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.

[mikeselectricstuff]

As available power is limited, I think LCD would be a better choice for display.

[theSuitMusic]

As an electronics engineering student, this is amazing.

Off the top of my head:
- Won't the 7seg-displays use up a lot of power? About 40mW * 6 = 240mW ~ 10% of your available power, or am I overestimating? Substitute for lcd?
- You could also use an attiny to make the supply effectively a usb keyboard, that outputs all the values as typed text (in notepad or something), you can even have two-way communication using caps/num/scrollock. Maybe for a future usb-stick size nanoSupply :) ?

[TheInternetwatcher]

Every one of your videos goes away beyond my understanding since I'm not into electronics. However, I really enjoy watching your videos and I look forward to your new series!

[kftkj]

Not only is this something I would buy I am very excited to see it develop! Brilliant!

[EEVblog]

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.

[EEVblog]

I developed these projects after hours when I was working full time.

[EEVblog]

No, it's not just two extra traces. You have to isolate the data too.

[EEVblog]

IIRC the dual muxed display at a reasonable brightness took about 40mA or so (200mW). It's a lot, but I was fixated on having LED displays at the time. I have a trick up my sleeve, those displays are not the final solution.

[EEVblog]

Read the silkscreen on the front of the prototype.

[TheChipburner]

Futher ideas (may be interesting):
1) Lead-acid battery powered regulated supply. Perfect isolation and overcome 2.5W power limit on USB
2) Redesign a cheap "electronic tramsformer" for halogen lamps into normal unregulated power supply. Power levels of 50-300W can be obtained easily at below $10.

[isashach]

Don't forget that newer 'iPad compatible' motherboards and computers can output 10w via USB. This is the case for all the ports on all of the 2011 MacBooks if I recall correctly. BTW, I love the thing, and I would totally buy it!

[Mr.Laidukas]

Congratulations Dave with this idea!! I promise my thumbs up :) I always enjoy these build videos with a cup of coffee :)

[sazhen]

Very nice! Software control of the voltage and current limits would be a useful feature.

[blink]

good idea, i'm gonna have to pick one up when you finish the design.

[alexanderet]

Nice! Would be really cool if u did a function generator as well. The power limits from the USB would not be as big a problem, and it would be super neat to have an easy to bring generator...

[SirBunghole]

Don't limit it to 2.5W as there are USB chargers that can do up to 10W and a dual USB cable (like those for some DVD drives) can do 5W. Neat idea and very handy.

[hikaru-live]

I actually got a recent design that combined the USB and batteries into one unit - USB charges the batteries, and batteries powers a boost converter capable of at most 36V at 2A. I called that MTD295A (5 cells, there was 3 designs differ in only the amount of Li-Ion cells used: 295A with 5 cells, 299A with 9 cells and 292B with 12 cells).

[ericjohnholland]

I like the idea. I would like a boost convertor in it so you could get roughly 0-12VDC output voltage. You could get fancy with computer controlled set points, but I think a two knob interface is a great starting point.

[deaggi]

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.)

[bgdwiepp]

USB ports on a computer are generally all centrally fused (usb_org/developers/whitepapers/power_delivery_motherboards_pdf), so are capable of supplying much more than 500ma/1000ma that most people quote;
Current limiting on the USB ports in a per port basis is generally only on hubs, it is not designed as a "current cop" to stop you going over the allowable current, only to protect you and your devices.

Idea related to above;
Put some form of adjustable current limiting on the sour