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'[EE] Nearly decided on a power source'
2008\06\26@130833 by Tomás Ó hÉilidhe

picon face


Two AA batteries in series gives me 3 V.

I'm thinking of going with the setup described on this page:

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1042,C1033,P1468,D4253

It uses the LT1173-5 chip to get 5 volts from 3 volts.

I will use the 5 V to power my microcontroller, but I'm thinking of
taking 3 V directly from the battery to power my LED's.

Does this sound good?

2008\06\26@132308 by Mark Rages

face picon face
On Thu, Jun 26, 2008 at 12:07 PM, Tomás Ó hÉilidhe <spam_OUTtoeTakeThisOuTspamlavabit.com> wrote:
>
>
> Two AA batteries in series gives me 3 V.
>
> I'm thinking of going with the setup described on this page:
>
> http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1042,C1033,P1468,D4253
>
> It uses the LT1173-5 chip to get 5 volts from 3 volts.
>
> I will use the 5 V to power my microcontroller, but I'm thinking of
> taking 3 V directly from the battery to power my LED's.
>
> Does this sound good?
>

Why not run the whole thing from 3V?  Most microcontrollers are happy at 3V.

Have you done as Olin suggested and traced the current v. voltage
curve for your LEDs?  I recommend you do this before deciding on a
power source.

Regards,
Mark
markrages@gmail
--
Mark Rages, Engineer
Midwest Telecine LLC
.....markragesKILLspamspam@spam@midwesttelecine.com

2008\06\26@134951 by Tomás Ó hÉilidhe

picon face


Mark Rages wrote:
> Why not run the whole thing from 3V? Most microcontrollers are happy
> at 3V.
> Have you done as Olin suggested and traced the current v. voltage
> curve for your LEDs?  I recommend you do this before deciding on a
> power source.
>  

I haven't picked my LED's yet, but I know their Vf will be less than 3 V.

2008\06\26@140827 by Mark Rages

face picon face
On Thu, Jun 26, 2008 at 12:49 PM, Tomás Ó hÉilidhe <toespamKILLspamlavabit.com> wrote:
>
>
> Mark Rages wrote:
>> Why not run the whole thing from 3V? Most microcontrollers are happy
>> at 3V.
>> Have you done as Olin suggested and traced the current v. voltage
>> curve for your LEDs?  I recommend you do this before deciding on a
>> power source.
>>
>
> I haven't picked my LED's yet, but I know their Vf will be less than 3 V.

How are you planning on regulating their current?


--
Mark Rages, Engineer
Midwest Telecine LLC
.....markragesKILLspamspam.....midwesttelecine.com

2008\06\26@144745 by Tomás Ó hÉilidhe

picon face


Mark Rages wrote:
>> I haven't picked my LED's yet, but I know their Vf will be less than 3 V.
>>    
>
> How are you planning on regulating their current?

Not sure if this is a trick question... but current-limiting resistors.

If the Vf is 2.2 volts and if I want 20 mA, then:

   3 - 2.2 volts = .8 volt

Then I just figure out what value resistor has .8 volt across it when
there's 20 mA thru it:

   R = V divided by I =     .8 divided by .02     =     40 ohms

2008\06\26@173555 by olin piclist

face picon face
Tomás Ó hÉilidhe wrote:
>> Have you done as Olin suggested and traced the current v. voltage
>> curve for your LEDs?  I recommend you do this before deciding on a
>> power source.
>>
>
> I haven't picked my LED's yet, but I know their Vf will be less than
> 3 V.

That's not the point.  Measure the current/voltage curve of any LED you
have.  It will demystify LEDs in general and you will learn something.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

2008\06\26@181035 by Tomás Ó hÉilidhe

picon face


Olin Lathrop wrote:
> That's not the point.  Measure the current/voltage curve of any LED you
> have.  It will demystify LEDs in general and you will learn something.

The problem I had was that I was thinking of LED's as being distinct
from normal diodes.

I understand diodes completely:
   * They only allow current to flow in one direction
   * They always have 700 mV across them (or thereabouts)

If the supply voltage is less than 700 mV though, then negligible
current flows.

Now the only difference with an LED is that instead of 700 mV, you have
maybe 2.2 volts.

