Burning a BS2 using 15 power Leds ( 40 mA each)

Markus

Hi,

I know that BStamps have a limited amount of mA.
Is it possible to burn the regulator if I turn on 15 or
more power Leds?

Power Leds consumption is about 40 mA each one.

( Of course, all the leds turned on at the same time using High and Low comands)

Thanks, Mark

Yanroy

If you don't have any driver circuitry (i.e. the stamp pin is connected directly to an LED and a resistor), then you will most certainly burn out the stamp by running those.· Purely by accident, I've discovered that you can push a stamp past it's 20mA per-pin limit for a brief second, but don't ever do it on purpose!· I imagine I've just always been lucky enough to turn it off before permanent damage was done.· But drawing 40mA from nearly all the pins would be pretty certain death for the stamp.· As for the regulator, if you're talking about the one on board the stamp, that wouldn't be able to source enough current... I don't know if it would let the smoke out or if it has a thermal shutdown.· If it's an external regulator, most of those can handle upwards of 500mA (especially the ones on the parallax carrier boards), so it's probably fine from a regulator standpoint.· You may want to look at using a transistor or FET to drive those LEDs.· There's plenty of posts here in the forums on how to do that.

Paul Baker

It's not the regulator you need to worry about, typically regulators will sag the voltage if you try to draw to much current (causing a brown out condition), problematic but usually not damaging, unless you have a regulator which does not provide thermal protection. The thing you need to worry about is overdrawing the maximum amount of current from the microcontroller on your BS. Doing this will cause individual pins to burn out or burn out the entire chip. And yes doing what you propose will cause one or both to occur. The max current per pin is 30mA (drawing at this current will dramatically reduce the lifetime of your stamp), the max total for all pins plus the microcontroller's current consumption is 70mA (I maybe slightly off on this number), your max total is 600mA! You can be sure you will have a dead stamp in no time flat. You need to buffer the output so the BS is not directly driving the LEDs. Your best option is using darlington transistor arrays.

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Post Edited (Paul Baker) : 7/16/2005 7:21:53 PM GMT

Electronegativity

Hook the stamp up to a MAX7219 LED driver:

http://www.parallax.com/detail.asp?product_id=603-00001

I did it last night and it works just fine.

It can drive as many as 64 individual LEDs.

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I wonder if this wire is hot...

allanlane5

I believe a 74HCT595 is rated for that much current. If it isn't, the popular Darlington Array certainly is.

Joe

Hi,

I will send 6 basic stamps to an school (studens about 14 years old)
and I would like to give some protection to the experiments.

Work with leds is very tipical and I would like to place sothething
like a fuse becasue I don't like to buy other 6 Basic Stamps again
each time the students add Leds or something that drain more than 40 mA.,
or each time they overload the Stamp regulator with any other little
motor.

It would be VERY USEFULL and I could send Parallax a lot of congratulations
if the basic stamps stop their operation or if the stamps could have
an autoprotection if somebody add big Leds, motors or any other hardware
that have a big comsumption (over the max. consumption of the Stamps)
to a Basic Stamp.


Thanks. Joe

Forrest

I'm pretty sure the BOE and the SuperCarrier boards will supply 1 Amp at 5V DC if attached to a 7.5V plug-in power supply

Jon Williams

Joe,

We're already criticized quite a lot for the price of BASIC Stamps -- if we added in all the protection you're asking for then the price would go even higher, and we certainly couldn't fit all those parts on a 24-pin module. We designed the Homework board for students with protection resistors on every IO pin to preven accidents, and we drove the cost down by buiding the module right into the board so that an "accidental" death would not be financially painful.

In the end, we are all dealing with electronic circuits here and a certain amount of design needs to come into play. Send your student specific guidelines (or user ours, we have lots of great experiments in "What's a Microcontroller?") so that you don't have any unhappy accidents.

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Jon Williams
Applications Engineer, Parallax

Paul Baker

allanlane5 said...
I believe a 74HCT595 is rated for that much current. If it isn't, the popular Darlington Array certainly is.

The per pin current should be close enough, but the combined current is too much for a 595.

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Joe

Thanks,


But, last time there was a mistake an I add some leds
to the Basic Stamp directly ( about 400 mA) and the Stamp
did not burn.

My question is then:

Does the Basic Stamps have protections?

I was a lucky man since the Basic Stamp did not burn?

Please, I am a little confused, a short "yes" or "not" answer to these
questions will be usefull

Thanks, Joe

KenM

Joe said...
Thanks,


But, last time there was a mistake an I add some leds
to the Basic Stamp directly ( about 400 mA) and the Stamp
did not burn.

My question is then:

Does the Basic Stamps have protections? NO

I was a lucky man since the Basic Stamp did not burn? YES LUCKY
Please, I am a little confused, a short "yes" or "not" answer to these
questions will be usefull

Thanks, Joe

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Ken

Paul Baker

If Im not mistaken, the voltage regulator is rated for 300mA. You may have been lucky because its possible you generated a near brown out condition, this lower voltage would have reduced the actual current of the LEDs. This is just a hypothesis, you should never count on such an occurance. Count your blessings and never do it again, it's never wise to tempt Murphy.

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