Driver Test

Discussion in 'General Electronics Chat' started by imraneesa, Jan 1, 2015.

  1. imraneesa

    Thread Starter Member

    Dec 18, 2014
    185
    4
    Hello everybody,

    I purchased buck driver of 1.8a, seller said it was tested. it gives 1.8a constant current. I tried to test it. I have ac to dc adaptor of varying voltage. I set it to 7.5V but when I check with Multimeter it was showing 13Volts. anyways I took the 7.5V for calculation because the adaptor shows that value. I calculated using ohms law.
    V=IR
    R=V/I, 7.5/1.8
    R= 4.166 say 4ohms
    I have 1 ohm resister (don't know how much wattage) on checking with multimeter I got the value around 1.5 and then I connected alligator wires and then checked it and it gave me 4ohms resistance.
    then I connected the AC-DC adaptor to the Driver and I connected multimeter in series with the output of the driver to calculate current.
    It suppose to show me current around 1.8A but I only get 700mA. I want to know what is the mistake and where will be the fault. Am I doing anything wrong. is that Adaptor wrong? Please clarify.
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,060
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    If you calculated that 7.5/1.8= 4.166 ohms, that should be the smallest resistor you can use with your power supply. You should not round down because you will draw additional current (more than 1.8 amps) and possibly damage your power supply.

    Next, a power supply needs some type of minimum load, 10 to 30 mA is typical, before the voltage is regulated. Sometimes power supplies have a resistor built in so it always reads the correct voltage with a meter and some power supplies do not have this resistor - so it appears to be some larger number when checked with a meter.

    Additionally, some (inexpensive) power supplies are designed for a specific load for some specific product and will only settle to the correct voltage when the correct load (correct device) is connected.

    Final note: some better power supplies also have an overload protection circuit built in. This protects the device from damage if someone tries to connect a 1 ohm load to a power supply that can support no more than a 4 ohm load. In this case, the current output will be greatly reduced.
     
  3. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Do you not think it strange that you have a nominally 1R resistor that measures 1.5R and then 4R when you connect wires to it? How long are the wires?
    I would expect a resistor marked 1R to be much closer to 1R than 4R even with a couple of short wires attached.
    If you know the value of the load resistor you can calculate current simply by measuring the voltage across it but your load resistor value seems to be all over the place.
    Are you sure that the resistor has not burnt out as a 4R it will dissipate 13W?
     
    Last edited: Jan 1, 2015
  4. imraneesa

    Thread Starter Member

    Dec 18, 2014
    185
    4
    The resistor I used is 0.5W 1 ohm resistor and when connected with the alligator wires to the multimeter the total resistance was 4.2ohms. also the person who sold me the driver told me that the driver was made for 4.5-5vf forward current diode. so he told me to use four numbers of 1N4007 diodes in series as load to check how much current the driver is giving. when I do the same thing I still getting current as some 500mA. I used power supply as two 18650 batteries and also I tried it with AC-DC adaptor.
     
  5. imraneesa

    Thread Starter Member

    Dec 18, 2014
    185
    4
    I am attaching the alligator wire and the resistor I used for your reference.
     
  6. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Ok, so you have a 1.8A constant-current LED driver, if you are only getting 500mA under load and the input voltage to the driver is maintained within spec. then the driver must be faulty or does not meet its claimed specification.

    Incidentally, a 1N4007 is only rated at 1A so connecting it across the output of a 1.8A current source is not good advice, neither can you load test the driver with a 0.5W 1R resistor as it would dissipate >3W at 1.8A.
     
  7. imraneesa

    Thread Starter Member

    Dec 18, 2014
    185
    4
    Then what is the best way to test it sir? input voltage is from the adopter I showed in the attachments. also I have tried it with two lithium 3.7V batteries in series as told by seller. so what wrong I am doing. do I need to buy 1ohm 3w resistor and try it again?
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Lose the clip leads. They have no place in any circuit were more than a hundred mA flows.
     
  9. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    4 x 3A rectifier diodes in series should provide a close enough approximation to the forward voltage of the LED that the driver expects to see; just measure the current with your ammeter.
     
  10. imraneesa

    Thread Starter Member

    Dec 18, 2014
    185
    4
    Can you give me a link of that where I can buy them. So if I use wire of 32 inch to connect multimeter and driver will it add any resistance? Do I need to consider that?
     
  11. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Diodes from the 1N5400 series are rated at 3A. The resistance of the wire should have no effect on the readings; the current should be the same regardless.
     
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