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How to calculate how many amps needed

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  • How to calculate how many amps needed

    I have LED lights to mount on my Polaris Ranger EV. There is a 12v tap available under the hood. It is limited to 10amps. My LED's are 12-30v 27w. How do I calculate how many amps they will draw?
    Location: The Black Forest in Germany

    How to become a millionaire: Start out with 10 million and take up machining as a hobby!

  • #2
    that's just over 2 amps. 2 amps at 12 v is 24 watts, so 2.2 amps would be really close.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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    • #3
      Per mister ohm: P=EI
      or power (in watts) is volts x amps
      So 27w = 12v x ?amps
      or 27w/12v = 2.25a each

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      • #4
        Thank you. After I made this thread I Googled it and found out it was very simple to calculate.
        Location: The Black Forest in Germany

        How to become a millionaire: Start out with 10 million and take up machining as a hobby!

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        • #5
          There are/can be some twists there. Naked LEDs are not 12-30 Volts. They will drop somewhere between 2 and 4 Volts. In packaging them to operate on higher Voltages different strategies are used. Individual LEDs can be connected in series to increase the Voltage rating of the string. But even that is not the end of it because, unlike other types of electric/electronic devices, they are not really rated by a Voltage. They are rated by and must be operated at a rated current. So, LED lamps, weather they use just one LED or many in a series or parallel or combined series/parallel arrangement, MUST have some kind of current regulation circuitry. That circuitry can take many forms from a single resistor to a IC (integrated circuit) that is designed to supply the needed current.

          The power rating (Watts) of the LEDs may be a constant over that 12 to 30 Volt range or it may just be the maximum rating at whatever supply Voltage the circuit determines. So yes, you can just divide 27 Watts by 12 or 14 (most auto batteries are closer to 14 V than 12 V) Volts and use the number of Amps that provides; I get a little under 2 Amps when I use 14 Volts. But other numbers are possible, especially at the top end of that Voltage range. It's the old trick of choosing your specs. with the most favorable conditions for each one individually, not one set of conditions for all of them.

          In this case, with 10 Amps available and if there is nothing else on that circuit, then you are probably safe to power one, two, or even three of these LED lights from it.

          If you pop a fuse or breaker, you can always run a new circuit from the battery with an in-line fuse for the needed Amperage.



          Originally posted by Black Forest View Post
          I have LED lights to mount on my Polaris Ranger EV. There is a 12v tap available under the hood. It is limited to 10amps. My LED's are 12-30v 27w. How do I calculate how many amps they will draw?
          Paul A.
          SE Texas

          And if you look REAL close at an analog signal,
          You will find that it has discrete steps.

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          • #6
            Originally posted by Black Forest View Post
            Thank you. After I made this thread I Googled it and found out it was very simple to calculate.
            And that would be why I managed to survive my electronics classes....

            Chilliwack BC, Canada

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            • #7
              usually they draw much less than advertised.

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              • #8
                I am very well versed in electrical / electronics,
                but I am also a big fan of reality. Most GOOD
                multi-meters can test in series up to 10 amps.
                As a feel good move, just test the current draw.

                -Doozer
                DZER

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                • #9
                  Given the rating of 12 to 30 volts and 27 watts means there must be some electronics inside to automatically limit the current to the design value. This means that at 30 volts you will be drawing under one amp- it also means that under 12 volts it will try to draw more than 2.25 amps. At 10 volts it will want nearly 3 amps. At that lower voltage it will also be starting to overheat the electronics- unless it's designed to actually shut off with a low input voltage.

                  If you want to actually measure the current draw, the meter may not read correctly since you're dealing with a switching power supply with high frequency pulses that could interfere with the meter. Just a possibility to keep in mind. The unit may also have an internal capacitor to stabilize the operation of the switching power supply. There is a chance that the initial inrush current will pop a fuse if the rating is too low. You might get away with a 3 amp fuse, but I'd recommend a 5 amp. Does it come with its own fuse?
                  I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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