LED technical info
- Thread starter don9GLC
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I think I am getting this now.... one last dumby question - If you get an LED working at an intensity you like on a stable voltage can you assume it will continue in service like that. Do they either pop or work as opposed to a bulb which may work but only for a short time if the voltage is too high...??
Yes maybe you could throw up a LM317 circuit.... I found them to be a far better solution than resistors.....don9GLC said:
http://www.electronics-lab.com/articles/LM317/
Can I bump this question....
I think I am getting this now.... one last dumby question - If you get an LED working at an intensity you like on a stable voltage can you assume it will continue in service like that. Do they either pop or work as opposed to a bulb which may work but only for a short time if the voltage is too high...??
I think I am getting this now.... one last dumby question - If you get an LED working at an intensity you like on a stable voltage can you assume it will continue in service like that. Do they either pop or work as opposed to a bulb which may work but only for a short time if the voltage is too high...??
I've not tried it myself but I'm sure you could also use a 317 as a current limiter so configuring one for say 20mA is yet another available method.ROSS said:If variation in voltage is the problem then yes...a 317 circuit will answer it.
The advice given I gave above if read properly was for steady lighting of a led with a resistor NOT about LEDs operating with a fluctuating voltage
CoggesRailway said:
As long as you are not exceeding the LEDs designed current limit, then yes, it will continue to work satisfactorily.
They will pop if the current is too high.
Put simply, you will know your max voltage supply, which in our case is no more than 24 V dc.
You must also be aware of the forward CURRENT, ask your supplier if you are not sure!
Your resistor can then be easily calculated:
R = V / I
Example
A 24 Volts track supply divided by a typical 20 mA ( 0.02 A) current for a red LED.
Thus R = 24 V divided by 0.02 A
Your required resistor will be 1.2 Kilo Ohms or the next nearest higher value available.
A higher value will reduce the brightness of your LED if required, so you can adjust to suit....
CoggesRailway said:
Ian - there's no simple yes/no answer to that. It depends on the LED's tolerances for current (milliamps) passing through. Exceed the maximum tolerance and there is indeed a likelihood the LED will be destroyed. It will also get hot. Some simple arithmetic will give you a good guess on how far you can stretch your LED for a given resistor and input voltage.
Best advice is to identify what brightness you need from your LEDs, buy one capable of that brightness together with the correct resistance to match your intended input current.
Short answer: yes.DoctorM said:
Longer answer: yes, so long as you do not exceed the maximum specified difference between input and output voltages. Typically, an LM317 specification is min input 3 volts, max 40 volts with a maximum difference between input and output voltages of 40 volts, so shouldn't be an issue. If you really want chapter and verse, see http://www.fairchildsemi.com/ds/LM/LM317.pdf
I suspect that you are looking at the suggested design for a 1.25V to 25V variable voltage supply. To guarantee getting 25V output from this you would need a minimum of 28V input due to the voltage drop in the 317 chip.DoctorM said:
As far as I'm aware the 317 will work with input voltages between 4.5 V and 40V with the maximum output voltage about 3V less than the input.
Take care with high input output voltage differentials and high currents that the heat sink is sufficient to dissipate the power loss.
Edit,
I see Whatlep replied whilst I was checking the data sheet!
Yes, these will work at any voltage up to 28V however they need I think its around 1.5 volts to work so say you want 3v out you need at least 5.5 v in. Does that make sense?DoctorM said:
It may be, but from my initial tests a -5% variation in voltage produces a 50% drop in luminance, which should be detectable. A 5% drop in current cannot be seen. I have used a current limiter on test, but voltage did not matter so much as current. So I (in theory, because I have not tested it) endorse current regulation.Neil Robinson said:
As someone else advised that may not be a problem with a 'one off' but produce 1,000,000 and sell them, then things have changed.
I'm not trying to say that voltage regulation is wrong, just understand where it is the best solution.
Meanwhile I'm busy testing, and unlike running locos, there IS a limit!
Don
One other solution, for the LED challenged amongst us, is to buy the LEDs with the resistors pre-wired into the circuit. Or buy loose LEDs that come with loose resistors. Check out Ebay. China has a bounty of sellers that supply LEDs with and without resistors included as described above. I have had very good luck with all of the LEDs that I have purchased from these sellers on Ebay.
For lighting buildings, I use the pre-wired LEDs. The wire leads are about eight inches in length which makes it simpler to install in structures. The loose variety, I find, are useful for loco headlights, since I can locate the resistors anywhere in the circuit. Usually inside of the loco body.
For lighting buildings, I use the pre-wired LEDs. The wire leads are about eight inches in length which makes it simpler to install in structures. The loose variety, I find, are useful for loco headlights, since I can locate the resistors anywhere in the circuit. Usually inside of the loco body.
I find that LEDs are very interesting. one point that I have found important to note is what the mfgs not about max voltage and current must be adhered to or you will get a burn out.I get My LEDs from Rapid suplies very efficient company.
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Roger
[h3]Beautiful weather here in Spain 19 c[/h3]
Roger