I know what the current/voltage curve of a diode looks like. For a 2.2
volt diode, it should look something like as follows:
   * When the supply voltage is less than 2.2 volts, the current flow
is extremely low (in the micro or nanoamps)
   * As the supply voltage hits 2.2 volts, the current curve starts to
rise, and then it quickly becomes a near-vertical line.

The current curve doesn't shoot up to the sky in a perfect straight
line, but it's pretty straight. Zeners of course have a straighter line
(but of course you use them in reverse bias).

2008\06\26@183502 by olin piclist

face picon face
Tomás Ó hÉilidhe wrote:
> I understand diodes completely:

Yeah, right.  With that kind of attitude you'll never learn anything new.
Do the experiment I talked about, noting the brightness at the different
levels.  I think a few details will surprise you.  The intuition gained is
something you won't get from any data sheet, and makes the distinction
between paper learning and experience that is so important in real design
situations.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

2008\06\26@190610 by Tomás Ó hÉilidhe

picon face

Olin Lathrop wrote:
> Tomás Ó hÉilidhe wrote:
>  
>> I understand diodes completely:
>>    
>
> Yeah, right.  With that kind of attitude you'll never learn anything new.
>  


Here we go again with your social difficulties. Figure out what you want
to say, then figure out a way to say it.

Do you not think you sound caustic when you respond with "Yeah, right.
With that kind of attitude you'll never learn anything new."? This is
why I called you a dickhead.

Are you really that arrogant/stupid/delusional that you think I'll
"never learn anything new"? I learn lots everyday. Just today I learned
about step-up regulators.

Do you realise that you're the *only* person I've had difficulty with on
this mailing list? Notice I haven't had a run it with Jinx, or Dennis,
or Tamas, or BJ, or Mark, or Alan, or Bob, or Byron. These people have
been polite and helpful to me, and I've been polite in return. You, on
the other hand, have just a belligerent old man.

I didn't invent the diode, granted, and I'm sure you could fill a
warehouse with what I don't know about them, but I know enough to use them.

Can I politely ask that you just don't respond to me in future if you
have nothing good to say. Speaking honestly, if you'd spoken to me that
derogatorily in real life then I'd just punch you in the mouth and be
done with you.

You're full of venom, and it's only to your own detriment.

2008\06\26@192503 by Tamas Rudnai

face picon face
> Yeah, right.  With that kind of attitude you'll never learn anything new.

He did not even click on links we sent, why would he switch on his soldering
station when you suggest that?

I think I will set up spam rules so that those topics created be Tomas, so
those will go to my spam folder without even reading them - at least I will
avoid loosing my temper just by reading from such an ignorant person.

Olin, I think now it's time get myself censored <grin> - I really do not
want to offend anyone here but that's the attitude I really do not like.

Tomas, knowledge starts at the point when started to realize how much not
knowing. If you think you know these things then DO NOT ASK HERE. If you do
not know or uncertain then do not just ask, also read what people here say.

Thanks
Tamas



On Thu, Jun 26, 2008 at 11:36 PM, Olin Lathrop <EraseMEolin_piclistspam_OUTspamTakeThisOuTembedinc.com>
wrote:

{Quote hidden}

>

2008\06\26@192912 by Jinx

face picon face
> I haven't picked my LED's yet, but I know their Vf will be less
> than 3V

Just when you thought it was safe to go back in the water ......

If you use PWM to drive the LEDs you may not need current-
limiting resistors. Adjust the duty cycle to suit the supply voltage

Up till now you've been talking about driving the LEDs with static
or standing current. But LEDs have another parameter - peak
repetitive current. This is in the order of a few amps for short
pulses. What mostly kills an LED is watts. Too much V or too
much I. By keeping W in spec you can take liberties with drive

Two examples -

Infra-red remote controls. To get distance, the IR LEDs are pulsed
with high current. In the schematics shown below, 500mA. Current
as DC that would fry them

http://home.clear.net.nz/pages/joecolquitt/txless.html

Moving signs. The LEDs in commercial units are strobed on a 12V
or more power rail, and may have as little as a few 10's of ohms and
draw significant current for short periods

The trick is speed and persistence of vision

2008\06\26@193703 by Tomás Ó hÉilidhe

picon face


Jinx wrote:
> If you use PWM to drive the LEDs you may not need current-
> limiting resistors. Adjust the duty cycle to suit the supply voltage
>  

This is what I originally thought, so I left out the current-limiting
resistors on my original project board. When I told them this on
comp.arch.embedded, they absolutely slated me.

