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phil.s

LED's

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I'm trying to run some 12Volt LED's of a PLC to make a test rig. Does anyone know how to wire them up correectly i.e. resistors and all of that? At the moment I've got 3 LED's running off Y0 Y1 and Y2 through a 2k2 resistor but as you turn more on they dim. I'm using a 24VDC 30Watt poer supply. HELP!

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LEDs operate with low voltage (1.8-3.6V depending on model) and low current (2-25mA, also depending on model). to operate properly they need something to regulate current. traditionally this was done with series resistor. say you have 2V, 20mA device and want to power it from 12V source. resistor will have to drop the voltage difference which is 10V. current in series circuit is same so it will be 20mA just like for LED. value of resistor is determined by Ohms law R=U/I=10/0.02=500 Ohm closest match are standard values such as 510, 470 or 560 Ohm. power dissipation is P=U*I=10*0.02=0.2Watt This means that standard 1/4 watt resistor will do even for continuous duty. Same approach can be used when calculating series resistor for any LED lamp even if it already has resistor built in (in your case for 12V operation). don't forget to check power requirement for the resistor or in might burn or else. For example if your lamp is 12V, 40mA device: R=(24-12)/0.04=12/0.04=300 Ohm ----> not standard value, could use two 150 ohms in series. P=(24-12)*0.04=12*0.04=0.48Watt ----> you need 0.5W size as minimum (it could be two 150Ohm resistors, each 1/4watt but if you can get bigger ones, i would go for it).

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I use 1K resistors and the LEDs are plenty bright enough. Also, 1/8 watt resistors are fine.

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note of warning: it may be fine for someting on your bench since all components have some safety margines. but this is not something you want to put in a project that goes 1000 miles away. with 1k resistor you might be driving most modern LEDs out of spec. Some 20mA used to me standard before high efficiency diodes came out. the high efficiency models only need 2-5mA and that's 4-10x difference. they are more tolerant to abuse compared to older LEDs (can survive bigger voltage and current swings) but pushing something far beyond spec might bite. many newer series of resistors are smaller in size (phisically) than classic ones so estimating power rating just by size is not simple task anymore. if resistor is really only 1/8W it won't last very long (i know, i tried it too. they got pretty hot quickly and lasted more than a week in a summer time which is quite impressive). Ohms law is not that difficult and if you buy new components, check the specs and do the math. It's not like you are going to save some money on smaller resistors...

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