Here's someone doing a project along these lines:
http://www.kickstarter.com/projects/210251816/usb-based-variable-power-supply-for-small-projects-0?ref=live
Well I'll be damned.
Testing leds with no current limit??
And what's the deal with D2 on the output after the feedback sensing divider?
No isolation, and less than 5V limit (which will drop significantly with the cable) doesn't make it very capable IMO.
Dave.
A curious situation happens with my iPhone. The proper iPhone power cube is marked 5 V 1 A and doubtless the iPhone takes this much current from it when charging.
If I plug the iPhone into a regular USB port on a computer it gets recognized as a USB device and also charges, but it doesn't shut down the USB port. So I presume it limits itself to 500 mA in this case.
If I plug the iPhone into a USB charging port on a particular Duracell battery charger, it overloads the port and shuts it down. At this point I do not know if the phone is trying to pull 1 A and causing an overload, or if it is trying to pull 500 mA and the USB port can't supply it.
I presume if I want to find out what is going on I need a USB breakout cable and to insert an ammeter in the circuit. Maybe I'll get round to doing that at some time.
There is no standard to this but most Android phones will accept the resistance offered by Apple chargers and go into full charge mode. If they don't recognize the value, they just drop down to a slower 500mA charge.
Here's someone doing a project along these lines:
http://www.kickstarter.com/projects/210251816/usb-based-variable-power-supply-for-small-projects-0?ref=live
Well I'll be damned.
Testing leds with no current limit??
And what's the deal with D2 on the output after the feedback sensing divider?
No isolation, and less than 5V limit (which will drop significantly with the cable) doesn't make it very capable IMO.
Dave.
What can be done about limiting the losses on the LED displays? Low voltage rail amounting to LED+switch drop and a miniscule resistor?
I know people think LED displays are a waste of limited power, but somehow I feel a power supply needs LED displays LCD does not feel right for some reason...
What can be done about limiting the losses on the LED displays? Low voltage rail amounting to LED+switch drop and a miniscule resistor?
Or some funny stuff with inductors and PWM power? I have seen some pretty small SMD 10uH inductors with 1Ohm resistance in 1206 size, but it would take something like 5MHz PWM frequency to use them as a lossless LED current limiter... and the EMI would be a mess I guess.
I've got a novel solution that saves power and cost, that I'm sure not everyone will agree with. Many will positively hate the idea (at least they'll forget about the USB negotiation )
There is an agreed standard http://www.usb.org/developers/devclass_docs
But i dunno if anyone is actually using it.
It's more about charger supported usb sockets i think.
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..
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..
Here are some inexpensive low-profile binding posts with colour coding:
http://cpc.farnell.com/pro-signal/psg02051/4mm-binding-post-gold-red/dp/AV15599
Well I'll be damned.
Testing leds with no current limit??
And what's the deal with D2 on the output after the feedback sensing divider?
No isolation, and less than 5V limit (which will drop significantly with the cable) doesn't make it very capable IMO.
Dave.
Yeah that's what I saw too. No current set/limit/readback, no isolation, and only 5V out, max... if that.
I've seen USB ports actually put out 4.8V, so they might have a 0-4.5V variable supply with a readout. Whoopee!
The writeup talks about charging batteries, so I assume D2 is to protect it in case someone leaves a fully charged
battery connected and pulls the USB power connector. They can't put the voltage sense after the diode because then it would feed back
into the (unpowered) regulator. But I don't think it would harm it. With the diode there as they've done it, they would have
to account for the (unknown) diode voltage drop in their micro. But now the accuracy has all gone to shit.
Nice to see them using a Renesas R8C/2K. I like any design bold enough to break away from the AVR's, and PIC's.
However, for all that they are doing with the micro (just an ADC to provide a voltmeter), a PIC might have been cheaper.
I like the idea of such a thing. If it is reasonably priced, small enough and in deed comes as a diy kit, then I'm in.
I like the idea of such a thing. If it is reasonably priced, small enough and in deed comes as a diy kit, then I'm in.
Well, it's almost all SMD, so a kit can't really be done. But I could let you solder the binding posts?
Dave.
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.
Dave, how isolated will the power supply be?