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.
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.
Fascinating how everybody wants Dave's small USB power supply to do something else...in the end it's going to end up making tea aswell .
Dave, don't let yourself be distracted too much. Just keep it simple, what you said so far sounds good .
That is a myth. USB ports do not require negotiation to supply 500mA, they must supply 500mA at all times regardless.
It would be even nicer if you publish the code for the micro controller (if there is one in) so we can have fun programming it too... i'd definitely buy one !
That is a myth. USB ports do not require negotiation to supply 500mA, they must supply 500mA at all times regardless.
You are (as usual) being overly tricky with words there.
The USB spec requires devices to negotiate in order to draw 500 milliamp. It does not require hosts to implement any sort of current limiting, although contrary to your statement they are allowed to.
A USB host would be perfectly within its rights to shut off a device that attempts to draw more than 100 milliamp without negotiating for it, although this does not happen in practice.
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
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.
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
I am pretty sure that no USB ports on consumer equipment actually does any sort of current limiting to 500ma.
I am pretty sure that no USB ports on consumer equipment actually does any sort of current limiting to 500ma.
Any well designed port will have some form of current limiting, it is very common.
Maxim for example make a range of USB current limit devices:
http://para.maxim-ic.com/en/search.mvp?fam=cl_usb_sw&tree=hotswap
Polyswitches are common too, although not as accurate as a current shunt measurement chip of course.
Dave.
I am pretty sure that no USB ports on consumer equipment actually does any sort of current limiting to 500ma.
Any well designed port will have some form of current limiting, it is very common.
Maxim for example make a range of USB current limit devices:
http://para.maxim-ic.com/en/search.mvp?fam=cl_usb_sw&tree=hotswap
Polyswitches are common too, although not as accurate as a current shunt measurement chip of course.
Dave.
Many modern microcontrollers have USB peripherals on them; presumably you are using a microcontroller on the board to drive the displays, so why not pick one with USB and do the enumeration to get 500mA?
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
Dave.
Many modern microcontrollers have USB peripherals on them; presumably you are using a microcontroller on the board to drive the displays, so why not pick one with USB and do the enumeration to get 500mA?
If it fails you could make something like the current display flash to indicate that the USB current is being exceeded. Just like I don't like using a 3A diode above 3A, or a 40 MHz micro above 40 MHz, it's just bad engineering practice. Sod's law will mean that some really popular series of laptops or desktops have current limiting.
No further correspondence will be entered into.
Dave.
Ehhh.. It depends.
Most desktop computers simply have a 500 mA polyfuse from the 5 volts rail to the USB Vbus pin.
Laptops do the same.
But... There are motherboards out there that use intelligent power switches typically from the TISP family (TI). These are digital mosfets in SO8 package with 2 feedback signals .. And these things detect 40,100 and 500mA. Combine that with a usb host controller that supports this and you have troubls. Truth be told , i haven't seen many of these around but they do exist. Some SuperMicro server boards use them as well as some intel workstation grade boards.
Try plugging a usb powered harddisk in that does not behave well and there is no starting it. Windows gives you a little balloon that say something along the lines of 'this device draws too much power and has been disconnected.'
And if they did actually limit 100mA to the first device plugged in, and it draws that 100mA, then there is nothing left to power the next device that is plugged in which is rightly allowed to also take another 100mA off the bat. That's the likely reason why they never implement limiting?
I've yet to see one or hear of a confirmed case of one that does 100mA current limiting.
To increase the cost and complexity on such an otherwise simple design just to do proper enumeration that is not needed in practice would just be silly.
--------
> 1.45V~
0.23A
--------
> OFF ~
--------
> 0.00V
O.LD.