The answer depends critically on what you meant by "in theory".

A battery could mean an ideal voltage source, or an ideal voltage source + internal resistance; A LED could be modelled as an ideal voltage source, or an ideal voltage source + dynamic resistance (which may be non-linear).

So a simplistic model for a battery + led + current limiting resistor would have a current of

I = (Vb - Vfwd) / (Rb + Rled + R), whereby Vb is the battery's voltage, Vfwd is the LED's voltage drop, Rb is the battery's internal resistance, Rled is the led's dynamic resistance and R is the resistance of the current limiting resistor.

As R -> zero, I -> (Vb - Vfwd) / (Rb + Rled). Obviously, it does not go to infinity if Rb + Rled is non-zero.

But here lies the problem with not using a current limiting resistor: if Rb + Rled is small (if for example it is powered by a set of rechargeable battery, or Li-on battery, or a regulated power source (Rb -> 0)), any small changes in Vb - Vfwd can cause large changes in I;

Worst yet, as the diode heats up, Vfwd decreases, causing I to go up further -> more heat -> a vicious cycle.

Two ways to control that:

1) A large Rb + Rled + R is a negative feedback mechanism in that case. Or
2) a negative feedback loop where you lower Vb as I goes up (or Vfwd goes down).

The 1st approach is typically used for signaling LEDs - simple but inefficient - and the 2nd approach is typically used for power LEDs.

Calculations are unimportant. The results depend on different factors of the LED and the battery which can't be known until you do it and measure the result.

 

Just for grins, years ago, I hooked up a red led to a lab power supply with voltage and current control. I opened up the current limit pretty wide, and started to raise the voltage. While doing that, I could measure the actual current on the supply. No current limiting resister. I was surprised to find that the LED was still working, and very brightly, up to about 12 volts. Raising the voltage even higher, the LED began to deteriorate. But it didn't just burn up like a fuse, it took a long time. Amazing how much abuse a red led could take before it quit working.
Robert, everything you need to know to answer your KVL questions is in post #13. What basic KVL doesn't deal with, however, is the effects of heat on active and passive components. That's advanced KVL. Heat changes the electrical properties of things. Just for grins, go into the lab and measure the internal resistance of a battery. Now set up to run that battery at full charge and measure again as the battery heats up. For extra credit, measure the internal resistance of the diode at normal operating current, and then at 1 Amp. Then solve your KVLs.

A great lesson it is for the classroom.
Destructive parts education. How parts will change during failure? Telling what a stressed part looks like with a meter.

 

I have been running a 4.5v AAA powered 12 white leds torch for a few years from a mains power supply at 4.5VDC/ 350mA as a garden light. I have set up three of them around the garden and they work fine. They are not lit continuously. I did not see any resistors in the torch so it appears they are wired series/parallel.

 

I have been running a 4.5v AAA powered 12 white leds torch for a few years from a mains power supply at 4.5VDC/ 350mA as a garden light. I have set up three of them around the garden and they work fine. They are not lit continuously. I did not see any resistors in the torch so it appears they are wired series/parallel.

Interesting. My flashlight (torch ?) is just 3 AA batteries and four white LEDs in parallel. I have replaced the LEDs before and know they are just simple LEDs, nothing special. I have other "hockey puck" lights that are three AAA batteries and smaller white LEDs. I really don't see what the engineering drama is all about.

 

I'm messing around with adding some LEDs to my car turn signals, and trying to calculate the right resistors, or simply go series with a few LEDs. Problem is the widely varying voltage from the low, when the engine is not running, to high when driving. About 3V variation or more. Anybody have any insights from experience actually doing LEDs on their auto?

 

About 3V variation or more.

A self-regulating LED or one with driver is likely the best for this case.

 

...Anybody have any insights from experience actually doing LEDs on their auto?

Yes, you gotta use current regulation. Trying to run bare LEDs from a car electrical system is folly (stupid!)

 

Hello,

Putting "normal" leds parallel with only one resistor is a bad idea.
Due to forward voltage differences the leds will get different currents if you place the parallel with one resistor.
The led with the most current will fail, the current in the rest of the leds will rise.
The next led with the most current will fail ........
For "normal" leds each led should have its own series resistor.

