LED driving question, Why do I need the resistor?

[johnsnd]

So here is the deal. My gut tells me this is wrong and bad but my brain cant seem to figure out why.

I have a 12V power supply that is set slightly less then 12 volts. Say 11.5V. The LED I am driving is MKRAWT-02-0000-0D0HH240H. It runs at 12V, 700mA.

The leds will be driven at the end of a 30ft 18awg wire. So I calculate the wire resistance to be 195mOhm.

During testing I see that the power supply drives only 400-500mA at 11.5V . Also on this same power supply are servo motors.

Now one thought is that the voltage might spike above 12V causing current to surge. So I was considering putting a 12 zener by the LEDs.

Please help me see the error of my ways, or reassure me that this isnt going to blow up. lol

Thanks for your help!
~Nick

 

[betwixt]

You might get away with it but equally you might fry the LED. This is why:

An LED has a steep voltage to current curve, in other words a slight increase in voltage might cause a large increase in current. Current will be low up to a certain voltage then suddenly rise. The wire resistance of 0.195 Ohms is almost insignificant in terms of providing protection so you are not giving yourself any safety margin. Adding a Zener will not help much as it also will pass excessive current if the voltage rises and could also burn out.

Ideally, you would start with a voltage higher than 12V so you can drop a little in a current limiting circuit (which could be nothing more than a resistor) but at the moment you seem to be relying on the current being limited by under-voltage to the LED. As such, any variation in voltage from the PSU or interference from the servos or even wire resistance change could have a bad effect.

Brian.

 

[johnsnd]

So in short it is a perfectly valid solution that requires the house of cards to stay up. Does the zener not provide some risk mitigation?

 

[Kfir Maymon]

i thinking you can use a low pass filter, and you change the value,

i hope i don't worng, let's wait and see what brian think i learn form him a lot.

 

[D.A.(Tony)Stewart]

These 12V 700mA LEDs perform just like a 10W Zener, so a Zener is useless.

The ESR is very predictable on LEDs and the only thing that changes is the threshold drops with rising temp.

At 85'C your response is predicted from the given chart in the specs.



I would highly recommend you mount this on an alum. Substrate or CPU heatsink and set the voltage higher. It should ever get too hot to burn your finger with a good heat-sink, an ideally mounted LED with a CPU fan might allow 1250mA current

BTW, The threshold voltage can be used to instantaneously compute Tj. ON..dim..quick measure by adding a 1A diode in series to drop to 10.5 or similar < 5% of normal current. For 12VLed, -8mV/'C

The resistance of your cable x2 incl. return is about the same as the ESR of the LED. Look at the curve. From 11.5 to 12.0V , current rises from ~530 to 1000mA ,so ESR= ΔV/ΔI=0.5/0.47= 1Ω and your cable adds 0.4Ω (Assuming you only measured 1 path)

I also suggest you put any other heavy loads on separate wires unless you WANT brightness flicker with load.

The best accidental protection from a different supply with overvoltage is a 700mA PTC Polyfuse. But if your supply is reliable , you should not need anything.

 

[andre_luis]

any variation in voltage from the PSU or interference from the servos or even wire resistance change could have a bad effect

I would strengthen this point of view. You need consider the fact that a LED - differently from an incandescent lamp - do not have the same ability to dissipate the exceding energy due to its tiny chip size, being therefore easily susceptible to burn.

 

[Audioguru]

The LED does not use 12V. Most need 11.7V. Some of them need less than 12V but they do not say which ones and do not say how low is their voltage requirement. Others need a voltage as high as 14V. Their voltage requirement drops as they heat up which causes their current to increase if you feed them a constant voltage. The increased current causes more heat which causes more current which causes more heat which causes .... thermal runaway and destruction.

LEDs should be fed a constant current, not a voltage. A series resistor provides almost a constant current.

 

[johnsnd]

BTW, The threshold voltage can be used to instantaneously compute Tj. ON..dim..quick measure by adding a 1A diode in series to drop to 10.5 or similar < 5% of normal current. For 12VLed, -8mV/'C

The resistance of your cable x2 incl. return is about the same as the ESR of the LED. Look at the curve. From 11.5 to 12.0V , current rises from ~530 to 1000mA ,so ESR= ΔV/ΔI=0.5/0.47= 1Ω and your cable adds 0.4Ω (Assuming you only measured 1 path)

The best accidental protection from a different supply with overvoltage is a 700mA PTC Polyfuse. But if your supply is reliable , you should not need anything.

