"Perfect" LED current limiting challenge

[Ron H]

Originally posted by jimandy:
I must have missed it but where did the quote come from that begins...
"HOWEVER, SOME CHEMISTS DENIGRATE PATENTS...."

The only source of this quote that Google could find is this site. Lots of interesting info here relating to the subject of chemical patents.

[Chris Smith]

From a Patent info website.

It might still be in my cookies?

[MicroRem]

I tossed my cookies from this thread long ago

let's put it to sleep

Tom

[v6a1a4]

I'm Embarrassed to find that "Ian" is from Toronto Canada, on behalf of all Canadians, I'm sorry, we are usually better behaved then that.

[ian]

Hey treebeard, isn't that the problem with Canadians? Aren't we polite to a fault?

I know a guy who used to be a consultant for companies going bankrupt. He worked for a company that would, when hired, evaluate your company, make recommendations, and take it from the red to profitable. He always says he hated working with Canadian managers because it was so hard to get them to make a damn decision.

As for my behavior, well I spent the first 100 posts in this thread trying to wake up the cut'n'pasters to an origional concept. I get fed up with so called electronic geniuses that can't follow a simple directive. I made it clear what I wanted and got 100 posts of something I repeatedly and explicitly said I did not want. Then I was mocked for not stating what I wanted, then I was told I didn't need it, then I was told there are "other ways", then I was told it was already solved and answered when it wasn't. How many goofball replies am I supposed to suffer before blowing a little steam?

<small>[ January 14, 2006, 07:28 AM: Message edited by: ian ]</small>

[Mike6158]

How many goofball replies am I supposed to suffer before blowing a little steam?

≈12.906323583200043 ±.000012118843

[jimandy]

How many goofball replies am I supposed to suffer before blowing a little steam?

Well to some, it sounded like hot air, not steam.

[philba]

Originally posted by jimandy:

How many goofball replies am I supposed to suffer before blowing a little steam?

Well to some, it sounded like hot air, not steam.

I had a much less charitable reaction...

[rshayes]

Here is a rough design for a switching type of LED driver.

<img src="http://home.earthlink.net/~hexibah/site ... ver_01.gif" alt=" - " />


I haven't tried breadboarding it yet, so it may need a fair amount of tweaking, since most of the references use the Vbe of transistors.

Two diodes, D1 and D2, are used to provide a crudely regulated supply for the control circuits. The 10 ohm resistor samples the LED current, It will have 200 millivolts across it when the diode current is 20 milliamps. An emitter follower, Q1, is used to shift this signal upward by Vbe. The 68K resistor uses the Vbe of the transistor to set the trip point of a schmidt trigger formed by Q2 and Q3. When the trip point is exceeded, Q2 turns on and Q3 turns off. Q3 provides the base drive for the switching transistor Q5 through a common base stage, Q4. This allows the transistor drive to be independent of the supply voltage. The 180K resistor sets the spacing between the trigger points of the schmidt trigger.

When Q5 is switched off, due to exceeding the upper threshold of the schmidt triggger, current will continue to flow in the LED due to the energy stored in the inductor. When this current falls to the lower threshold of the schmidt trigger, the switching transistor will be turned on, and the LED current will begin to increase.

The current in the LED will be continuous, and will cycle between the upper and lower limits set by7 the schmidt trigger. Current will be drawn from the battery only during the switch on period, resulting in lower battery drain when the battery voltage is higher. If effect, constant power is drawn from the battery independent of the battery voltage.

<small>[ January 14, 2006, 12:40 PM: Message edited by: stephen ]</small>

[Chris Smith]

What was the original question?

It changed soo much through the posts I got lost.

I do know the original post was met at least a dozen times over but the goals kept moving one step ahead of the answers.

Even in electronics this is called the "elusive problem",..... one with out a solution.


And remember you disqualify your self the second you change or add to your goals especially after the solution to the original problem has been met.

If you fail to state ALL of your parameters up front, You failed and you became the problem. Not the other way around.