Bertus

 

I'm messing around with adding some LEDs to my car turn signals, and trying to calculate the right resistors, or simply go series with a few LEDs. Problem is the widely varying voltage from the low, when the engine is not running, to high when driving. About 3V variation or more. Anybody have any insights from experience actually doing LEDs on their auto?

For your application, you should stick with commercially available replacements for incandescent bulbs. They will, hopefully, have been designed to be robust and you avoid liability by using a DOT approved light.

That being said, why are you concerned with brightness variation when the engine isn't running? Turn indicators are only meaningful when the engine is running.

Any advice you've read in this thread about operating LEDs without some form of current limiting/regulation should be ignored for most applications. In automotive applications, there will be sufficient current to destroy LEDs in any poorly designed circuits.

 

So then, I should use 13.8V as my voltage value, and not series 2 LEDs into a single resistor? This is for my own car, so DOT means absolutely nothing to me, especially since these are supplemental to the poor OEM arrangement that obviously did meet DOT requirements.

 

True, just LEDs with no current limiting is a bad design. My flashlight is a bad design. Not all LEDs are just LEDs. Many have built in circuitry for current limiting. Some have circuitry to blink the LED or flash in multi-colors. Some are designed to work at 5 V or 12 V. No resistor required.

 

This is for my own car, so DOT means absolutely nothing to me, especially since these are supplemental to the poor OEM arrangement that obviously did meet DOT requirements.

If you want your car to be street legal, you need to use DOT approved lamps or LED replacements. If you don't, you could be responsible for any liability caused by your design.

 

This is for my own car, so DOT means absolutely nothing to me

This is from betterautomotivelighting.com


This was specifically for headlights, but it applies to any lights on a vehicle. DOT also regulates brightness and the specific colors used. Now forget about DIY and buy an approved aftermarket LED for your vehicle.

 

This is from betterautomotivelighting.com
View attachment 97736

This was specifically for headlights, but it applies to any lights on a vehicle. DOT also regulates brightness and the specific colors used. Now forget about DIY and buy an approved aftermarket LED for your vehicle.

Re: "This was specifically for headlights, but it applies to any lights on a vehicle. DOT also regulates brightness and the specific colors used. Now forget about DIY and buy an approved aftermarket LED for your vehicle."

You can't go wrong giving good advice. Yes, this applies to headlights, turning signals, running lights and such. Does it apply to lighting for aesthetic purposes, dashboard lights or convenience lights?

 

Does it apply to lighting for aesthetic purposes, dashboard lights or convenience lights?

You're in a gray area of the law. Say for instance, you mounted an LED laser inside of your vehicle for decoration and that laser happened to shine into the eyes of another driver and cause an accident. What do you think the NHTSA or state DOT would do if it became known that a consumer modification caused the accident?

 

You're in a gray area of the law. Say for instance, you mounted an LED laser inside of your vehicle for decoration and that laser happened to shine into the eyes of another driver and cause an accident. What do you think the NHTSA or state DOT would do if it became known that a consumer modification caused the accident?

Your exception is correct, of course. I guess the question is what type of lights is the OP considering.

 

Your exception is correct, of course. I guess the question is what type of lights is the OP considering.

In post #45, he said turn signals.

 

In post #45, he said turn signals.

In that case the conservative answer is the best.

 

Again this is supplemental. But, since someone has chosen to drag this thread off into the land of bureaucracy and legispeak, I have absolutely no regard for DOT in my own designs for my own use. Would you like my address and phone number to send them to? I am a technician and tradesman; the laws that concern me when dealing with electrical circuitry are those of physics, and ethics/morals. Bureaucrats struggle to understand and describe sound practices, and typically fail. When they succeed, my work will not suffer. I see this all the time. Just the simple facts.

 

But, since someone has chosen to drag this thread off into the land of bureaucracy and legispeak, I have absolutely no regard for DOT in my own designs for my own use.

Laws are intended to protect the rights of the many from the stupidity of the few. If you don't like the laws; work to have them changed or leave. The choice is yours.