It’s not that I’m lazy, it’s just just I can either design my PCB to have 12 resistors connecting to 12 LEDs (2 sets of 6), or 2 resistors connecting to each set. Fewer componets means less prone to error in my situation where soldering will be manual.
The LEDs belonging to each set are identical. Is there any reason why I still need a resistor for every LED?
No you do not require a resistor for each LED, but you will have to size the resistor accordingly. If the led’s all come on at one time…the resistor will be different than if they come on separately.
Here is a calculator to figure the resistor sizes.
http://www.quickar.com/bestledcalc.php?session
Hope this helps,
James L
propellanttech:
If the led’s all come on at one time…the resistor will be different than if they come on separately.
I didn’t think about that. They will go on and off independently, so I guess that would mean I’ll have to use 12 resistors. Just hoping.
James,
Are you sure that you can do that? I thought LEDs were the sort of devices that have a lower resistance as they heat up. If so, then if one LED gets slightly hotter than the others, it’s resistance will get slightly lower, which means that in a parallel circuit it will start drawing slightly more current, which will make it get even hotter, resulting in even more current, and pretty soon it overheats and burns out. Now all the other LEDs have a higher voltage drop, which means they have more current, which means more heat, which means they all eventually end up going into thermal runaway.
I’ve always understood that you do need a separate resistor for each LED. But maybe I’ve understood wrong?
Keith
You can reduce assembly time and PCB space a little by using SIP resistors.
Khearn,
I do not think leds change their amperage based on heat. Also…the resistor will limit the current. Hence the reason we put it there.
If leds couldn’t go together in parallel…the calculator that I linked to would not have it on the page. At least I would think a person would omit something that could cause problems…but this is based on the way I think.
I may be wrong about this…but I’ve seen a lot of circuits built with parallel leds.
James L
oPossum:
You can reduce assembly time and PCB space a little by using SIP resistors.
Yes, that does look like a time saver. Not sure if I want to use through hole parts though. And I can’t really find a great variety at Mouser or Digikey. Oh well, no big deal. Maybe eventually I’ll be fortunate enough to have a shop do it.
propellanttech:
Khearn,I do not think leds change their amperage based on heat. Also…the resistor will limit the current. Hence the reason we put it there.
If leds couldn’t go together in parallel…the calculator that I linked to would not have it on the page. At least I would think a person would omit something that could cause problems…but this is based on the way I think.
I may be wrong about this…but I’ve seen a lot of circuits built with parallel leds.
James L
The resistor limits the total current, but not the current to any individual LED. The total current passed by the resistor is far beyond what any one LED can handle. The resistor basically says to the LEDs, “Here’s a bunch of current guys, you decide how to split it up.” As long as they split it equally, everything is fine. But if one starts drawing more than the others, the resistor isn’t going to keep things even.
As for the calculator… Well, just because someone knows how to set up a web calculator, that doesn’t mean he knows whether or not LEDs should be used in parallel with a common resistor.
I just did a little web searching, and mostly I’m seeing advice not to use them in parallel with a common resistor.
http://www.kpsec.freeuk.com/components/led.htm says that you shouldn’t do it.
Here’s another link that says it’s a bad idea, and he provides more links to others that say not to do it. http://wolfstone.halloweenhost.com/Ligh … InParallel
He also says “Of course, this kind of thing works often enough that you can often get away with it. But why not buy a couple more resistors and do it the right way?”
So it looks like it can work in some situations, but probably isn’t a good idea.
Keith
Keith,
Well…we learn something new everyday.
Thanks for the sites…it was infomative.
James L
No problem. I wasn’t 100% sure, either.
Keith
djohnson:
It’s not that I’m lazy, it’s just just I can either design my PCB to have 12 resistors connecting to 12 LEDs (2 sets of 6), or 2 resistors connecting to each set. Fewer componets means less prone to error in my situation where soldering will be manual.The LEDs belonging to each set are identical. Is there any reason why I still need a resistor for every LED?
