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Author Topic: Correct Relay Selection?  (Read 606 times)
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Hey Forum,

I'm setting up 20 LEDs in series and powering the circuit using a 12v battery. I am attempting to use a Mega2560 to signal a relay to control the lights turning on and off but I'm not sure what power specs I need to look for when selecting a correct relay.

Each LED has a vDrop of 2.4v and they are wired in series with a 40 ohm 5w resistor.

Thanks for the help!
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Each LED has a vDrop of 2.4v and they are wired in series with a 40 ohm 5w resistor.

Did you measure this voltage from a single LED with 40 ohm resistor on the 12V battery? If each LED drops 2.4V than 20 of them in series with do 48V. Tell more. The answer to your question might simply be a MOSFET, no need for a relay.
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Did you measure this voltage from a single LED with 40 ohm resistor on the 12V battery? If each LED drops 2.4V than 20 of them in series with do 48V. Tell more. The answer to your question might simply be a MOSFET, no need for a relay.

The LED specs say there is a 2.4 voltage drop but I've set up the circuit using all 20 LEDs in series with a 40 ohm 5watt resistor to the 12v battery and it works. Now I simply need a relay go to all 20 LEDs in series and turn them on or off.

What are the power specs I need to figure out for selecting a correct relay in this setup?
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Basically I'm trying to find a simple explanation for correct relay selection. I've searched here:
http://arduino.cc/playground/Main/Relay

But it's not much help as you can see. Are there any simple relay selection instructionals out there?
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Relays have a coil rating and ratings for the contacts (the switch part).

The coil rating is a voltage and often a resistance, so that you can determine how best to switch it.

The contact ratings are given as a voltage and a current, usually with DC values and AC values.  The circuit that you're switching should not exceed either the voltage or current rating specified.
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If they say the LEDs work at 2.4V and you connected them up in series, then none of the LEDs are at 2.4V so I have no idea what voltage they are at unless you can measure them. Why not using a multimeter to get the current of the setup and use current * 12V as power rating for the MOSFET? You don't need a mechanical relay if the current is not too big.
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You have all 20 in series? Not strings of 5, with 4 strings in parallel?
I'm surprised they light up.
Or do you have 20 in parallel, each with a resistor?
Or, 20 in parallel, with 1 resistor for the group?
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Post a picture to clear the confusion, would you?
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I guess any general purpose relay with contact rating 250VAC 10A will work - but to drive relay you still need a transistor, so why not use transistor alone. You can drive MOSFET's directly from Arduino pins and those in TO220 package has pretty good characteristics - I have lot of IRF530 at home and I am using them anywhere - 14A peek drain current and 100V
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