raschemmel:
they should not be in series because if one fails it will most likely be an open circuit.
Yes, this is a potential problem, just like the proverbial string of Christmas lights that goes dark when one burns out. But it's countered by the benefit of requiring much less current than a parallel circuit, and is much less of a problem than the proverbial Christmas lights because an LED has a much longer lifespan than an incandescent bulb. A failure is relatively rare.
If leds share a current limiting resistor, when one fails the current increases through the others . If another fails , the current increases again. Eventually it could lead to a cascade catastrophic failure.
This will only happen if you have parallel LEDs that all share a single current limiting resistor. And that is exactly why such a configuration is NOT recommended. This is not a concern with a series string of LEDs, since a failure is likely to cause an open circuit which will stop the current and prevent a spectacular cascading failure.
There is a slight chance that an LED could fail with a short, so there is a chance that a cascading failure could happen with a series string: if an LED were to short, it's forward voltage would be taken out of the string, increasing the current through the string. That may stress the remaining LEDs and cause another failure, but the odds are that the next one will fail open, breaking the chain.
For absolute reliability, a parallel orientation with a current limit resistor for each LED will allow the others to remain on when one LED fails. The cost for this is greatly increased power consumption, and increased cost in parts and complexity. The extra parts and connection points actually decreases the mean time between failures, making the overall system less reliable, but more tolerant of a partial failure.
There are trade-offs with everything. I seriously doubt that this is an application that warrants the concern about losing a string of LEDs. Power savings and simplicity is likely a better goal.
ShapeShifter:
There are trade-offs with everything. I seriously doubt that this is an application that warrants the concern about losing a string of LEDs. Power savings and simplicity is likely a better goal.
Sounds reasonable. Most 7 segment displays consist of a string of LEDS in series for each segment. All sharing one common input that requires one current limiting resistor for that string. It seems pretty reliable.
My comment was that I am pretty sure raschemmel was - for whatever reason - somewhat addled when he posted this, and might wish to come back and correct it.
My comment was that I am pretty sure raschemmel was - for whatever reason - somewhat addled when he posted this, and might wish to come back and correct it.
What comment ? (the one about why do we have to repeat this dialog?) (maybe I should have asked "why do people always want to put leds in series ?)
This will only happen if you have parallel LEDs that all share a single current limiting resistor. And that is exactly why such a configuration is NOT recommended
Here is the entire paragraph:
You can parallel leds that each have a series current limiting resistor . (as many as you want) but they should not be in series because if one fails it will most likely be an open circuit.
If leds share a current limiting resistor, when one fails the current increases through the others . If another fails , the current increases again. Eventually it could lead to a cascade catastrophic failure.
If leds share a current limiting resistor
was a reference to the parallel led configuration.
I thought that would be obvious.
Since we are still beating the dead horse, we might want to ask if the leds are going to be placed in a display that prohibits replacing a bad led.
Paul__B:
Hang on! If you are going to use 12V, it makes sense to use series chains of LEDs; three or four. Now if you have five LEDs per channel, you do need two chains, and to control and match the current most precisely, you want the resistor to take as much of the voltage as possible and as similar as possible in both chains, so you would (as ShapeShifter explained) use three and two instead of four and one.
raschemmel:
Leds should not be operated in series for several reasons.
You can parallel leds that each have a series current limiting resistor . (as many as you want) but they should not be in series because if one fails it will most likely be an open circuit.
If leds share a current limiting resistor, when one fails the current increases through the others . If another fails , the current increases again. Eventually it could lead to a cascade catastrophic failure.
Okay. I have decided to go with series. To make all the LEDs in each group as equal as possible, I used 3 an 2 LEDs per line setup. Then I used a combination of resistors to get the resistance within 6 ohms of the exact calculations. I attached a new diagram to this post.
ShapeShifter:
Yes, this is a potential problem, just like the proverbial string of Christmas lights that goes dark when one burns out. But it's countered by the benefit of requiring much less current than a parallel circuit, and is much less of a problem than the proverbial Christmas lights because an LED has a much longer lifespan than an incandescent bulb. A failure is relatively rare.
