Help out a noob

[GForce2010]

Hi guys, i'm quite new to this and wanted som help on an idea i have. I'm ding a project on my car and want to run a few UV LED's but the only ones i can find can only handle 4v. So is there anyway i can hook these up to my 12V feed using some sort of converter.

Sorry if this is a stupid question.

Thanks.

 

[ljcox]

Put a resistor in series.

 

[fingaz]

Ok,

what you need to do is use a resistor in line with each led. This will drop the current to the required level to safely power the led. In order to work out the resistance you will need, you need to know the supply voltage the voltage drop of the led and the current at which you want the led to operate.

ok supply voltage is 12v. voltage drop of led is 4v(approx) and typical operating current for led's is 20mA.

resistor required = supply voltage - voltage drop / current(in amps)

12-4/0.02 = 400 ohm resistor (next closest value is 470 ohm)

so you would need a 470 ohm resistor in series with each led in order to operate them from 12v at a forward current of 17mA(0.017A)

 

[Andy1845c]

Keep in mind a cars voltage will be around 14v when running. I would size your resistor for 14v or use a voltage regulator.

 

[fingaz]

Keep in mind a cars voltage will be around 14v when running. I would size your resistor for 14v or use a voltage regulator.

yeah, I agree.

But even at 14v, resistor would be 500 ohm to run at 20mA. Using 470 ohm would mean the led is only running at 21mA, most uv led's have a maximum rating of 30mA so 21mA isn't going to cause too much of a problem

NOTE: these calculations may not be accurate for your specific LED's. You will need to check on the data sheet, or ask whoever you buy them from for the Voltage Drop, and Operating Current as LED's from different manufacturers can vary.

 

[GForce2010]

Specs.

Forward voltage max. at IF=20mA: 4V
Forward current max.: 10mA
Reverse voltage max.: 4V
Wavelength @ peak: 410nm
Light output min.@ 20mA: 300mcd
Light output typ.@ 20mA: 500mcd

thanks

 

[audioguru]

Stupid, stupid specs.

Its max voltage and light output are rated at 20mA but its max allowed current is 10mA.

Maybe you can blink it briefly at 20mA but 10mA is its max continuous current???
Its max voltage is 4V but what is its typical voltage when its current is less???

Maybe its typical voltage is 3V at 10mA. 3.5V???

They don't tell you the important specs because they want you to burn them out then buy more.

 

[GForce2010]

ok well i've found this other one.

Forward Voltage 3.9 +-0.3v
Continuous Forward Voltage 30mA
Power Dissipation 120 mW
Peak Forward Current 100mA
Reverse Voltage 5V

Would this be a better choice?

 

[audioguru]

The forward voltage has a plus and minus limit but they forgot to say at what current. 1ma, 10mA, 20mA or 30mA??

When a Chinese company spec's a Continuous Forward Voltage of 30mA then run away from them as fast as you can. If they don't proof-read their spec's then they don't test their LEDs.

 

[GForce2010]

Sorry...that was my mistake when i copyed it from the pdf....it actually is Continuous forward Current.

and looking at the next page of spec it says that the voltage +- is at 30mA

 

[audioguru]

So the LED is 3.6v to 4.2V at 30mA. Calculate its current to be less than its max of 30mA.

 

[GForce2010]

Well i've had a loook on a few sights and have come up with this idea.

**broken link removed**

Two leds off the same 12V feed with one resistor, please let me know if i've made a mistake somewhere.

Thanks

 

[audioguru]

With 260 ohms (not a standard resistor value) the current is only 16mA. With 150 ohms the current will be 28mA.

Use 9V instead of 12V then there is less waste. Then a 47 ohm resistor makes a current of 26mA.

 

[Sceadwian]

The nominal voltage of a 6 cell NiMH pack is 7.2 volts, no resistor would be needed.

 

[GForce2010]

This is for an in car project so it has to be able to deal with 12 - 14V

I'm proberly wrong but if i ran say 4 LED's in secquence like the two above would i actually need to have a resister in the circut?

Sorry to be a pain.

Thanks.

 

[audioguru]

The nominal voltage of a 6 cell NiMH pack is 7.2 volts, no resistor would be needed.

Two 3.9V LEDs need 7.8V. If they are 3.6V ones (as per the spec) then they might light up without blowing up.
If they are 3.9V ones or 4.2V ones (as per spec) then they probably won't light up.

I have a keychain white LED flashlight. It has 3 button batteries making 4.5V without a load. The white LED is 3.5V and the internal resistance of the battery cells reduces the current to about 22ma. No resistor is needed.

 

[Sceadwian]

You're either going to waste the 4 volts the resistor needs to drop need a seperate switching power supply.

 

[fingaz]

I would just put 3 in series, and use a 150 ohm resistor. this assumes a forward voltage of 3.6v and operating current of 25mA (to err on the side of caution)
Maybe some will be slightly brighter than others, but hey, who's perfect?

 

[Nigel Goodwin]

The nominal voltage of a 6 cell NiMH pack is 7.2 volts, no resistor would be needed.

Really, really BAD idea - you must include current limiting of some kind with LED's, directly across NiMh they would either not light at all, or blow instantly.