Author Topic: EEVBlog #285 - Isolated USB Lab Supply  (Read 48799 times)

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Offline tom66

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EEVBlog #285 - Isolated USB Lab Supply
« on: May 30, 2012, 09:19:22 am »


Interesting.

I wonder how the isolation is performed?

An isolated flyback SMPS? That would allow for a voltage tracker so power dissipation could be limited in (presumably) the linear output regulator.

What would the level of isolation be - will the output be truly isolated (e.g. with optocoupler) or will it just have a floating ground limited to +/-12V (for one additional output)?

USB ports can supply 5V @ 500mA but only with negotiation - are you going to worry about this Dave - I know most systems don't really care or enforce it but there are a few ones which do. Also for a 12V output this is ~200mA which is probably okay for most circuits but will this current limit be the same at lower output voltages?
 

Online Rerouter

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #1 on: May 30, 2012, 09:51:41 am »
looks interesting, like on the "other" usupply, another 7mm between the knobs would be ideal, (enough to push your index finder between to turn and not have the finger joint catch on the upper one)

also like the look of how simple it looks, (perhaps add a Vin and ground pad for people who might want to make this battery powered, but nothing more on that front i would think)

still the back looked rather bulky, but not quite bulky enough for a transformer, i would then guess a charge pump?

still if the box it end up using is about as thick as a fat calculator, then i would think it would make a great kit / tool, likely using 90 degree binding posts, or...? binding posts mounted to box and a small connector used? but thats for you to enlighten us on i suppose :)
 

Offline Bored@Work

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #2 on: May 30, 2012, 10:06:19 am »
USB ports can supply 5V @ 500mA but only with negotiation - are you going to worry about this Dave

In previous discussions Dave indicated he gives a flying fart through a rolling doughnut about this limitation.
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Offline codeboy2k

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #3 on: May 30, 2012, 10:16:10 am »
Notwithstanding Dave's unmeasured skill at passing gas through mobile loops of bread,  could one take power from 2 USB ports if one actually needed to?
I am pretty sure that no USB ports on consumer equipment actually does any sort of current limiting to 500ma.

 

Offline tom66

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #4 on: May 30, 2012, 10:21:32 am »
My laptop won't run a 1A HDD from one USB but will from two, so I guess mine is current limited. It's probably a nonissue for a desktop computer which usually uses a 2A PTC for all the ports combined.
 

Offline Bored@Work

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #5 on: May 30, 2012, 10:23:58 am »
Notwithstanding Dave's unmeasured skill at passing gas through mobile loops of bread,  could one take power from 2 USB ports if one actually needed to?
I am pretty sure that no USB ports on consumer equipment actually does any sort of current limiting to 500ma.


Legally, i.e. while staying within the USB specs, no.
For those who like their computer occasionally reset or like to risk to burn it, there is junk like this http://www.walmart.com/ip/Startech-6-USB-Y-Cable-for-External-Hard-Drive/14659917
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Offline dcel

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #6 on: May 30, 2012, 10:25:51 am »
USB ports can supply 5V @ 500mA but only with negotiation - are you going to worry about this Dave

In previous discussions Dave indicated he gives a flying fart through a rolling doughnut about this limitation.

Or you can use something like a usb phone charger at 1A.

Or like the attached image.

Cool idea none the less, first thing that I thought of is college students using it for their simple labs.
Potentially a huge market there...

Chris

« Last Edit: May 30, 2012, 10:29:37 am by dcel »
 

Offline mikeselectricstuff

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #7 on: May 30, 2012, 10:34:02 am »
A suggestion that would both solve the negotiation issue and provide extra functionality - put an FTDI chip in there. 
Not necessarily for controlling the PSU (although the ability to display and log current draw may be handy), but just as a TTL level serial port that will often come in handy when debugging.

Something else worth considering is that there are plenty of USB supplies that can provide more current, so if there is a 500mA current limit, there should be a way to override it. I think there is some sort of standard that uses resistor values to tell a device how much current is available from chargers.
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Offline miceuz

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #8 on: May 30, 2012, 10:35:12 am »
i think you can shave some of the price by removing those bulky LED displays and displaying voltage and current in software on the computer (it will be connected to computer after all, right?)

i would leave rotary encoders though.

Offline EEVblog

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #9 on: May 30, 2012, 10:46:53 am »
USB ports can supply 5V @ 500mA but only with negotiation - are you going to worry about this Dave - I know most systems don't really care or enforce it but there are a few ones which do.

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.

Dave.
 

Offline mikeselectricstuff

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #10 on: May 30, 2012, 10:48:29 am »
i think you can shave some of the price by removing those bulky LED displays and displaying voltage and current in software on the computer (it will be connected to computer after all, right?)

i would leave rotary encoders though.
But you'd need to pass the data over the isolation barrier - not hard but more cost.
I think local display is essential - laptop screen space is small enough already. Maybe should be LCD though to avoid eating into the available power.
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Offline codeboy2k

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #11 on: May 30, 2012, 10:49:02 am »
i think you can shave some of the price by removing those bulky LED displays and displaying voltage and current in software on the computer (it will be connected to computer after all, right?)

i would leave rotary encoders though.

I wouldn't.  I would like it as compact as possible, with just 2 ports, power and gnd . oh and the usb port of course

should be software controlled and displayed on the attached PC. but the outputs should be isolated as he did already, and fully protected. voltage and current limited. cc/cv modes.

make it thin with red/black croc clip leads, either removable or permanently attached.
binding posts are overkill.
 

