UV LED

[High4Volts]

what would i need to power this led from a 12vdc? the led info is here----
5mm, T 1-3/4 Water clear lens
Operates on 3.7 - 4 VDC @ 20mA
700 MCD luminosity
InGan Source Material
395NM Peak Emission Wave Length <p>This will be a project for my truck. Does anyone have a schematic for this? If more info is needed let me know. Thanks for any help.

[dyarker]

510 Ohm, 1/2W resistor will do the job.

[High4Volts]

that would be in series dale?

[jwax]

12-4=8 volts drop @ .02 amp looks like 400 ohms to me. (Use a 390?) 1/2 watt. In series.

[jwax]

Dale- are you figuring an actual 14 volt supply? Then, yes I agree- 510 ohms.
Sorry.

[dyarker]

Yes, I should have said 14V for when truck is running. Can actually be like 14.5V at the battery, but I used minimum LED drop of 3.7V.

[High4Volts]

ok guys edumacate me. lol I guess i dabble more with AC than DC.

[High4Volts]

ah ok now it's making sense. guess i'm going to have to order that intro to electronics course soon after all. lol

[dyarker]

A 12V lead-acid battery fully charged, but has rested since the charging was stopped will actually be about 12.2V to 12.4V.<p>To recharge the battery after use, the voltage must be greater than the battery voltage. The battery has internal resistance to overcome, but mostly it takes more voltage to split the molecules that formed during discharge than voltage created by atoms combining. (I don't remember the chemistry.) The recharge voltage is 14.5V to 14.9V.<p>When the engine is running, the voltage regulator senses the battery voltage and adjusts the alternator field winding current to keep voltage at around 14.5V. On a truck I had 30 years ago the regulator was 3 resistors and 2 relays. I would hope all modern regulators are solid-state.<p>----------------------------
To calculate the resistor needed for an LED, decide the maximum current you want at the highest supply voltage. This might come out a little different than what I said before. Let's see.<p>The max current for that LED it probably 30mA. We want the LED to last a long time, so lets say our first guess for max design current is 22mA. The voltage across the the resistor is the highest supply voltage minus the lowest LED drop.
14.6V - 3.7V = 10.9V
R = 10.9V / 0.022A = 495.45 Ohms
A close standard value is 510 Ohms.
10.9V / 510 Ohms = 0.02137A (21.3mA)<p>Now check to see if its bright enough with lowest expected supply voltage and highest LED drop. You didn't say, so I'll assume 13V with engine at idle. If this LED is powered while engine is off, recalculate with like 11.5V.
13V - 4V = 9V
9V / 510 Ohms = 0.01764A (17.6mA) If this is bright enough we're done.<p>If not, change the resistor to next lower standard value of 470 Ohms. Go back up and check that max current is still well below 30mA.<p>To figure the resistor power rating:
P = 10.9V * 0.02137A = 0.233W
Resistors come in 1/8W, 1/4W, 1/2W, 1W, etc.
0.233W is 93% of 1/4W so use 1/2W. A rule of thumb I was taught, said next larger size if over 50%. 60% to 70% is okay with low ambient temperature and good ventilation. In cars and trucks the larger resistor stands the vibration better.<p>Cheers,

[EngineHog]

I have a question about the UV LED's-
Do they emit visible light? what is the intensity relative to a blacklight?

[Chris Smith]

http://ledmuseum.home.att.net/leduv.htm

[jollyrgr]

EngineHog,<p>Yes, the UV LEDs I have seen emit a visible light. They are slightly white/blue light and are more visible than a fluorescent tube. They work pretty well. I have a keychain light that is meant for checking US currency for counterfit bills.<p>When hooking up an LED the resistor is in series.<p>Don't mess with the incandescent "blacklights". They do not work very well.

[cato]

Great Page Chris. Thanks for posting the link!