Do I really need a curent limiting resistor

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spinnaker

Joined Oct 29, 2009
7,815
I have a project in mind where I plan to switch some of those 1W LEDs with a logic level MOSFET. One LED per MOSFET.

My supply will be 5V. Forward voltage of the LED is 3.1V @ 350ma. If I did my calculation correctly that gives me a current limiting resistor of only 5.6 ohms. A little more than a half watt so I'd probably need to go to a watt watt.

With those smaller LEDs I would probably risk no current limiting resistor for a project that doesn't need to last forever and is just a fun project anyway. But I have not worked with the higher power LEDs before.

Since this is just a fun project and don't care if it lasts forever can I get by with just leaving the 5.6ohm out as a current limiter?
 

danadak

Joined Mar 10, 2018
3,573
If burning out parts not a problem, go for it. But not a safe practice,
for parts or people.

Otherwise limit drive to specs.

Regards, Dana.
 

WBahn

Joined Mar 31, 2012
24,684
How much current can that 5 V supply deliver, because that LED is probably going to take all of it. It is going heat up quickly, which will drop the Vf significantly and result in even more current flowing. Now, LEDs typically have a strong ohmic-like characteristic so it won't go exponential, but it will likely go high enough so as to burn out the LED pretty quickly (again, unless the voltage supply just can't put out enough to destroy it).

You might at least consider putting a couple of diodes in series with the LED.
 

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spinnaker

Joined Oct 29, 2009
7,815
How much current can that 5 V supply deliver, because that LED is probably going to take all of it. It is going heat up quickly, which will drop the Vf significantly and result in even more current flowing. Now, LEDs typically have a strong ohmic-like characteristic so it won't go exponential, but it will likely go high enough so as to burn out the LED pretty quickly (again, unless the voltage supply just can't put out enough to destroy it).


You might at least consider putting a couple of diodes in series with the LED.
I did not think about the what the supply can deliver. I will be using a wallwart as I typically use for projects. I have a tone of them so I will need to check around for a lower current supply. Plus the same supply might be driving a motor controller and hard drive motor so would that not also help to limit the available current?

I did consider using a couple of diodes to drop the supply voltage.
 

WBahn

Joined Mar 31, 2012
24,684
I did not think about the what the supply can deliver. I will be using a wallwart as I typically use for projects. I have a tone of them so I will need to check around for a lower current supply. Plus the same supply might be driving a motor controller and hard drive motor so would that not also help to limit the available current?

I did consider using a couple of diodes to drop the supply voltage.
If you are using the same supply to power other things, then don't be surprised when the LED drags the supply voltage down to the point where it can't properly drive the motor controller or the motor.

If you REALLY want to go that route, about the only thing you can do is try it. You might get lucky enough to get away with it.
 

DickCappels

Joined Aug 21, 2008
5,876
I would not encourage this kind of design, but...

Decide what power you want to run your LED at then get the datasheet look at the I vs V curve. Most of them have this. Fro the curve you can figure how dead you LED will soon become :)

Alternatively connect the LED through a resistor to a variable power and creep up the voltage (and current) and see if the LED gets too much power for the degree of reliability you desire <= if you don't have the datasheet, maybe you can monitor the temperature failing that, just guess.
 

WBahn

Joined Mar 31, 2012
24,684
I would not encourage this kind of design, but...

Decide what power you want to run your LED at then get the datasheet look at the I vs V curve. Most of them have this. Fro the curve you can figure how dead you LED will soon become :)
And if you can find a decent datasheet, keep in mind that most of the curves assume a constant temperature. Be sure to look at them with the knowledge that this LED is going to get pretty hot.
 

ebp

Joined Feb 8, 2018
2,332
I killed a moderate power LED once by being careless. I was using a bench power supply with the current limit set to what the LED required but had the voltage set too high, though not by much - which is not a problem as long as everything is connected when the power is first applied. I opened then reconnected the connection to the LED with the supply turned on. Of course the output caps in the supply charged to the voltage setpoint of the supply, then discharged into the LED before the current limit could do anything. Dead LED, face red.
 

ArakelTheDragon

Joined Nov 18, 2016
1,265
I have a project in mind where I plan to switch some of those 1W LEDs with a logic level MOSFET. One LED per MOSFET.

My supply will be 5V. Forward voltage of the LED is 3.1V @ 350ma. If I did my calculation correctly that gives me a current limiting resistor of only 5.6 ohms. A little more than a half watt so I'd probably need to go to a watt watt.

With those smaller LEDs I would probably risk no current limiting resistor for a project that doesn't need to last forever and is just a fun project anyway. But I have not worked with the higher power LEDs before.

Since this is just a fun project and don't care if it lasts forever can I get by with just leaving the 5.6ohm out as a current limiter?
Let us explain properly.

This is why we hate the programmers approach(at least me): yes it will work, it might even last for long, but at 1 point it will burn and start causing problems. Its true that if you do not know why, it LOOKS like its working, but it will become a serious problem at one point.
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,815
I think I am going to go with a 3.3V supply if I can get away with having a low enough actual forward voltage. That would give me a 1 ohm and only take 70 mw on the resistor. I need to see if I have some 3.3V regulators around here.
 

