What happened to my thread?
What happened to my thread?
(OP)
I had a question on this forum concerning a SM resistor in a LED circuit. It has since disappeared? There was a poster that posted an admonishment (to whom I do not know), and I checked today and it has since disappeared. Was there a violation in posting rules?





RE: What happened to my thread?
Dan - Owner

http://www.Hi-TecDesigns.com
RE: What happened to my thread?
RE: What happened to my thread?
As a Mechanical Engineer in a small company without much of an Electrical Engineer in house (supposedly coming) I wanted to seek the help of knowledgeable resources on this board. I thought this is what these forums allow - dissemination of information (both technical and informational) to help those in need regardless of knowledge standing. With this knowledge I would have designed a reliable and successful product.
I suppose Google is my friend then.
RE: What happened to my thread?
RE: What happened to my thread?
I thought it was specific and detailed. I did all the ground work up front. I just needed reaffirmation on the specifics of a SM resistor - how to calculate wattage for proper sizing.
I would have liked at least an explanation for the deletion. It was a bit sudden and abrupt.
RE: What happened to my thread?
Dan - Owner

http://www.Hi-TecDesigns.com
RE: What happened to my thread?
Did you note down the answer already?
RE: What happened to my thread?
That said, in a nutshell figure out the power dissipated in the resistor (voltage times current, or current squared times resistance), double it for safety, then find the closest (but higher) wattage resistor. For example, a 100 ohm resistor passing 100mA... that's 1W, so find a 2W resistor.
Dan - Owner

http://www.Hi-TecDesigns.com
RE: What happened to my thread?
RE: What happened to my thread?
http://www.allaboutcircuits.com/
RE: What happened to my thread?
@VE1BLL (Military)
I now understand your concern. And I'm as concerned as well
Here is my circuit in a nutshell:
3.2 Vf typ (3.6 Vf MAX)
.150 mA
24 volt CV PS supplying 3.3 A
7 series in parallel with 22 chains
I calculated using typical Vf as the following:
24/3.2 = 7.5 devices so I rounded down to 7 devices in a series chain.
7*3.2 = 22.4 volts required
24 supplied - 22.4 volts = 1.6 volts to dissipate.
Resistance = 1.6/.150A = 10.67 ohms
Wattage across resistor = 1.6*.150 = .24 W
So I found a SM resitor 10.7 Ohms at .25 W.
I have a concern with Vf variance. Would this circuit not work if I get Vf's higher than 3.2 Vf? So if I change my circuit to 6 devices per series chain I need 19.2 volts and need to dissipate 4.8 volts. This just seems like a waste.
I still do not understand Vf variance. If I design for 6 and dissipate 4.8 volts with a resistor - won't the series chain be starved if Vf goes higher? So how do you accommodate this variance?
I just have worries about this design (regardless if I design for 6 or 7). I know some about LEDs but I do not know what happens when I get Vf variance.
@MacGyverS2000 (Electrical)
By doubling the resistor value don't I starve the series circuit? I.e. I dissipate needed voltage (or wattage) that is required by the LED series circuit in order to light. If I double to 3.2 volts to dissipate I'm stealing this from the series chain.
In my design I'm being held to optimizing the efficacy of the entire system -> total lumens/total wattage. So I'm trying to maximize total lumens and minimize total wattage.
RE: What happened to my thread?
Does a SM resistor go at the end of the series chain? I always assumed at the start of the series chain. I thought a resistor dropped the voltage prior to it entering the series chain - thus limiting how much voltage is supplied.
If it goes at the end then the required voltage needed by the LED series will be used by the time it gets to the resisor - thus the resistor may or may not need to disippate voltage depending upon what is left over.
Sorry for the "noob" questions. I have searched the internets to try to piece all this together.
I promise once you answer these concerns/questions I'll be on my way
RE: What happened to my thread?
You have encountered the biggest downsides to controlling LED current with a simple resistor... changes in power supply voltage or LED Vf can create wide variances in current through the string. It's a games of numbers... how much variance in current are you willing to allow for other parameter changes. If you put 7 in a row, even a small spike of 2V in your supply will nearly the current in your string. Losing 1V will cut your design current in half. This is why no quality/efficient display circuit will control current in this manner (they'll use a current-controlled device).
Plenty of chips out there to do the job for you, but here's an example of a constant current source using a cheap and readily available part, the LM317 voltage regulator:
http:
Dan - Owner

http://www.Hi-TecDesigns.com
RE: What happened to my thread?
Dan - Owner

http://www.Hi-TecDesigns.com
RE: What happened to my thread?
RE: What happened to my thread?
1) Fixed resistors mean large variability in string currents.
2) The first failed-shorted LED results in most or all the LEDs in a string going down in flames. With a controller one shorted LED means that the remaining LEDs in a string continue with the original current - unaffected.
Keith Cress
kcress - http://www.flaminsystems.com
RE: What happened to my thread?
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: What happened to my thread?
They take those rules seriously.
RE: What happened to my thread?
Also, posts are often removed for an entirely different reason than you think. There are certain IP numbers that will get nixed if they ever show up. That means you could see a post and possibly respond to it before the powers-that-be see it and remove the entire thread.
Keith Cress
kcress - http://www.flaminsystems.com
RE: What happened to my thread?
Its probably the shilling that got me tossed. Why they deleted the original post I can't figure. The guy seemed to have a legit question, even if it was one of terminology and not straight engineering.
I find it hard to believe that they remove posts based on IP addresses manually. That's a lot of work for something so easily automated.
RE: What happened to my thread?
Most likely, the OP was found out to be a student.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: What happened to my thread?
For instance a school might have hundreds of IPs that are in a block. Sometimes a banned individual tries to come back on all the adjacent IPs. Or you could have a shill working from the same corporate block as the original spammer.
Keith Cress
kcress - http://www.flaminsystems.com
RE: What happened to my thread?
I got my boards in and they work fine.
Though I have one last question:
I designed in 11 SM resistors for each series chain to drop 1.6 volts. Did I make a mistake?
Why wouldn't I have just one SM resistor at the start of the circuit to drop 1.6 volts and the deliver the remaining 22.4 volts down stream in a series parallel design? I designed the circuit as I see depicted on the web.
Wouldn't it be the same if I just used one resistor and disapated .25 W * 1 device instead of .25W * 22 devices?
Again I am a Mechanical designing LED circuits. I really do not have an Electrical of knowledge on board. Hopefully this will change.
RE: What happened to my thread?
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If we learn from our mistakes I'm getting a great education!
RE: What happened to my thread?
Because I have to disappate 3.3 A * 1.6 VDC instead of .150 A * 1.6 VDC, right? Now I get it
Thanks for the answer.
RE: What happened to my thread?
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If we learn from our mistakes I'm getting a great education!
RE: What happened to my thread?
The LEDs in each series string will pass equal current, by definition. But between strings, differences in the sums of the LEDs' Vf drops could result in each string drawing more or less current. In most cases, differences between individual LED Vfs will average out within each string.
Another problem to consider is that your current regulating resistor is dropping only 1.6V out of a total string voltage of 24 Volts. The 22.6 V LED drop assumes some nominal operating temperature. As the ambient temp and/or the LED temperatures change, so will their Vf. And since the regulation slope of the resistor (the inverse of its resistance ) is pretty steep, a small change in the strings Vf will result in a large change in current.
Not knowing the device parameters (temp coefficients) and the thermal operating characteristics, I can't say whether this will be important. But this is the sort of stuff that people with advanced degrees in LED strings have to deal with every day.