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Thread: DC Voltage & Current

  1. #1
    Join Date
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    Default DC Voltage & Current

    I installed a 40 WATTs solar panel connected to a 12-24 volts controller and a 12 Volts battery to power fans in my greenhouse. The controller also come with two USBs and I have 3 fans running simultaneously, (two run on USB and another on a 12 volts plug from the controller. All the fans are working good but not powerful enough. Yesterday, I bought a 12" dual power 12Vx700 mA fans with a transformer. It works great when I plugged it into the AC power outlet.
    but didn't work when I connected directly (no transformer) to the 12 volts supply from my controller. The fan is not working anymore when I replug back to the AC outlet with the supplied 12V transformer. Does that mean the fan has been fried?
    I researched online and people was saying that higher voltage than required would kill the device but not the amperage as the device would only draw the required amperage. Please explain while the fan is not running now. My solar controller shows 13.7 volts. Thanks.

  2. #2
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    Not enough information to answer. Can you verify if the fan transformer is 12VDC or 12 VAC? Generally, a 10% to 15% over voltage will not hurt a motor as long as it isn't overloaded. But it depends on what type of motor this fan has. If it has an AC motor and you put 13.8 VDC on it, you may have killed it (stalled rotor and higher than normal voltage allows more current to flow; Even a stalled rotor at 12V could burn it up). If the motor did in fact run but turned faster than normal, it would draw more current at 13.8V and probably burn up because the load of a fan increases as fan speed increases. If there are digital controls in this fan, running it without some reverse voltage protection could burn it up (energizing a coil of wire makes a big voltage spike when the voltage is removed -- you need something to absorb this or diodes or transistors could be fried).
    Last edited by suemarkp; April 14th, 2018 at 11:24 PM.
    Mark
    Kent, WA

  3. #3
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    Default 40 WATTs Solar Panel and 12 Volts DC Fan

    Quote Originally Posted by suemarkp View Post
    Not enough information to answer. Can you verify if the fan transformer is 12VDC or 12 VAC? Generally, a 10% to 15% over voltage will not hurt a motor as long as it isn't overloaded. But it depends on what type of motor this fan has. If it has an AC motor and you put 13.8 VDC on it, you may have killed it (stalled rotor and higher than normal voltage allows more current to flow; Even a stalled rotor at 12V could burn it up). If the motor did in fact run but turned faster than normal, it would draw more current at 13.8V and probably burn up because the load of a fan increases as fan speed increases. If there are digital controls in this fan, running it without some reverse voltage protection could burn it up (energizing a coil of wire makes a big voltage spike when the voltage is removed -- you need something to absorb this or diodes or transistors could be fried).
    The fan is a 12VDC 700mA. My solar panel is 40 WATTs, the charge controller is for 12-24 Volts DC. I use a 12V battery. I guess W =VA, A= 40/12=3.33 Amps;
    Will the fan draw 3.33 AMPS instead of 7oomA and got fried? Seem like I need to solder a 3.6 Ohms of Resistance, is it correct?
    V=IR, R=12/3.33 = 3.6 OHMs

  4. #4
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    No. You have a voltage source not a current source (the motor will pull the amps needed based on the applied voltage -- you could have a 100 amp source and things would be no different). The voltage is 13.8V so you need to drop 1.8V (13.8 - 12) to get the voltage applied to the motor to be correct (although this approach is wasteful dissipating that unwanted power as heat and is not ideal for motor loads because the inrush current will cause an even larger voltage drop than intended). R = E/I. So 1.8/.7 = 2.6 ohms. This resistor must be at least 1.5 watts in rating (power dissipated is I^2(R) which is 0.7*0.7*2.6 = 1.27).

    If you want to take this approach, I would look for two 5 ohm 1 watt resistors and put them in parallel. That will create a 2.5 ohm resistor rated at 2 watts.

    Note - I'd try to find an automotive fan that draws the power you're looking for. Automotive things are rated to work between 12V and 14.4VDC and solar systems typically emulate automotive voltages. Something like a heater blower fan (but I have no idea how much current those draw).
    Last edited by suemarkp; April 15th, 2018 at 01:12 AM.
    Mark
    Kent, WA

  5. #5
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    Default

    Thanks.I will go to automobile junk yard to find a radiator fan.

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