EEVblog #285 – USB Lab Supply – Part 1

Part 1 of a new design series on developing an isolated USB Lab Power Supply.
Convert your USB port into an isolated lab type power supply.
This is just the introduction announcing the intention and showing off some old concept prototypes. The design videos will follow as Dave develops them.
The finished project is planned to look quite different to the prototypes.

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  1. [..YouTube..] Neat! Would buy one!

  2. [..YouTube..] Ooooh, I always wanted one like these. Especially together with a USB isolator like ADuM[34]160.

  3. [..YouTube..] Also, there are USB Power Delivery (USB PD) which allow a USB port to supply much more than 2.5 Watts. Though probably no one implements this at the current time and afair it is still a draft, it might be good to look at the coverage of this and take a future implementation of this into consideration. (You are of course right that 2.5 Watts is really neat, but if you really think about it – very often this is not enough.)

  4. [..YouTube..] Dave don’t forget to do PC control through the USB

  5. [..YouTube..] did you design this from scratch or is this a kit .. looks professional

  6. Five words: Dump the -bleep- LED display.

    Just the thought of using 10 to 15% of the rather limited 2.5W power budget for the display doesn’t feel right to me.

  7. sounds awesome!
    * you could use two usb inputs that would double the power available. there are also high-power usb ports on some computers – you might also design for that

    * an terminal output for the pc might be nice to be able to control the thing also via terminal program.

    * as the other two guys said I would also prefer a more low-power solution for the display 🙂

    • Yes, more ports for higher power — or even just a jumper that sets the maximum current. That way a multiport cable (like the ones that come with 2.5″ USB hard drives) could be used in-place.

      Also USB3 allows for 900mA draw using one socket. I think it should be designed with that in mind.

    • With an isolated supply, you can’t just magically increase the current capability. High power isolated converters cost extra $, and it’s often not a linear price relationship, plus there are size issues. With loses, this version of the supply will be limited to 1.5W output.

      • As above mentioned. USB3.0 ports offer more power allready, and If i’m not mistaken, USB also has the ability to supply even MORE power. I forgot its exact name, the USB3 charge spec? I know my bios has an option specifically for USB3 charging.






    • Read the silkscreen on the prototype. I’d never design that was limited to 5V. Not that you’d be able to get 5V direct out of the USB port anyway (i.e. that Kickstarter one) due to cable drop and tolerance.

  9. Fantastic concept!

    I’m also concerned about the LED displays and their power consumption. What do they draw? There must be a way of driving them at low power.

    I’m looking forward to the design videos, especially if they are anywhere near as detailed as the previous lab power supply!


  10. Yes of course I want one. Looking forward to future videos on this.

    One suggestion, there should be a USB output port too.

    • Why?

      USB devices require 5v, so there would be no purpose for a variable output USB socket.

      • I’ve designed a USB power supply as well, but its much more simple. It just has multiway switches on it:

        1.8V – 3.3V – 5.0V

        100mA – 250mA – 500mA

        Works for about 95% of the things I want it for.

        I think your LCD displays would make the device too bulky.

        Definitely don’t use the box from your uCurrent, WAY too big!

  11. [..YouTube..] I designed and built it myself of course.

  12. [..YouTube..] I developed these projects after hours when I was working full time.

  13. [..YouTube..] USB ports do not limit the current to 100mA before negotiation. There is no requirement in the standard for them to do this, and it is almost unheard of for them to limit the current 100mA before negotiation. It is just a “play nice” software request thing.

  14. The VariPower-USB-500A now on Kickstarter is a very similar concept:

    I have a feeling I’ll end up with both Dave’s device and the VariPower. The one that is smaller will probably be the one the stays in my bag. 🙂

  15. [..YouTube..] It would be cool to allow the voltage and current limit to be set over a serial connection over USB, but I may be being a bit greedy there, but this seems cool and I would definately be interested in one in kit form.

  16. [..YouTube..] Pretty simple idea. I feel it needs either a low power LCD or a way to turn off the LED display so you can save a few precious ma for the project, but honestly if you’re going to be going that close “to the rails” this may not be the best option.

  17. It’s a good idea to switch display mode while setting up the maximum possible output current for a while, even if there’s no consumer connected to it. I own a power supply with digital instruments, that shows actual current only when it’s really been drawn by the adapted consumer. Not only because of this information, the video was worth.

  18. Sorry about poopooing this, but… Personally, I see a little use of such a complicated approach. Most simple MCU development boards already come with the USB interfaces, theynaturally draw power from there, and contain a proper regulator. If I’m using my own breadboard, and it needs a 3.3v, I just add an LM1117-3.3 that is already soldered on top of a small nice DIP stub. Same with a USB connector to draw power from. Galvanic isolation is a good thing, but fraknly speaking, I’m unsure how frequently it could be necessary when using a laptop. Drawing higher voltage, e.g. a low power +/-15V when working with OPAMPs could be useful though. Remember, this is all for the cases where you are away from your bench, with your laptop on your own, i.e. in front of a TV or on a bus to work. And the last thing you need in this case is another bulky device to complete your setup.

