Using low voltage led on 17 volts

[Gstring]

What would the value of the resistor have to be to address this issue? Thanks

[Peter Snowberg]

When you power an LED using a higher voltage supply and a dropping resistor, it's important to note that the resistor doesn't "cut off" the power flow. The resistor absorbs the excess power.

When you get into higher voltages like 17V, you're better off moving to a "constant current source" which can be made with a single JFET and a resistor. A constant current source will directly limit the current flowing where as the resistor will limit the current indirectly by trying to "balance" where the current is spent.



In this example, the LED would sit inbetween the battery and the drain of the JFET (point I on the diagram).

Google for JFET current source for more info. You'll see them all over the place once you start looking.

There are also "constant current diodes" which are like the JFET with a resistor built in.

[toneman]

but one  resistor will work fine.
figure @ 5V, & regular LED,  need about 330ohm, 1/4watt.
figure @ 10V, & regular LED, need about 680ohm, 1/4watt.
figure @15V, & regular LED, need about 1Kohm, 1/4watt.
4 your 17V, & highbritness LED(lower current),
start with about 1.5K.  If it's too dim, go 2 next value lower until
U achieve the brightness U want, without blowing the LED, of course.
Bsure 2 observe the *polarity* of the LED.
One way won't glow at all....
stayLit
tone

[Paul Perry (Frostwave)]

If the 17v supply is stable, there isn't any advantage using a transistor constant current source, over a (suitable) resistor. THe unwanted 15.5 volts or so (supply minus led voltage) is either dropped across the transistor or the resistor, same losses.
But if you have a variable supply, then the transistor constant current is a Good Idea, you would have constant brightness no matter what the battery was (down to a couple of volts).

[Peter Snowberg]

I heard 17V and thought unregulated and unstable, but I could be far off in which case Paul is right.... 20mA is 20mA is 20mA. Watch your power rails with a meter and see if they're stable at 17V during power up and down. If it's in an amp, check with the standby switch on and off just to be sure.

Ohms law is what will allow you to figure out the resistor value.



Whoooooppps...... not that Ohm, this one....



Let's assume you want to run 10mA through an LED with a Vf of 1.5V.

If the supply is 17V, then the resistor is left to deal with 15.5V and we want the current flowing through the resistor to be 10mA.

Above is a handy Ohms's law chart. You can always find one at Google with the image search. This is a good one... save it for later reference and print it out!

We want to find a resistor so we go to the resistance section... then we look for the equation that we have the values for. In this case V & I (voltage in volts and current in amps). So....

R=V/I

is what we use..... now we add the values in volts and amps....

R=15.5/.01

Now we do the division and we get the results in ohms.

R= 1550 ohms

You see above Toneman suggested 1.5K to start.

Depending on how bright you want it, 10mA might be too much, or just right, or not enough, but you can be sure you won't fry any color of LED at 17V.