Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
  #1   Report Post  
KILOWATT
 
Posts: n/a
Default Blue leds drive circuit...needed?

Why bugger about tarting up a bloody computer
with fancy lights. I cant see the point.


A computer is just a tool to do a job.


LUSER !!!

Anyone serious here at least about electronics? TIA


  #2   Report Post  
KILOWATT
 
Posts: n/a
Default Blue leds drive circuit...needed?

Hi Sam. Nice to have some knowledgeable people here!
But assuming you have the zeners backwards, that circuit
is actually a constant current source[...]

Ok Sam i've double-checked and the diodes are not shown
backwards on the schematic,they're really oriented this way.
I've tested two of them with about 30mA of current flowing through
them to be shure they where indeed 6.2V zeners (C6V2 is shown
on their casing) and...they are. In fact i got a reading of 6.4V with
the 30mA current.

Not a complicated circuit but the more i think of it, the more i'm asking
myself the utility of this "regulator". Even if the diodes conduct @ 6.2V,
the voltage should rise then at least near 12.4V before they conduct. I asked my
friend to fire-up his ASUS pc Probe program today, without using anything
nor running any programs...and asked him to check the voltages. The 12V
portion indicated 12.11V. This isn't close to 12,4V at wich the zener diodes
would conduct and then play their role in the regulator.I tested many pc power
supplies in and out of computers since almost two years...and never seen the 12V
portion going higher than 12.2~12.3V. So the only reason i can see yet to introduce
such "regulator" between the 12V and the expensive blue leds, no matter if it would
be for current or voltage limit, is probably just to protect them in case where the
12V because excessively higher.

That's what i figured yet in the past hours...i can't see any other reasons,unless
someone have a better explanation to offer. I contacted Addison electronics
here in Montreal and they do have 5mm blue leds... 1.75$ (X 4) wich is not
a bad price for someone who love modded computer cases! :-)

Thanks a lot also to you Jeff for the description of
how you managed to connect your blue leds in series.
--
Alain(alias:Kilowatt)
Montréal Québec
PS: 1000 excuses for errors or omissions,
i'm a "pure" french canadian! :-)
Come to visit me at: http://kilowatt.camarades.com
(If replying also by e-mail, remove
"no spam" from the adress.)


  #3   Report Post  
Franc Zabkar
 
Posts: n/a
Default Blue leds drive circuit...needed?

On Thu, 10 Jul 2003 23:54:59 -0400, "KILOWATT"
put finger to keyboard and composed:

Hi Sam. Nice to have some knowledgeable people here!
But assuming you have the zeners backwards, that circuit
is actually a constant current source[...]

Ok Sam i've double-checked and the diodes are not shown
backwards on the schematic,they're really oriented this way.
I've tested two of them with about 30mA of current flowing through
them to be shure they where indeed 6.2V zeners (C6V2 is shown
on their casing) and...they are. In fact i got a reading of 6.4V with
the 30mA current.


The circuit is that of a current source whose value is given by

I = (2 * Vf - Veb) / 47

where Vf is the forward voltage drop of the zener diode, probably
around 0.7V at 5mA. Assuming Veb is about 0.65V, then I = 16mA.

I'm not sure why zeners are used as they are. I know that two 6V2
zeners have a lower combined temperature coefficient than a single 12V
diode, at least in the reverse direction. But I'm not sure whether
this is relevant in this case.

Anyway, here are two datasheets:
http://www.dectel.ru/analogues/stabil/pdf/BZX79_2.pdf
http://www.fairchildsemi.com/ds/BZ/BZX79C22.pdf


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.
  #4   Report Post  
Sam Goldwasser
 
Posts: n/a
Default Blue leds drive circuit...needed?

Franc Zabkar writes:

On Thu, 10 Jul 2003 23:54:59 -0400, "KILOWATT"
put finger to keyboard and composed:

Hi Sam. Nice to have some knowledgeable people here!
But assuming you have the zeners backwards, that circuit
is actually a constant current source[...]

Ok Sam i've double-checked and the diodes are not shown
backwards on the schematic,they're really oriented this way.
I've tested two of them with about 30mA of current flowing through
them to be shure they where indeed 6.2V zeners (C6V2 is shown
on their casing) and...they are. In fact i got a reading of 6.4V with
the 30mA current.


The circuit is that of a current source whose value is given by

I = (2 * Vf - Veb) / 47

where Vf is the forward voltage drop of the zener diode, probably
around 0.7V at 5mA. Assuming Veb is about 0.65V, then I = 16mA.

I'm not sure why zeners are used as they are. I know that two 6V2
zeners have a lower combined temperature coefficient than a single 12V
diode, at least in the reverse direction. But I'm not sure whether
this is relevant in this case.

Anyway, here are two datasheets:
http://www.dectel.ru/analogues/stabil/pdf/BZX79_2.pdf
http://www.fairchildsemi.com/ds/BZ/BZX79C22.pdf


That makes sense but it's still a mystery as to why they would use
zeners at all unles the manufacturer got a great deal on them. I can't
imagine such a circuit caring about super stable performance with
respect to temperature.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the Feedback Form at repairfaq.org. Thanks.

  #5   Report Post  
KILOWATT
 
Posts: n/a
Default Blue leds drive circuit...needed?

Hello all!
Frank your absolutely right...it's a contant current source
and i tested it tonight(with cheap red leds...not the expensive
replacement blue leds!) and it regulates the current so it provides
a constant 17mA approx. to the blue leds from 6.5V and up.(below
that of course the leds begins to glow dimly because they start to
operate under their normal operating voltage,wich is about 3.2V(X2
in series) ) The circuit nominally operates @12V...it can operate up to
14V in fact. But higher than that...excessive power dissipation begins.

It regulates the current pretty well...especially for such a simple circuit.
It varies from 17 to 20mA for the range of 6.5~14V. I was very
surprised when i tested it (1st wich cheap reds leds) to see that it survived
the thousands volts from the inverter's output where my friend "accidentaly"
plugged it.
Sam wrote:
That makes sense but it's still a mystery as to why they would use
zeners at all unles the manufacturer got a great deal on them. I can't
imagine such a circuit caring about super stable performance with
respect to temperature.

Yes Sam...it's probably a large batch they got and they decided to use them
this way(forward biased). To finish...the front panel is now lighted just as it
used to be...and my friend is proud again of his modded computer case! :-)
Thanks a lot to all of you for the usefull replies.


Reply
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules

Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Typical domestic electrical circuits John Aston UK diy 21 January 21st 04 04:10 PM
low voltage PIR controlled lights for drive - where? keith UK diy 11 December 24th 03 09:49 PM
Convert radial (cooker) circuit to socket circuit Chi UK diy 3 December 23rd 03 06:48 PM
Case of the unknown circuit David Hearn UK diy 4 November 10th 03 05:12 PM
Is it a radial or ring circuit? Paul UK diy 14 September 4th 03 05:48 PM


All times are GMT +1. The time now is 11:34 AM.

Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 DIYbanter.
The comments are property of their posters.
 

About Us

"It's about DIY & home improvement"