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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. |
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#1
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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
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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
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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
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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
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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. |
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