Author Topic: EEVBlog #285 - Isolated USB Lab Supply  (Read 48158 times)

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Online mikeselectricstuff

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #125 on: May 31, 2012, 05:33:02 pm »
Also, about the whether it should be 100ma vs 500ma vs ???ma, spec for it to only draw 500ma at most but leave a footprint beside the resistors to put in a small trimpot, that way you stay inside the rules, but people who want to push it can, and can use it with devices like the iPad charger to their full advantage.

People forgot get that isolated DC DC converter cost so many $/watts, and it's not often linear. To design the thing worst case around some of the figures being thrown around, it could double the BOM cost or more.

Dave.

Easy - multiple footprints.  Most DC/DCs are through-hole so easy to allow for a few different ones. Won't add much cost to make your regulating back-end capable of, say, 1 amp for use with an external PSU,

Quote
Voltage drop across a standard usb mini/micro cable of 1m length is horrendous!

There  are a LOT of cheap crap cables that are well below spec - USB standard does specify maximum voltage drops, but the proliferation of junk cables means it can be hard to find a decent one, as even manufacturers that want to do it right can't afford to because nobody wants to pay for a properly made cable.

Perhaps a kit should include a known-good (distinctively coloured) cable, to avoid problems caused by junk cables. 
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Offline peter.mitchell

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #126 on: May 31, 2012, 06:21:30 pm »
Also, about the whether it should be 100ma vs 500ma vs ???ma, spec for it to only draw 500ma at most but leave a footprint beside the resistors to put in a small trimpot, that way you stay inside the rules, but people who want to push it can, and can use it with devices like the iPad charger to their full advantage.

People forgot get that isolated DC DC converter cost so many $/watts, and it's not often linear. To design the thing worst case around some of the figures being thrown around, it could double the BOM cost or more.

Dave.

The more I think about it, the more I wonder if it really needs to be isolated, a significant portion of the devices connected will be isolated themselves, mainly because they are just things like arduinos, timers and other small devices. Would it not be more benificial to just offer some input and output filtering (and flyback protection) and more flexability at the cost of isolation?

Quote
Voltage drop across a standard usb mini/micro cable of 1m length is horrendous!

There  are a LOT of cheap crap cables that are well below spec - USB standard does specify maximum voltage drops, but the proliferation of junk cables means it can be hard to find a decent one, as even manufacturers that want to do it right can't afford to because nobody wants to pay for a properly made cable.

Perhaps a kit should include a known-good (distinctively coloured) cable, to avoid problems caused by junk cables. 

Indeed, a known good cable would be VERY helpful, and if you are going to supply one, I would consider offering a "y" cable as well.
 

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #127 on: May 31, 2012, 07:10:06 pm »
Easy - multiple footprints.  Most DC/DCs are through-hole so easy to allow for a few different ones. Won't add much cost to make your regulating back-end capable of, say, 1 amp for use with an external PSU,

Sorry, not that easy. Bigger isolators and more heat sinking requirement takes more space in an already tight design.
Much better leave it to a bigger battery powered design that uses the USB for just charging (like the bench PSU), then you can still have the small cheap isolator as well because the battery can provide the amps.

Dave.
 

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #128 on: May 31, 2012, 07:12:36 pm »
The more I think about it, the more I wonder if it really needs to be isolated, a significant portion of the devices connected will be isolated themselves, mainly because they are just things like arduinos, timers and other small devices. Would it not be more benificial to just offer some input and output filtering (and flyback protection) and more flexability at the cost of isolation?

No, I deem isolation to be an essential feature. That way you can connect say multiple devices to the same PC/USB hub (that use a common ground) and you can get a split supply, or put them in series etc.

Dave.
 

Online tom66

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #129 on: May 31, 2012, 08:09:42 pm »
Dave, how isolated will the power supply be?

Standard 1KV

Dave.

Even my lab supply is only 240V between any terminal and earth, but I suppose the isolation will just be achieved with a decent gap between USB and output sides. I don't know if you will need EMI filtering or not, like a Y-cap, or if that's only important for mains powered stuff.

Will you be using a tracking pre-regulator, or just use a big heatsink on your pass transistor?
 

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #130 on: May 31, 2012, 08:13:31 pm »
Even my lab supply is only 240V between any terminal and earth, but I suppose the isolation will just be achieved with a decent gap between USB and output sides. I don't know if you will need EMI filtering or not, like a Y-cap, or if that's only important for mains powered stuff.