WBahn

Joined Mar 31, 2012
24,684
I have no idea what LED you are using, so I just grabbed the first 1W LED datasheet I could find:

http://www.farnell.com/datasheets/1636581.pdf

Notice that once it starts conducting at about 3 V, it has a very ohmic characteristic of about 900 mA/V. So you go and put an extra 2 V across it and you are looking at something in the range of 2 A of current (and 10 W of power dissipation). Unfortunately, they don't show how much that curve shifts with temperature.
 

MrSoftware

Joined Oct 29, 2013
1,556
If code is easier for you than hardware, and you're using a processor to control the LEDs, then you can use PWM to limit the rms current to the LED. Assuming your processor is capable of PWM on the outputs that you're using.
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,815
If code is easier for you than hardware, and you're using a processor to control the LEDs, then you can use PWM to limit the rms current to the LED. Assuming your processor is capable of PWM on the outputs that you're using.

I am using a Pic for the project. That is an interesting idea. And I think I already have it designed in. ;) What I am building is a POV clock from an old hard drive. I have 8 compartments where there will be LEDs inside. I have a disc where I have 0-9, the : character and the / character etched into them. The disc spins and the correct LED lights when the correct character is over the correct compartment. So I pretty much have PWM already built into the design. ;)
 

ebp

Joined Feb 8, 2018
2,332
PWM does not work well for this sort of thing. When the switch is ON, the current will still be very high, unpredictable and vary with temperature. What would work moderately well is PWM with a lower resistance than you would use for always-on operation - perhaps 2 or 3 ohms.

I don't quite follow the intent in terms of lighting the LEDs, but if only one LED is on at a time then they can all share the same current limiting resistor. e.g. all the anodes are tied together and through a common resistor to +5 V, each cathode is switched to ground to turn the LED on. No harm would result from multiple LEDs having their cathodes grounded at the same time, the LEDs would just share the current to a somewhat unpredictable degree.

As long as the average power is below the resistors's rating, it won't care if the peak power is higher as long as it isn't too much higher. What constitutes "too much" depends on the type. Metal oxide film are fairly tolerant, wire wound are very tolerant. Other film types are more fussy.
 

crutschow

Joined Mar 14, 2008
23,304
you can use PWM to limit the rms current to the LED.
That can limit the average current through the LED but not the peak current.
That will still be determined by the supply voltage and circuit/LED resistance, so you typically still should add some series resistance to limit that peak.
 

nsaspook

Joined Aug 27, 2009
6,264
I am using a Pic for the project. That is an interesting idea. And I think I already have it designed in. ;) What I am building is a POV clock from an old hard drive. I have 8 compartments where there will be LEDs inside. I have a disc where I have 0-9, the : character and the / character etched into them. The disc spins and the correct LED lights when the correct character is over the correct compartment. So I pretty much have PWM already built into the design. ;)
You have variable pulse positioning from timing marks unless you also vary the width of the strobe pulse at each compartment. While that's possible, it tends to blur the strobe image if too long and dim the image if too short if used instead of a optimized LED current for the strobe width.

From my simple POV demo of a single strobe LED. Top trace timing pulses, bottom trace time varying strobe pulses to timing mark.

Four strobe line demo.

I used this RGBA led with current limiting resistors to keep the peak forward current below the Max rating.
https://github.com/nsaspook/hd_pov/blob/xc8/ov4zrgba-1210202.pdf
https://github.com/nsaspook/hd_pov/tree/xc8
 
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WBahn

Joined Mar 31, 2012
24,684
If code is easier for you than hardware, and you're using a processor to control the LEDs, then you can use PWM to limit the rms current to the LED. Assuming your processor is capable of PWM on the outputs that you're using.
This can still leave you open to problems. First, the peak current itself can be too high and significantly shorten the life of the LED via mechanisms other than heat. Second, the high peak current draw can drag down the supply rails enough to cause problems throughout the circuit. So you would likely end up strengthening the supply rails to reduce the second problem but, in doing so, only make the first problem worse.

IF it can be determined that the peak current is tolerable as a result of the largely ohmic response, then for a low-importance hobby project it might be okay. But it is poor design, especially since the parts are almost certainly not well enough spec'ed to back up the approach
 

BobaMosfet

Joined Jul 1, 2009
782
I have a project in mind where I plan to switch some of those 1W LEDs with a logic level MOSFET. One LED per MOSFET.

My supply will be 5V. Forward voltage of the LED is 3.1V @ 350ma. If I did my calculation correctly that gives me a current limiting resistor of only 5.6 ohms. A little more than a half watt so I'd probably need to go to a watt watt.

With those smaller LEDs I would probably risk no current limiting resistor for a project that doesn't need to last forever and is just a fun project anyway. But I have not worked with the higher power LEDs before.

Since this is just a fun project and don't care if it lasts forever can I get by with just leaving the 5.6ohm out as a current limiter?
Let's rephrase this question another way? Does my car really need brakes?

This isn't let's pick-n-choose which parts we use. This is electronics. It is an engineering science. There are rules you follow, for a good reason. Not following them is well....
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,815
Let's rephrase this question another way? Does my car really need brakes?

This isn't let's pick-n-choose which parts we use. This is electronics. It is an engineering science. There are rules you follow, for a good reason. Not following them is well....

All right enough already. I get it. I am fully aware of the purpose of a current limiting resistor. I was just wondering if I might be able to get way without it in this case.
 
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