Offline EEVblog

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #12 on: May 30, 2012, 10:52:40 am »
And of course the first comments all say the same thing, can it supply more current and work with higher current ports etc. Feature creep ::)

As we will no doubt see, it's all a trade-off with the size of the unit, and the cost, complexity and size of the isolated DC-DC converter required to handle it all.

For those interested, no data comms ability, it's just not worth it. I want a simple lowest possible cost unit that is just a PSU.

Dave.
 

Offline EEVblog

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #13 on: May 30, 2012, 10:55:22 am »
i think you can shave some of the price by removing those bulky LED displays and displaying voltage and current in software on the computer (it will be connected to computer after all, right?)

May not be a computer.
PC control needs data across isolation, and can't be used standalone with any 5V USB supply.

Dave.
 

Offline EEVblog

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #14 on: May 30, 2012, 10:56:42 am »
binding posts are overkill.

I deem them to be essential.

Dave.
 

Offline mikeselectricstuff

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #15 on: May 30, 2012, 10:57:03 am »
And of course the first comments all say the same thing, can it supply more current and work with higher current ports etc. Feature creep ::)

As we will no doubt see, it's all a trade-off with the size of the unit, and the cost, complexity and size of the isolated DC-DC converter required to handle it all.

For those interested, no data comms ability, it's just not worth it. I want a simple lowest possible cost unit that is just a PSU.

Dave.
Simplicity makes sense, and avoiding doing anything with data makes a lot of problems go away - can I suggest you put a USB type A pass-though socket on - some laptops don't have enough USB ports, so not losing one for the PSU would be handy and not cost much.
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Offline EEVblog

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #16 on: May 30, 2012, 10:58:12 am »
Simplicity makes sense, and avoiding doing anything with data makes a lot of problems go away - can I suggest you put a USB type A pass-though socket on - some laptops don't have enough USB ports, so not losing one for the PSU would be handy and not cost much.

Not enough room I'm afraid.

Dave.
 

Offline Bored@Work

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #17 on: May 30, 2012, 10:58:55 am »
That is a myth.

No, it isn't. Last time we had that discussion we even quoted the standard to you. You could have used that as an opportunity to educate yourself. This time I am to lazy to dig out the standard again. One can lead a horse to the water ...
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Offline mikeselectricstuff

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #18 on: May 30, 2012, 10:59:06 am »
binding posts are overkill.

I deem them to be essential.

Dave.
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.
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Offline JoannaK

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #19 on: May 30, 2012, 11:06:02 am »
I like the idea.. power/gnd isolation is good to have, saves some hassle at later times. Of course it might be nice to have usb-controllability (and back-read) but isolated data does add cost and needs board space.

Led-displays are quite nice, but they tend to eat quite a lot power (many mA per each segment on). I wonder if one could find somekind of suitable smallish LCD (seperate segments, not generic text/matrix) that could be used instead for those two readings.
 

Offline EEVblog

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #20 on: May 30, 2012, 11:25:50 am »
No, it isn't. Last time we had that discussion we even quoted the standard to you. You could have used that as an opportunity to educate yourself. This time I am to lazy to dig out the standard again. One can lead a horse to the water ...

I've read the standard, you simply did not understand what I was saying.
You could not show me where it said that the USB port cannot actually provide 500mA at all times. That is the myth I'm talking about. People actually think the USB port somehow limits itself to 100mA output current and cannot/will not provide any more until 500mA is negotiated. That is simply not the case, 500mA is available at all times.

Dave.
 

Offline tom66

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #21 on: May 30, 2012, 11:26:08 am »
Dave, you are using a commercial isolated DC-DC?

I think you're missing out on a great opportunity to design a low voltage flyback. It's not too difficult and there's a lot to be discussed. Many different ways to do it. Thermal calculations, and an introduction to magnetic "circuits". You can even get pre-made transformers for chips such as LM2577 so there's no need to design or wind your own (Coilcraft sell some IIRC.)

And an app-note here: http://www.ti.com/lit/an/snva028a/snva028a.pdf
« Last Edit: May 30, 2012, 11:31:22 am by tom66 »
 

Offline EEVblog

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #22 on: May 30, 2012, 11:27:12 am »
Led-displays are quite nice, but they tend to eat quite a lot power (many mA per each segment on). I wonder if one could find somekind of suitable smallish LCD (seperate segments, not generic text/matrix) that could be used instead for those two readings.

I've got something clever in store in the display front. (but nothing that hasn't been done before)

Dave.
 

Offline EEVblog

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #23 on: May 30, 2012, 11:28:59 am »
Dave, you are using a commercial isolated DC-DC?

I think you're missing out on a great opportunity to design a low voltage flyback. It's not too difficult and there's a lot to be discussed. Many different ways to do it. Thermal calculations, and an introduction to magnetic "circuits". You can even get pre-made transformers for chips such as LM2577 so there's no need to design or wind your own (Coilcraft sell some IIRC.)

Yes, commercial DC-DC.
I don't really have the time or inclination to dick around designing my own I'm afraid, as educational as it would be.

Dave.
 

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #24 on: May 30, 2012, 11:32:50 am »
dave has yet to tell us what method of isolation he is using, transformers arent the only way :) and at low voltages and currents transformers usually arent the most ideal method,

while i do agree about 7 segment displays sucking down a bit of juice, i think they are better for this application, as generally they are far easier to read from a distance, and something like this is very likely to be used in barely lit rooms, perhaps using a few smd current limiting resistors or somehow through the driver chip, but once again unless its built into whatever is driving it, it adds parts,

though i am curious as to what dave has in store for it :?
 


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