Greetings djohnson et al.,
I’m here a little late - I see others have weighed in on the topic, which is a good one.
An LED is a diode (duh…) and has a non-linear impedance (resistance) depending upon the applied voltage. If the voltage is too low no current flows (expect some very small leakage current). As the voltage rises the current starts to flow - we’re on the knee of the diode’s VI curve. With little increase in voltage the LED (diode) draws ever greater current until the device is destroyed by self-heating.
The ballast resistor is used to limit the maximum current and prevent damage (or to set a lower current when less light is required for a given application). A particular diode has a knee voltage that varies with temperature, manufacturing tolerance (no two LEDs are the same), and LED construction (different colour LEDs have quite different voltages). Here’s a typical [high brighness LED data sheet.
LEDs have a negative temperature coefficient, meaning forward voltage decreases by about 20 mV for every 10° C rise in junction temperature. As noted by someone else, this could lead to a runaway failure of the LED.
Here’s an [LED app note on the topic.
If several LEDs are operated at the same time they may be placed in series - the same current flows through each one. The voltage across the string is the sum of the individuals and a single ballast resistor can be used to make up the difference from the supply voltage to the total of the LED voltages. Some LEDs in the string may be brighter for the given current (great effort is made for LED video screens to blance out the variations - sometimes you may see blotches of brightness variation or even a fixed rectangle that is out of adjustment).
If multiple LEDs are driven by DC and remain on at the same time, then a series string has the fewest components.
If multiple LEDs are not on at the same time, but driven by DC (via switches for example) they should have a separate resistor each.
A special case is if the LEDs are strobed (Multiplexed or MUX’d) above visual persistence (above 15 to 20 Hz) then a single resistor may serve mulitple LEDs. In this case each LED may be on for it’s time-slot, and the resistor is in play for just that LED. The LED will be off most of the time while the MUX driver services the other LEDs on the common resistor, so the current will fall to 1/n th the DC value. A smaller resistor is required to boost the LED current and bring up the brightness.
For example, if eight LEDs are MUX’d together, each LED is on for a max of 1/8th the time, so eight times the current must pass, and the resistor value is now 1/8th that required for DC operation of the same brightness. There is a practical limit to how many LEDs can be MUX’d as the on current must not exceed the abs. max. current for a given LED type.
In general modern LEDs are quite tough compared with the first examples - which stopped making light if you touched the leads for too long with a soldering iron.
Comments Welcome!](http://www.optekinc.com/pdf/AppNote228.pdf)](http://www.optekinc.com/pdf/ovfsrac8.pdf)
Peter did a wonderful job explaining the various issues involved. The general rules for parallel, simultaneously-active diodes are:
The last point is so that when one diode enters this positive feedback cycle of increasing current and decreasing resistance, it heats the other diodes. This causes the other diodes to have decreased resistance as well, so they take more of the current. Basically, it forces the diodes to play tug-of-war over the current. This only works if the diodes are similar.
Nobody piped up about putting the LEDs in series. If you have the voltage, this is a way to safely save on resistors, and is more efficient. At least, I’m not aware of any problems with this approach.
[edit] Should have read Peter’s post more thoroughly.
On page three is a creative way to drive way too many LEDs with 8 I/O:
Also, if brightness isn’t really an issue, ie, the LEDs are just status LEDs, you can get by with using a lower value resistor when in parallel.
For instance, lets say you have your 12 LEDs. You know that there will always be at least 5 on at a time. You can use a resistor which will produce under the minimum current draw of 5 LEDs all on at once, thus protecting from thermal runaway, and still providing enough current to light up the maximum number of LEDs that will ever be on, even though they will be significantly dimmer.
This is just an example, and the above values may not be the best, but I think I got the point across.
Something like this really does depend on the application and tolerances of the components themselves.
Good luck!
-Nate
Thanks everyone for the help. I didn’t realize this was such a deep question. I probably will go with the traditional approach for now of having one resistor per led. May be more creative later. I’m having enough trouble just completing the PCB. Thanks again.