Here's a challenge for you hacks.........
I need a battery operated very efficient(bordering on perfect) circuit for diving a LED but it has to be safe. Right now I'm using a PIC to control the a DC to DC converter voltage to a LED, but the LED "on" state is still controlled by a 2n3904 transistor. I intend to use a large enough resistor into the base of the transistor so that the current through the LED can only be a maximum of 30mA, then control the LED with PWM.
The transistor only creates a voltage drop of .1V allowing the DC to DC controller to be efficient. I need an average of 20mA through the LED but if the micro goes haywire I can't have the LED burning out so I need a max 30mA through the LED in a worst case scenario.
My concern is that the hfe of a 2n3904 transistor may vary so much that a design resistor producing a nominal 30mA may yield less than 20mA or more than 40mA depending on the hfe variance. Is that true? Are there transistors with "set" hfe's? Is there a better way to limit the current to 30 or so mA without creating a large voltage drop?


"Beauty is transitory" Spock

[ December 02, 2005, 05:04 AM: Message edited by: ian ]

I see no mention of how many batteries you intend to use?
I see no restriction about changing the battery configuration as to voltages or size requirements?
I see NO power requirements or percentage ratios that you want to achieve?
I see no limits or real goals as to what your trying to accomplish?
I see no mention if brightness or longevity is your goal?
I see no mention if pulse or straight DC is part of your design?
I see no mention if this is a signal project, a illumination project, or just a indicator lamp?
“Safe” is a subjective term, blowing out or blowing up?
Starting a fire or saving the life of the Led?
I see no mention of what this project is even for?

In fact, I have asked more of your post than you have actually stated in your original post, yet were all supposed to Read your mind AND solve this equation?

Nope, You failed to state your objective up front and thus keeping one step ahead of the answers serves no purpose. That is called “elusive” and has no home in the electronics field. Don’t apply for upper management just yet, you WILL confuse the poor staff.

<small>[ January 14, 2006, 01:53 PM: Message edited by: Chris Smith ]</small>

[ian]

Ian said....

"In experimenting with this circuit I used a DC to DC converter. With 4 new D cells outputting 6.4 volts, and a white LED with a forward drop of 3.1 volts I was able to drive the LED with 20mA while drawing 12.5mA from the battery.
Thats about 80% efficient?"


Will said...
"80% efficiency ?? 20 mA at 3.1 volts divided by 12.5 mA at 9.0 volts looks to me like 55% efficiency"

Ian said.....
"I guess reading isn't your strong suit Will.
77.5% efficiency. 20 mA at 3.1 volts divided by 12.5 mA at 6.4 volts (peak voltage of 4 "D" cells)."

And will said....
" I Guess reading isn't your strong suit " - You ignorant clown - I read your portion about the 9 volt battery - More to the point - how come you said 80% efficiency first time then, after I gave you a tutorial on how to calculate it (I remember from last year or earlier this year, that you didn't know the difference between Watts and Watt-seconds) you got it to 77.5% ? - No need for a reply - the answer is fairly obvious"

Geez, I missed that earlier

[Chris Smith]

No actually that is not what Ian said.

Ian revised, and revised, and revised.

Here is what Ian really said.

"Here's a challenge for you hacks.........
I need a battery operated very efficient(bordering on perfect) circuit for diving a LED but it has to be safe. Right now I'm using a PIC to control the a DC to DC converter voltage to a LED, but the LED "on" state is still controlled by a 2n3904 transistor. I intend to use a large enough resistor into the base of the transistor so that the current through the LED can only be a maximum of 30mA, then control the LED with PWM.
The transistor only creates a voltage drop of .1V allowing the DC to DC controller to be efficient. I need an average of 20mA through the LED but if the micro goes haywire I can't have the LED burning out so I need a max 30mA through the LED in a worst case scenario.
My concern is that the hfe of a 2n3904 transistor may vary so much that a design resistor producing a nominal 30mA may yield less than 20mA or more than 40mA depending on the hfe variance. Is that true? Are there transistors with "set" hfe's? Is there a better way to limit the current to 30 or so mA without creating a large voltage drop?"

[jimandy]

" I Guess reading isn't your strong suit " - You ignorant clown

What a great line! It reminds me of the one on SNL when Dan Akroyd turns to to Jane Curtin and says...

"Jane, you ignorant ####4!"

[ian]

Chris you fool, I quoted my post directly. AND it was the DIRECT reference of my 80% efficiency reference. Talk about not being able to stay on topic! Sheesh

[cadstarsucks]

Well you could use a single transistor with two resistors from the pwm output and an emiter resistor but you still have the efficiency problem.

With a PIC you could feed back the emiter current to the onboard comparator and use it with an inductor for a peak current boost triggered by the PIC.