Will you be using a tracking pre-regulator, or just use a big heatsink on your pass transistor?

The 1KV converters are really noisy, so do need a suppression cap across the isolation, yes.

Tracking pre-reg, yes.

Dave.
 

Online mikeselectricstuff

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #131 on: May 31, 2012, 08:57:56 pm »

Tracking pre-reg, yes.

Dave.
Have you calculated overall efficiency of the DC-DC and pre-reg combined yet?
In an ideal world you could combine the pre-reg with the DC/DC to maximise efficiency.
I suspect however there aren't any cheap DC-DC's that expose the feedback input to do this. 

 
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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #132 on: May 31, 2012, 10:40:59 pm »
I suspect however there aren't any cheap DC-DC's that expose the feedback input to do this.

Correct.
But if anyone knows of one, I'll all ears.
The overall system efficiency isn't great. The isolated converters do 80% at best. Tack on the tracking pre-reg ad then linear reg, and you are lucky to get 1.5W out.

Dave.
 

Offline david77

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #133 on: May 31, 2012, 10:59:20 pm »
What's the max. output voltage of your supply anyway? I don't think you mentioned it yet. About 10V maybe?
 

Online tom66

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #134 on: June 01, 2012, 01:06:20 am »
In addition, if you adjust the isolated DC-DC, then you need some way to find the 5V or 3.3V for the microcontroller; you might have to use a higher voltage tap for the micro (say 10V out when the main out is 3V) then use a regulator.
 

Offline Eliminateur

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #135 on: June 01, 2012, 02:22:36 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.
But Dave, you already have a micro in it, you can use a USB enabled one or put a FT232 which means you can do 100% software control using a simple terminal software(no need to write anything fancy), which enables you to do away with *at least* 1/2 knobs+rotary encoder, 6 LED digits(clunky, big and they eat lots of power which from the already low supply).
Because if you're going to plug it to a notebook, the notebook is on and chances is you're already using it to develop/program/whatever it is you need for your electronic thing, so what's the harm of doing it software controlled?.

I think the cost of those parts is way more than a simple ft232 or a usb enabled micro.

Also, you can use a "dongle" cable like in several common USB HDDs which will net you 1 AMP combined (but adding some sort of detection based on consumption could be tricky, adding a pusbutton or switch or software key at start to select between "hi/normal power" could solve it)

Plus the size and weight shedding means you can use a much slimmer package in the same footprint (half the height or less) and no protruding stuff means easier shipping!.

anyway, those are my 2(3) cents!
 

Offline T4P

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #136 on: June 01, 2012, 02:47:21 am »
He mentioned it already. It doubles the BOM cost and needs more space.
AND it must be isolated ISOLATED !


Physical knobs and screens are much better, what if your laptop would supply USB power but wouldn't get boot to windows/linux?
Plus 7segs are physically proven to be the best solution. Cheap,simple and what would it do to the 500mA supply?
 

Offline rr100

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #137 on: June 01, 2012, 03:03:17 am »
(only) Software controlled is no go for me. I have a bus pirate which I bring with me each time I have to fly somewhere and I don't expect to do any EE work at all (weight is always an issue as I tend to pack everything so I try to cut down at least on multimeters, interfaces, cables, adapters, etc). It can deliver 5V and 3.3V but you need to give it a couple of commands first. And it starts with installing the FT32 drivers (oh, I don't have the Administrator password?). Where's the com port, what you don't have hyperterminal? Of course google for the manual because I forgot the command to enable 5V. You get the picture.

Now software controlled in addition to knobs will be nice, but it seems to be "no go" because of the "isolation" requirement. Also logging would be nice, maybe basic power and mAh totals can be implemented on the existing micro without PC communication ?
 

Offline free_electron

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #138 on: June 01, 2012, 04:14:07 am »
i'm also in favor of at least having the option to install an ftdi chip, not so much for 'control' but for logging !
if i run something overnight i may want to be able to chart power consumption ..
or if i detect an overcurrent in the script power cycle the target.
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Offline Rutger

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #139 on: June 01, 2012, 05:09:35 am »
How about adding a couple of AA batteries/rechargeable batteries in the case and having the option of running without the USB connector?
 

