newbie PSU / USB question

[defred]

Hi there, just had the idea that it might be possible to use an USB link with a PC to get the 5 V required by the Midibox hardware. no ? yes ?

The idea is, of course, to use it with a good laptop computer, headphones, anywhere !

thanks.

defredoo

[uclaros]

Well, this issue has been discussed again some time...

The problem is that the USB port can supply pretty low current to power a full midibox, lcd, backlight etc.

(500mA I think... correct me someone if I'm wrong).

So, a kind of minimal midibox could be usb powered, as long as it consumes less than 500 mA. (you wouldn't want a blown motherboard especially on your laptop...)

Maybe for the laptop you could take power staight from the battery...

Uclaros.

[defred]

I see, but actually I am doing a minimal Mbox64 core+ain+din+bankstick that's it.

I'll see this later, always possible to insert the option.

sorry all for my bad scan of the forum (there's tons of posts !)

you can erase the topic, since it has been already discussed, I will find it.

thanks.

[Guest OzMax]

I see, but actually I am doing a minimal Mbox64 core+ain+din+bankstick that's it.

I don't know what people's opinions are of USB power, but I use it all the time. I have a special cable which is firstly a USB extender cable, and then the power lines split off to a little power connector.

I power all my little projects on my desk using USB power.  Now I use a little HUB to isolate my projects from my main machine, and if I short out a line, or generally abuse the power off a USB port, it just shuts down that port (VERY specific to the type/brand/make of the USB hub. YMMV - Your Mileage May Vary).

However I was also under the impression that PC USB ports do the same thing. They use a polycap (??? is that correct??) to act as a dynamic fuse. It just shuts down the port if abused..

However (caveat), I feel that a larger projet in the long term (and close to the 500mA current consumption) shouldn't really use USB power (others may disagree, and are welcome to comment), and of course if it's well above 500mA, no joy there. Instead, I would really use a dedicated power supply and a very large T03 style 5V regulator. (like a large 25mm metal oval discrete component with legs coming out of the bottom)

(Begin Off Topic...

I have found that (in my case) even a simple 16x2 backlit LCD (almost full brightness = sucking alot of current) and a core module push the thermal limits of the TO220 case regulators I have been using (noting that this is also dependant on the incoming voltage, i.e. how many watts the linear regulator has to thermal dump, and perhaps my regulators are just well, junk).. In fact has anyone disintegrated their regulators with these large projects? Mine were shutting down regularly until I installed a HUGE heat sink. Now it just melts the plastic case.  ;-)

So I (personally myself) would only use a TO3 in the future and ignore the TO220 case regulator versions completely.

End Off Topic)

So as others will state, yes, no problem with using USB, provided your total current is below 500mA, probably 400mA (add a little safety margin), but (as with everything) there are risks.

Oz

[Guest OzMax]

The idea is, of course, to use it with a good laptop computer, headphones, anywhere !

Ahha, also (and you probably checked this) some USB laptop ports only supply ~80mA. Sufficient for a mouse, but not for anything else.

The keyboard connector is a good place but not much current protection usually makes it's way there. The PCMCIA slot can also be used (I think 200mA is the maximum however through that? sorry don't have the specs with me) Firewire port can give up to ,what, 15 watts?? (~5V @ 3 A ignoring losses, P=VI). That's AC and might work directly into a CORE module  (I take no responsibility for these suggestions, usual deal..   "if you break both legs, don't come running to me" sort of stuff  :-)  ).

But a $5 plug pack which can be easily replaced is looking good.

[pay_c]

First: Yes, 500 mA per USB is correct. Although I wouldn´t push that to the limit.

Perhaps a USB or normal solution would be cool (just dont use both at once). That way you have maximum flexibility.

For the current question: The MOST (!!) of the current is drawn by the backlight of the LCD (appr. 200-500 mA). So if you cut down that (to lets say 100 mA)  you´re on a veeery low current side (in the whole 200-300 mA with not to much LEDs lit up).

Also consider low current LEDs. They need 3mA (instead of 20 mA) to light up. You just have to put another resistor in there (appr. 1k - 1,6k instead of 220 Ohm).

Hope I could help. Greets!

[Guest OzMax]

For the current question: The MOST (!!) of the current is drawn by the backlight of the LCD (appr. 200-500 mA). So if you cut down that (to lets say 100 mA)  you´re on a veeery low current side (in the whole 200-300 mA with not to much LEDs lit up).

Quite right.

Now I haven't worried about any of this yet, so sort of speculating off the top of my head...

There are lots of variables to consider, but certainly with the backlight at full brightness it would chew most of the power (provided any LEDS aren't at ridiculous output brightness levels, and people aren't using ~5W LEDS :-)...

I guess one way would be to say..

+ Right, going to use USB power :. < 500mA (factor in safety, < 400mA)

+ design the LED output stages to use an appropriate amount of power (do the same with sensible values for the DIN, and AIN modules)

+ power the unit up, then push the backlight up till the total current consumed meets your limit, then back it off a lttle for safety.

Off the top of my head... Total current consumption would depend on points including the following but not necesarily limited to...

+ Resistors used for pullups

+ Pot values used for the knobs (if each pot is a 1K, that's alot of power lost, noting that all of the pots are effectively paralleled together (VCC to ground), which result in large power loss through them. (correct? or do I need a coffee)

+ Current limiting on LEDS

+ Backlight level

+ CPU speed

+ CPU type

+ Ambient temperature

+ How the sun, moon and stars are aligned, etc,etc

So it's probably easier to build conservative (if that's your goal) and then trim the largest current sucker (probably the backlight...) of all at the end to reach the target.

YMMV (Your Mileage May Vary)

Oz