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### Topic: multi chip led series resistor math help (Read 812 times)previous topic - next topic

#### grendle

##### Mar 31, 2013, 08:00 pm
does this piece require a resistor off of 6volts? if so, whats the math, straight forward like any other led?
http://www.lumex.com/specs/SSL-LX20R6GD.pdf

thank you.
happy holidays

#### JimboZA

#1
##### Mar 31, 2013, 08:08 pm
My understanding is that an LED won't work unless the supplied voltage is greater than Vf, which according to that sheet is more than 6V in the first place.

(Purpose of the series resistor is usually to drop the surplus voltage... you don't have enough in anyway  . )Or it's marginal.))
Johannesburg hams call me: ZS6JMB on Highveld rep 145.7875 (-600 & 88.5 tone)

#### grendle

#2
##### Mar 31, 2013, 08:31 pm
ok hmm, its a strange little device. ya it lights at 6, at 7 any res (tested low ohms) dims it alot. i guess for this to be bright enough im going to have to go with 9 volts with a proper resistor. trying to incorporate a nice bright led into my dog collar, maybe this device is the wrong choice, im trying to keep the package small(its a little dog). too bad i got a bunch of em.

#### JimboZA

#3
##### Mar 31, 2013, 08:37 pm
Quote
i guess for this to be bright enough im going to have to go with 9 volts with a proper resistor.

Well that's the same as putting 7 in... your calculation will be based on 7V across the LED, so you'll lose 2 in the resistor and get 7 across the LED just the same as if that's what you were supplying. (Unless the point you're making is that 9V is more convenient with a PP3 than trying to hook 5-6 AAs together.)

Johannesburg hams call me: ZS6JMB on Highveld rep 145.7875 (-600 & 88.5 tone)

#### grendle

#4
##### Mar 31, 2013, 10:14 pm
ya 9 volt supply i meant purely for convenience. should be an interesting project, im still in the 'messing around with what works good phase'. i appreciate your advice Jim thank you.

#### cmiyc

#5
##### Mar 31, 2013, 11:07 pm

(Purpose of the series resistor is usually to drop the surplus voltage... you don't have enough in anyway  . )Or it's marginal.))

The purpose of the resistor is to limit the current since the LED turns into a short when its forward voltage is applied.  This works because the resistor drops the surplus voltage of the series circuit (LED in series with Resistor).

---

Vresistor = Supply - Vforward = 9V - 6V  = 3V

Resistor = Vresistor / Iforward = 3V / 20mA = 150ohm

Datasheet says you can run the LED to max continous current of 25mA.
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#6
ok thx James

good info

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