Offline Eliminateur

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #140 on: June 01, 2012, 05:16:16 am »
(only) Software controlled is no go for me. I have a bus pirate which I bring with me each time I have to fly somewhere and I don't expect to do any EE work at all (weight is always an issue as I tend to pack everything so I try to cut down at least on multimeters, interfaces, cables, adapters, etc). It can deliver 5V and 3.3V but you need to give it a couple of commands first. And it starts with installing the FT32 drivers (oh, I don't have the Administrator password?). Where's the com port, what you don't have hyperterminal? Of course google for the manual because I forgot the command to enable 5V. You get the picture.

Now software controlled in addition to knobs will be nice, but it seems to be "no go" because of the "isolation" requirement. Also logging would be nice, maybe basic power and mAh totals can be implemented on the existing micro without PC communication ?
you can do software controlled ISOLATED, with a couple optos for the datalines.
i don't see how shedding all the excess would double the BOM...

every computer has a terminal program, if you use an arduino you already have one even!
about hte manual, unnecessary, simply make a rudimentary "help" command or a simply hello message that says "Vxxx and Axxx to set current and voltage, ON/OFF toggle output, LOG: show current values", done, that's it, a simple string in the program.

anyway, if Dave makes it open hardware similar to the ucurrent anyone can change it to suit their need afterwards and compare the BOM
 

Offline rr100

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #141 on: June 01, 2012, 05:24:43 am »
Oh, yes, I got too tied up in my story about the bus pirate to mention independent use without computer (as in a huge obvious disadvantage for a software-only-controlled PSU).
On the same note: there are already plenty of USB "powerpacks" of all sizes and prices, you can easily use one of those for portable operation.
 

Online tom66

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #142 on: June 01, 2012, 05:26:18 am »
Yeah with FTDI you can do the current negotiation from EEPROM, and just isolate the TX and RX low speed (9600 baud) with cheap as chip optos. Instantly solved the current problem and the now you have an interface to use with a computer.
 

Offline kek

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #143 on: June 01, 2012, 10:16:29 am »
I like LED's too but how about a sexy OLED display ...


Ken
 

Offline JoannaK

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #144 on: June 01, 2012, 11:24:48 am »
I like LED's too but how about a sexy OLED display ...


Ken

Oo.. Looks sexy  :-*
 

Offline codeboy2k

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #145 on: June 01, 2012, 11:35:45 am »
I like LED's too but how about a sexy OLED display ...

Ken

OLED's are nice to look at. Sharp contrast, easy to see in different lighting, extremely wide viewing angles.  Aren't they power hungry though?
I thought the OLED multimeter Dave reviewed once was a power pig.  Maybe no worse then LEDs though.

A year or so ago, I wanted to use an OLED on a hobby project of mine. I found they were just too expensive in small quantities.
I think they are insanely expensive still.
 

Offline NiHaoMike

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #146 on: June 01, 2012, 12:26:28 pm »
What about allow the option of installing a FTDI and optoisolators (on a secondary board, perhaps) so those who want it can install it?

And I think there should be the option of adjusting the input current limit. USB 3 can do 900mA and it's becoming very common nowadays. There are also various motherboards out there that can supply as much as 2A for charging tablets.
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Offline Eliminateur

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #147 on: June 01, 2012, 01:31:28 pm »
What about allow the option of installing a FTDI and optoisolators (on a secondary board, perhaps) so those who want it can install it?

And I think there should be the option of adjusting the input current limit. USB 3 can do 900mA and it's becoming very common nowadays. There are also various motherboards out there that can supply as much as 2A for charging tablets.
with software control it would be very easy to do (some: SET INPUT xxx command in milliamps).
I wonder if Dave would use an energy micro for this project, a directly programmed pic/AVR or an arduino ;)
 

Offline PeteInTexas

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #148 on: June 03, 2012, 01:56:34 pm »
IMHO, the ability to power from USB should be rolled into the other PSU project, the one that is already powered by batteries.
 

Offline T4P

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Re: EEVBlog #285 - Isolated USB Lab Supply
« Reply #149 on: June 03, 2012, 04:06:43 pm »
STOP shifting the project into the weirdest areas :
1) Cheap simple and not complicated
2) Isolated

All he wanted was a simple Isolated USB Lab Power Supply without all the added bonuses like microc's and such

Let me ask you, does your wing-wang-pong power supply have a microc monitoring system? Yes?
 


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