This will only happen if you have parallel LEDs that all share a single current limiting resistor. And that is exactly why such a configuration is NOT recommended. This is not a concern with a series string of LEDs, since a failure is likely to cause an open circuit which will stop the current and prevent a spectacular cascading failure.
There is a slight chance that an LED could fail with a short, so there is a chance that a cascading failure could happen with a series string: if an LED were to short, it's forward voltage would be taken out of the string, increasing the current through the string. That may stress the remaining LEDs and cause another failure, but the odds are that the next one will fail open, breaking the chain.
For absolute reliability, a parallel orientation with a current limit resistor for each LED will allow the others to remain on when one LED fails. The cost for this is greatly increased power consumption, and increased cost in parts and complexity. The extra parts and connection points actually decreases the mean time between failures, making the overall system less reliable, but more tolerant of a partial failure.
There are trade-offs with everything. I seriously doubt that this is an application that warrants the concern about losing a string of LEDs. Power savings and simplicity is likely a better goal.
I chose to go with series just because of the risks of parallel. Series I find is easier anyways. Diagram attached
Leave no stone unturned Award ? (we did after all help the OP to get his solution , right ?) (although it would have been nice to see the GND symbols on the left pointing downward but I guess we can't be to picky at this point)
raschemmel:
Leave no stone unturned Award ? (we did after all help the OP to get his solution , right ?) (although it would have been nice to see the GND symbols on the left pointing downward but I guess we can't be to picky at this point)
Ground symbols should always point down?
I'll keep that in mind for later times.
So because this is my first time with something like this, I attached a diagram that has the ground symbols removed, and there just wires going to negative. Yes, I know it is not to "diagram code", but it's just for my understanding!
So let me try to explain this with words!
The power supply of 12 volts positive wire(unregulated) goes to the resistors to go to the LEDs. The LEDs are in paris of 2 and 3 to avoid reaching a low voltage. The positive then comes out the last LED and goes into the collector of the transistor. With power, this does nothing by itself, because the transistor breaks the circuit.
Next up, a negative and positive wire come out of the power supply, and go through a regulator to produce 5 volts. The positive 5 volts(regulated) goes to the Vcc pin of the ATMega chip. The negative 5 volts(regulated) goes to the ground pin of the ATMega chip. This connects the chip to ground. Three wires from the chip go through a resistor and into the base of the transistor.
Both circuits are now setup and ready to run. With power on, the LEDs will not light at all, but the chip is on. Once the chip receives the instruction that one of the color of LEDs need to be turned on, it will send a current through one of the three wires(complying with the LED color) that goes through a resistor(to not overload the transistor) and then to the base of the transistor. The transistor then completes the LED circuit, turning on the line of LEDs by allowing the positive to run to the negative(ground).
HeyAwesomePeople:
Then I used a combination of resistors to get the resistance within 6 ohms of the exact calculations. I attached a new diagram to this post.
No! That was the point of my discussion (in reply #74). You will simply not be able to distinguish a 20% variation in LED brightness (current). Even if you did, component variation would require you to "cut and try" values to trim it. And I am not even sure you really know what the LED voltage drops will be as the datasheet gives an estimate only. But there is no need whatsoever to do so. Use the single resistor closest to what you calculate.
HeyAwesomePeople:
Sound about right?
Sorry, TL;DR. It is the diagram that means something to me. The present version, minus the ridiculous "trim" resistors, looks pretty good to me.
Paul__B:
No! That was the point of my discussion (in reply #74). You will simply not be able to distinguish a 20% variation in LED brightness (current). Even if you did, component variation would require you to "cut and try" values to trim it. And I am not even sure you really know what the LED voltage drops will be as the datasheet gives an estimate only. But there is no need whatsoever to do so. Use the single resistor closest to what you calculate.
Yeah I know but I am that type of person that has to make everything as perfect as possible. The resistors I need only cost a cent each, not that much of an issue price wise.
I've used LEDs with resistors of a size that gets the current down below 10mA, and I bet you can still see the bloody things from the moon, they're so bright nowadays.