    • Isn’t it strange, to lodge your own dis-interest? “Most simple MCU development boards already come with the USB interfaces” – Most.. not all? The current limiting on this device is a feature not implemented on many MCU dev board PSUs, which could result in a broken power supply making the board useless, should a peripheral draw too much power. It’s useful for situations without MCUs as well. For a hobbyist, this might be sufficient as a bench power supply substitute, which it is significantly less bulky than.

    • It’s got current limiting which is useful, it’s got isolation which can be useful (as you said), and it doesn’t need another mains point which can be useful for those who like to move around a bit. Finding free mains outlet can be pain sometimes, so the USB power is handy.

  19. [..YouTube..] See chapter in “Universal Serial Bus Specification Revision 2.0”: “It must first come up in a reduced power state of less than one unit load.” (one load: 100 mA). Then it asks for more power and the host can grant it.But sounds like a usful gadget. Would be nice if I could read and control the voltage and current from the PC, too. A HID device with a custom report would be the easiest way, avoiding complicated HID descriptors. Openmoko gives away free USB product IDs, if you need one.

  20. [..YouTube..] Beware that engineers (well MBA’s, actually) cut corners to increase profit. There may be a lot of computers in the wild that cannot source much current and do not adhere to any specs. Second, if you are using USB, might as well exchange data with a client on the PC to show current draw and voltage. You can make it programmable and save the BOM on the LEDs and drivers. Trivial to program and you can have a window with those same readings and maybe even TREND data for under-voltage.

  21. [..YouTube..] The slimmer version look very good, but will be nice not to be limited by 5VDC from the USB. Make the input with bigger range and why not to be AC+DC.. input.Then I will buy one.

  22. [..YouTube..] Can it be adjusted above 5v? Maybe a pin and sleeve connector that comes out at a right angle from the board to keep it slim. Do you plan on leaving the connections to the load just solder able?

  23. [..YouTube..] Hi Dave,I’d definetely want one of those. Thought about building one myself but was curious in terms of safety for my laptop USB-ports. So please pay special attention to this issue. For connection of the supply leads I would recommend just a simple pin header. When you interface it to a breadboard or evaluation board, there is nothing more needed. In the case someone needs a special connecter, it can be easily done by oneself.

  24. * Ditch the display, try some low power monochrome.

    * I spy a new toy on your telescope eyepiece.

  25. [..YouTube..] I like I like.

  26. Looks very cool!

    The 500mA limit for USB ports has been superseded though. Modern ports can allow up to 1.5A with high speed communication, and up to 5A otherwise.

    An interesting alternative to physical knobs might be capacitive touch sliders. Seems like the form factor you chose would have plenty of room for two of them, and I’d be really interested in seeing what can be done with the modern libraries!


  27. Suggestion #1 – put in a proper USB port. Drawing 5V from a USB port blindly isn’t a wise idea, and it’s best to negotiate with the host PC when you do this. Since you have a micro on there anyhow, it should be easy to make it full featured. As a bonus, you can make the PC control the power supply! (or interrogate it, etc).

    USB ports are only guaranteed to provide 5V 100mA unless enumerated properly. People often abuse this though (hard drives are a popular one – most hard drives will need more than 2.5W to start up and they will trip PCs with 500mA enforced monitors).

    The other benefit is some USB ports can provide more than 500mA, but to do so requires active negotiation with the host. It’s still 5V, but we’re talking 1-2A out (10W).

  28. [..YouTube..] I WANT, at least, one, and probably, about 2, to 5, to 10 of these puppies.I’d like to be able to pick the color of your 7 – DIGIT LED’s, I want yellow or orange, for reasons of nostalgia, from a day at a Radio Shack in Kentucky.

  29. [..YouTube..] could be made also usb programmable???? like I can program vout via usb serial interface maybe??? could it be very useful!!!!! 🙂 good project dave!!! And I have concern about usb maximum supply current as well….

  30. [..YouTube..] also range: 0.5-15 volt maybe? with stepup regulator?

  31. [..YouTube..] I would like to get one someday as I have loads of USB ports, but not only one remotely regulated power supply.

  32. [..YouTube..] can someone give me the link of the old usb power supply ? 🙂

  33. [..YouTube..] Cool, this looks cute. Would be great to make it lower profile, including the knobs and display. Also a standard A connector would last longer. How about including logging capabilities?

  34. [..YouTube..] Send me one now, please. 🙂 Can’t wait until you tell us all how to build one ’cause I really need one (realized while playing with a 3V laser pointer this past weekend).

  35. [..YouTube..] Hi Dave,You could incorporate a data logger into it. It will require to make a PC program and I don’t know if you’re willing to do so, but I think this could be a neat feature.BR

  36. I want! I will be throwing this guy in my backpack. Ruggedness will be key!

  37. [..YouTube..] where do i preorder!!!!!! also would it be possible to have a dual usb port so you can perhaps get 5volts from 2 ports?

  38. I say keep the display (but only 3 – 4 digits + some led – uses ~300mw? ) How about an input for messuring voltage? should not cost too much, especialy if you have a free ADC pin?

  39. Since USB chargers are available with more power (and often carried along + probably already isolated), perhaps bypassing a means to also bypass the 500mA isolation chip and provide more power/voltage options? Or if the connectors are cheap, just have 2 connectors, one for isolated 500mA and one for charger or rogue running …

    Personally I’d love to see this become split supply, but I guess that will never happen.

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