2008\06\26@194241 by Tamas Rudnai

face picon face
> Here we go again with your social difficulties. Figure out what you want
> to say, then figure out a way to say it.

Tomas, I have to disagree with you, really. Olin was helping you. He said
measure and make some experiments. You just completely ignored him, and told
him that you understand diodes completely. That is rude and ignorant. You
open many topics, sometimes 2 or 3 a day, many of them belongs to the same
topic you previously opened, and then you did not even make attention to the
answers. He just told you that this kind of attitude does not lead to
anywhere. And you are using rude words? I do not know what to say, good luck
for everything and I really hope you will think it all over again and will
show some respect - not to people here, but to the science.

Regards
Tamas



On Fri, Jun 27, 2008 at 12:05 AM, Tomás Ó hÉilidhe <toespamspam_OUTlavabit.com> wrote:

{Quote hidden}

>

2008\06\26@194930 by peter green

flavicon
face

> If you use PWM to drive the LEDs you may not need current-
> limiting resistors. Adjust the duty cycle to suit the supply voltage
I wouldn't do this for the intial prototype. If the code hangs or you
are single stepping with a debugger you don't want the LEDs driven
beyond thier continuous maximum. Maybe do this later if you want to get
a bit more efficiancy or brightness out of the system and are confident
that your code is solid.

Another problem is consistancy. The brightness for a given current is
usually fairly constant. The relationship between current and voltage is
not. So a slightly higher input voltage with a series resistor while
giving lower efficiancy should give much better consistancy between then
LEDs in the array.

2008\06\26@195658 by Tomás Ó hÉilidhe

picon face

peter green wrote:
>> If you use PWM to drive the LEDs you may not need current-
>> limiting resistors. Adjust the duty cycle to suit the supply voltage
>>    
> I wouldn't do this for the intial prototype. If the code hangs or you
> are single stepping with a debugger you don't want the LEDs driven
> beyond thier continuous maximum.

In my original project board I used higher value resistors when
debugging. Then when I had it working, I got rid of them.

2008\06\26@200045 by Jinx

face picon face
> > If you use PWM to drive the LEDs you may not need current-
> > limiting resistors. Adjust the duty cycle to suit the supply voltage

> I wouldn't do this for the intial prototype. If the code hangs or you
> are single stepping with a debugger you don't want the LEDs driven
> beyond thier continuous maximum

Tomas has initimated he's better at s/w than h/w and intensive code
would not be a problem. I totally agree wth what you say though for
the development stage

The other thing is that the LEDs are on the output of drivers, not a
PIC and he's thinking of a 3V supply. I don't recall what the driver
IC is but I bet it's got more of a voltage drop than a PIC o/p

> Another problem is consistancy

Agreed

2008\06\26@213755 by Jinx

face picon face
> I wouldn't do this for the intial prototype. If the code hangs or
> you are single stepping with a debugger you don't want the LEDs
> driven beyond thier continuous maximum

In this application the current is already limited by the power supply
and driver semis. Both a benefit and a drawback at times

2008\06\27@014531 by Wouter van Ooijen

face picon face
>> Yeah, right.  With that kind of attitude you'll never learn anything new.
>
> Here we go again with your social difficulties. Figure out what you want
> to say, then figure out a way to say it.
>
> Do you not think you sound caustic when you respond with "Yeah, right.
> With that kind of attitude you'll never learn anything new."? This is
> why I called you a dickhead.

Have a look in the mirror.

If that is not sufficient, reread:
- your statement about "understanding diodes completely"
- Olin's firm but polite (and most of all: totally correct!) response
- your totally out-of-line response to that

--

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu

2008\06\27@065747 by olin piclist

face picon face
Tomás Ó hÉilidhe wrote:
>>> I understand diodes completely:
>>
>> Yeah, right.  With that kind of attitude you'll never learn anything
>> new.
>
> Are you really that arrogant/stupid/delusional that you think I'll
> "never learn anything new"?