@SunnySkyguy:
Wow clearly you know your stuff, in an effort to better myself and share the knowledge, could you expand on a few of those points.
First the diode in series, by adding a diode in series with the LED we would effectively double the voltage drop (0.7+0.7=1.4V; 12V-1.4V=10.6V) and slightly increase ESR. How do you come up with the 5% drop in current? What is the reference to -8mV/'C ?
To your second comment, and feel free to correct … The Effective Series Resistance (ESR) of the LED decreases by 1ohm. Because at 11V to 11.5V we see .5V/.25A = 2 Ω. What does that tell us? VR = I so 12V * 1.4Ω = 16.8A … something’s not right there … I bet I can’t use ESR in Kirchhoff's law?
After looking into polyfuses, they seem quite sensitive to heat. So mounting these on the aluminum made pcb might be a futile effort.

@AudioGuru:
So you are saying that running it at 11.5 Volts still presents a thermal runaway possibility?

Thanks for all the help!

 

[betwixt]

Gets complicated quickly doesn't it!

Firstly, the low pass filter idea is a non starter. You can't lower in frequency than DC and to accept the concept of filtering implies AC is present which would kill the LED anyway!

LEDs are like most diodes, they are 'constant voltage devices' which means that given current above a low threshold, they try to hold the same voltage across themselves as the current increases. So if you feed 12V to your LED it will have 12V across it and pass the 700mA or so the manufacturer specifies. If you connect 13V across it instead, it will try it's hardest to pull it down to 12V by passing more current. That current may be many times more than 700mA and will damage the LED by fusing it's junction or simply overheating it and melting something.

We normally limit the current through an LED rather than the voltage across it so it find it's own voltage by drawing whatever current it wants. Using the same analogy as above, if the voltage was increased to say 15V and a series resistor was used to limit the current, the LED would set it's own voltage (called Vf) and the remaining 3V would be dropped in the resistor. Because the LED tries to hold the voltage across itself constant, if the voltage applied is changed, the resistor 'takes up the slack' by dropping the remaining voltage across itself.

Now the even more complicated bit: the voltage the LED maintains across itself depends slightly on the current through it (in other words it isn't a perfect constant voltage) and it's junction temperature. When you consider that with 12V and passing 700mA it dissipates around 10W and the temperature will rise quickly unless you can remove the heat somehow. As the temperature rises, the Vf across the diode drops so it tries to draw even more current. Without current limiting that means it will enter a vicious circle and get hotter, in turn drawing more current which makes it even hotter again.

Brian.

 

[Audioguru]

AudioGuru:
So you are saying that running it at 11.5 Volts still presents a thermal runaway possibility?

The LED operates at ABOUT 12V when its current is limited by a series resistor or something. The datasheet says its "typical" voltage is 11.7V at an internal temperature of 85 degrees C, 700mA of current and a pretty good heatsink. Its maximum voltage is 14V so some of them operate at 14V. They don't tell you what the minimum voltage is for some of them. Their operating voltage is not marked.

What if the LED you have actually operates at 10.5V and you feed it 12V? It will explode because its current is not limited by anything.
What if the LED you have actually operates at 14.0V and you feed it 12V? It will not light.

Maybe you are lucky enough to have an LED that operates at the supply voltage you are feeding it. It gets hot and since its operating voltage is reduced by heat (which causes its current to increase) then it will have thermal runaway.

 

[D.A.(Tony)Stewart]

I do ESR from a threshold at 5 or 10% to 100% current for estimating variations due to cable resistance.

Of course 0-50% gives higher ESR and 100-150% gives lower. But ohms law works in the incremental ESR as a linear approx.

When it comes to tolerances, I would say most LEDs follow typical, but they have wide specs to preclude yield loss in case of batch variations in ESR. So you can't guarantee future replacements to be the same, but the ones you HAVE will be predictable in their VI curve.

They can be purchased with tighter Vf bins. But for DIY project this is ok. I run my 12V LEDs at 16V thru 100ft of awg18 to get 12.5 at the end around the fence and garden. I use a selectable V unversal laptop charger 65W .