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?
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What happened to my thread?
- Thread starter oharag
- Start date
MacGyverS2000
Electrical
Without seeing the post I cannot say for sure, but an SM resistor in an LED circuit sounds like a hobbyist-level or student-level question. If that's the case, it was probably removed for being a violation of the site's TOS (posts by engineering professionals only).
Dan - Owner
Dan - Owner
CorBlimeyLimey
Mechanical
See faq1088-1600 for a list of possible reasons.
- Thread starter
- #4
It was a novice question since I'm a Mechanical Engineer by degree.
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.
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.
CorBlimeyLimey
Mechanical
I did not see the removed thread. Try reposting, but this time explain your situation (as above) and maybe the person(s) who Red Flagged the thread will be a bit more understanding.
- Thread starter
- #6
@CorBlimeyLimey (Mechanical)
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.
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.
MacGyverS2000
Electrical
I don't recall the thread offhand, but Google will give you a million hits for determining resistance values (and proper wattage) for LED circuits.
Dan - Owner
Dan - Owner
It was a very simple question, but I noticed the "(Mechanical)" and generously assumed it was probably a legitimate question. These forums tend to be on hair trigger for student posts at this time of year.
Did you note down the answer already?
Did you note down the answer already?
MacGyverS2000
Electrical
I'll flag such simple questions if it's the OP's first post... but oharag has quite a few in the appropriate forums (I always check before RFing), so I doubt I'm the guilty party.
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
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
If you have any question about the validity of the question on an electrical circuit one might want to direct the OP to this site.
- Thread starter
- #12
Ok now we are getting somewhere.
@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.
@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.
- Thread starter
- #13
Maybe this will answer my question:
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
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
MacGyverS2000
Electrical
You double wattage handling capability, not resistance.
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:
Dan - Owner
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:
Dan - Owner
MacGyverS2000
Electrical
Doesn't matter what end of the chain it goes on... you need voltage dropped, it does the job wherever it may be placed.
Dan - Owner
Dan - Owner
You should not do it the way you're planning.
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 -
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 -
PHovnanian
Electrical
Back to the original topic: I replied to a post regarding a Google translation of a Russian vendor's web site. The poster (probably Russian) was searching for the 'English name' of the piece of equipment. I replied and included a link to an equivalent American piece of equipment, which may have been interpreted as shilling for that company. The whole thread promptly disappeared.
They take those rules seriously.
They take those rules seriously.
PHov; If you think a post like that might be seen as a shill-ing, you can state, "I have no association with this company."
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 -
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 -
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