HeyAwesomePeople:
Yeah I know but I am that type of person that has to make everything as perfect as possible. The resistors I need only cost a cent each, not that much of an issue price wise.
In that case, take note of what I was explaining.
If you actually wanted (despite what you are being advised) to match the currents closely, you would have to determine the currents and add appropriate resistors after you constructed the system as you do not know the LED voltage drops in advance.
A useful way to determine the current without disturbing the circuit is to measure the actual voltage across the resistance, and calculate the current according to its specified - or measured - resistance.
This thread is an extremely valuable resource to those of us new to circuitry, such as myself. I do not mean to go off topic, but a huge thank you goes to the OP and everyone who responded for the wonderful information!
In response to the "What is GROUND ?" question I submit it is a name for the common connection of all the returns. The current direction is spoken of as flowing from positive to negative. ( without getting caught up in technicalities). All roads should lead to Rome. ( all the returns (neg ends of loads connected across the common power source). Each load has a "source" and a "return".
In general, any electrical ( or electronic) system has a single main power source ( we are only talking about DC circuits here) or multiple sources ( voltages). If there is a bi-polar circuut such as an op amp amplifier running off +/- 12V , it will still need a "ground" reference for the input and output signals. This is when you would draw a distinction between "negative" and "ground" since the - rail for op amp is not a return but a negative source . So if you said make sure all the returns are connected to a common ground you would not be referring to the negative voltage (-Vee) of the op amp.
The common ground concept would include everything controlled by that uC. If interfacing to another uC system you would use opto- isolators to separate the grounds of one system from the ground of the other uC system. There are times when the opto-isolator barriers are used to separate sensor circuits from the uC system or motor drivers from the uC because the nature of the system being measured makes it difficult if not impossible to have a common ground (for physical reasons). The application of opto-isolators to separate ground is also applied to prevent one electronic circuit from "taking down" the entire system. A failure in one circuit is confined to that circuit and easier to debug.
By encorporating Enable signals sent via opto, you can turn on/off the power for one circuit indepent of the others. Motor controllers almost always have such an enable signal. By adding micro-switches, the system can disable any motor driven mechanics ( for safety reasons) when someone opens the cover.
Paul__B:
In that case, take note of what I was explaining.
If you actually wanted (despite what you are being advised) to match the currents closely, you would have to determine the currents and add appropriate resistors after you constructed the system as you do not know the LED voltage drops in advance.
A useful way to determine the current without disturbing the circuit is to measure the actual voltage across the resistance, and calculate the current according to its specified - or measured - resistance.
Well I don''t have all the resistor sizes just sitting in my room to be able to do that. I would have to order them seperatly after I did the exact calculations. That is also the reason that I stressed so much on getting everything right the first time, or atleast trying to. The place I order parts from takes up to two weeks to arrive at my house, but I use it because it is very cheap. If I had the parts to test in front of me, I would test different resistor sizes correlating to the actual voltage drop. But I do not have that parts.
raschemmel:
In response to the "What is GROUND ?" question I submit
it is a name for the common connection of all the returns. The current direction is spoken of as flowing
from positive to negative. ( without getting caught up in
technicalities). All roads should lead to Rome. ( all the
preturns (neg ends of loads connected across the common power source). Each load has a "source" and a "return". In general, any electrcal ( or lectronic) system
has a single main power source ( ww are only talking abour DC circuits here) or multiple sources ( voltages).
If there is a bi- polar circuut such as an op amp amplifier running off +/- 12V , it will still need a "ground" reference fir the input signal and output signal. This is
when you would draw a distinction between "negative"
and "ground" since the - rail for op amp is not a return but a negative source . So if you said make sure all the
returns are connected to a common ground you would
not be referring to the negative voltage (-Vee). The common ground concept would include everything
controlled by that uC. If interfacing to another uC system
you would use opto- isolators to separate the grounds
of one system from the ground of the other uC system.
That helped clear up some confuson for me(except it took a while to read that. What happened to the formatting?) Thanks