If you know something "completely", then by definition there is nothing left
to learn about it.

> I didn't invent the diode, granted, and I'm sure you could fill a
> warehouse with what I don't know about them, but I know enough to use
> them.

That's rather less than knowing them "completely" as you claimed before.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

2008\06\27@071229 by olin piclist

face picon face
Tamas Rudnai wrote:
> at least I will avoid loosing my temper just by reading from such an
> ignorant person.

This is probably a language issue, but "ignorant" only means someone who
doesn't have knowledge.  That's OK.

The problem with Tomas is that he thinks the way we do things is stupid
without understanding them, thinks he already knows everything, and is too
lazy to learn any information on his own.  He just wants us to tell him the
answer.  Fork it over already.  He doesn't have time for those data
thingies.  He'd have to sit down and actually read something.  Eewe.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

2008\06\27@145022 by Cedric Chang

flavicon
face

On Jun 27, 2008, at 1:00 PM, Jinx wrote:

>> If you use PWM to drive the LEDs you may not need current-
>> limiting resistors. Adjust the duty cycle to suit the supply voltage
>

I am sorry.  I do not understand the above statement.  Does this  
presume the power supply is current limited ? Or what ?  What am I  
missing ?  What if the supply is not current limited ?  Do the traces  
provide enough resistance ?  Or is there enough inductance to limit  
current ?  Tell me why this would work in a practical application.
Best
cc

2008\06\27@183700 by Jinx

face picon face
>> If you use PWM to drive the LEDs you may not need current-
>> limiting resistors. Adjust the duty cycle to suit the supply voltage

> Does this presume the power supply is current limited ?

It IS current-limited. (a) small batteries (b) two semis to get current
through and (c) heat dissipation in the LEDs can be calculated

> What if the supply is not current limited ?

Then you do it another way or limit incoming current. The insistence
seems to be for batteries

> Do the traces provide enough resistance ?  Or is there enough
> inductance to limit current ?

Expecting resistance from traces is futile. AFAIK there is no
useable inductance on the LED side, but you'd probably not want
to be wastefully filtering carefully-generated PWM with inductance
known to be in the way

2008\06\27@235910 by David Meiklejohn

face
flavicon
face
Tomás Ó hÉilidhe wrote:

> Olin Lathrop wrote:
> > Tomás Ó hÉilidhe wrote:
> >
> >> I understand diodes completely:
> >>
> >
> > Yeah, right.  With that kind of attitude you'll never learn anything
> new.
> >
>
> Do you realise that you're the *only* person I've had difficulty with
> on
> this mailing list?

Do you realize that Olin was going out of his way to try to help you?
He suggested a way for you to learn more about LEDs, and you respond with "I
understand diodes completely", which is quite arrogant on your part.  I'm
not surprised that Olin responded with "Yeah, right".

> Notice I haven't had a run it with Jinx, or Dennis,
> or Tamas, or BJ, or Mark, or Alan, or Bob, or Byron. These people have
> been polite and helpful to me, and I've been polite in return. You, on
> the other hand, have just a belligerent old man.

There was nothing in any way belligerent in the post from Olin that
suggested ways for you to learn more.

> I didn't invent the diode, granted, and I'm sure you could fill a
> warehouse with what I don't know about them, but I know enough to use
> them.

Perhaps, but the point that you have missed is that the behavior of a LED is
more than just electrical, so your knowledge of diodes won't tell you all
about how LEDs behave in the real world.

As Olin said, data sheets can't really give you an idea of how bright a LED
looks (largely because human perception is not linear).  For example, does a
LED driven with 20mA look twice as bright as one at 10mA?  The data sheets,
and your understanding of diodes, won't answer that one for you.  Nothing
beats personal experience, and that was all that Olin was pointing out (very
politely).

> Can I politely ask that you just don't respond to me in future if you
> have nothing good to say. Speaking honestly, if you'd spoken to me that
> derogatorily in real life then I'd just punch you in the mouth and be
> done with you.

You come across as being very immature.  Maybe you'd punch me in the mouth
too, for saying that, but it's just an honest observation.


David Meiklejohn
http://www.gooligum.com.au



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