portable and I could use in the field (literally) now that would be different.

Design Goal : To be used with a laptop and fit inside a laptop bag for a reason.
 

Offline johnpatcher

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #64 on: May 30, 2012, 06:27:52 pm »
Great idea, definitely would buy one of these ;).
 

Offline Zad

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #65 on: May 30, 2012, 06:33:50 pm »
Dave's other portable power supply will have a rechargeable battery and all the other stuff (USB control etc). No point duplicating it in this, which is supposed to be cheap, simple and small. In fact, I would drop the secondary display and just illuminate an LED when it was in current mode.

Offline IanB

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #66 on: May 30, 2012, 06:41:57 pm »
With all the talk above about power negotiation, I'm surprised nobody has mentioned the USB Battery Charging Specification. This allows compliant devices to supply and draw up to 1.5 A by simple hardware negotiation, with no USB protocol negotiation necessary. Some summary details are quoted below, extracted from a Maxim data sheet.

Dave, I wonder would your power supply be able to take advantage of this capability in devices that support it?

Charging downstream port (CDP) BC1.1 defines this new, higher current USB port for PCs, laptops, and other hardware. Now the CDP can supply up to 1.5A, which is a departure from USB 2.0 because this current can be supplied before enumeration. A device plugged into a CDP can recognize it as such by means of a hardware handshake implemented by manipulating and monitoring the D+ and D- lines. (See USB Battery Charging Specification, section 3.2.3.) The hardware test takes place before turning the data lines over to the USB transceiver, thus allowing a CDP to be detected (and charging to begin) before enumeration.

Dedicated charging port (DCP) BC1.1 describes power sources like wall warts and auto adapters that do not enumerate so that charging can occur with no digital communication at all. DCPs can supply up to 1.5A and are identified by a short between D+ to D-. This allows the creation of DCP "wall warts" that feature a USB mini or micro receptacle instead of a permanently attached wire with a barrel or customized connector. Such adapters allow any USB cable (with the correct plugs) to be used for charging.

Additional details on these port types are described in the USB Battery Charging Specification, Rev 1.1, 4/15/2009
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Offline T4P

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #67 on: May 30, 2012, 06:51:24 pm »
I haven't even seen a "CDP" on a laptop yet
 

jucole

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #68 on: May 30, 2012, 06:51:53 pm »
Dave's other portable power supply will have a rechargeable battery and all the other stuff (USB control etc). No point duplicating it in this, which is supposed to be cheap, simple and small. In fact, I would drop the secondary display and just illuminate an LED when it was in current mode.

I was missing something, :-) thanks for putting me right Zad
 

Offline adrianblack

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #69 on: May 30, 2012, 08:14:54 pm »
What a great idea!! I agree binding posts are a must.

It would be cool to be able to set the maximum current draw from USB -- and perhaps be able to go over 500mA@5v if you use this with a mobile phone charger that can supply more. (Like 1000mA@5V from iPhone chargers and 850mA@5.1v from most Motorola chargers.)

Like a momentary you can hold down then turn the current rotary encoder to set the max.

Neat neat neat!
 

Offline Chryseus

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #70 on: May 30, 2012, 09:02:56 pm »
I don't know why everyone seems to think the more current the better, most small size projects only end up drawing 50-100mA.
Really if you need more current you should be at your bench or carry a few pocket size batteries with you.

Personally I'm more interested in the output noise and ripple, USB is typically pretty nasty so I'd be interested in seeing what Dave does to tackle it, assuming he decides to with this appearing to be more targeted towards 'digital' users where noise is not so much an issue.
 

Offline TbayBoy

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #71 on: May 30, 2012, 09:20:36 pm »
Well you've probably forgotten more about electronics than I have learned so far so I'll stay away from that end, but since you are thinking about something that can be tossed in a laptop bag might I suggest something a little bigger than a mint tin, but in water resistant plastic and see thru so only the connectors and nobs are outside to cut down on gunk and such that traveling can accumulate.
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Offline JoannaK

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #72 on: May 30, 2012, 09:28:47 pm »
With all the talk above about power negotiation, I'm surprised nobody has mentioned the USB Battery Charging Specification. This allows compliant devices to supply and draw up to 1.5 A by simple hardware negotiation, with no USB protocol negotiation necessary. Some summary details are quoted below, extracted from a Maxim data sheet.

Dave, I wonder would your power supply be able to take advantage of this capability in devices that support it?

There is quite direct comment against Dave-method ... (Ie using 500mA blindly without any negoation)

Quote
"Cheating"—Noncompliant USB Charging

A bolder noncompliant scheme assumes that 500mA will be available and instructs users to plug only into powered ports and hubs that are capable of 500mA. Again, since most USB ports do not disconnect power, this approach can work in most cases. When such a device is plugged into a port that cannot support 500mA, the port is supposed to shut down. However, the overload behavior of a USB port is not always well defined and can lead to system reset or damage. Fortunately, this level of desperation is no longer required since battery charging is now an active part of the USB specification.
 

Offline codeboy2k

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #73 on: May 30, 2012, 09:29:27 pm »
I think this is a fantastic idea! The only feature I want is a low, low price. I'm looking forward to the kit, and I hope you do a video explaining how it works too! Keep up the great work.
what exactly does a "low, low price" mean to you. Quantify it, with how much are you willing to pay, that will help Dave set pricing and budget for features.

 

Offline codeboy2k

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #74 on: May 30, 2012, 09:58:53 pm »
binding posts are overkill.
I deem them to be essential.
Dave.
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)

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.

So there's some extra thoughts for you.
(Have you got a headache yet from everyone's different design ideas :)  designed by Internet committee. )
Cheers!
 


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