Resistor load bank

Discussion in 'General Electronics Chat' started by superway, Jul 27, 2011.

  1. superway

    Thread Starter Active Member

    Oct 19, 2009
    128
    0
    Hello,

    I am going to build a rersistor load bank to test a product.
    I have 4 fixed resistors and one rheostats. Please see my attachment,
    in order to vary the small increment of the load ( like 0.5 amp per increment) I want to add a Rheostats to do this job. Would you please give me the advice what is the specs. of rheostats should it be? range of Ohm and watts?

    Thanks

    ken
     
  2. Robert.Adams

    Active Member

    Feb 16, 2010
    112
    5
    For the schematic given, the resistance and power dissipation rating of the rheostat depends on the other resistors which are turned on.

    If, for example, you have CB1 on under test with the 90V, 750W unit, you could burn up the resistor if the unit delivers over the 250Watts.

    Since the resistances are in parallel, multiple resistances on will lead to less dissipation in each as the current is divided like a current divider and the power through each is I^2*R.

    If I was making a bank like that, I would prefer to have every branch be rated at least 750W if the switches are user controlled. That would keep anyone from inadvertently frying a big power resistor.
     
  3. superway

    Thread Starter Active Member

    Oct 19, 2009
    128
    0
    For safety concern, a 750 W for each branch is fine.

    but if I turn CB1 and it is only 45 ohm 250W, the UUT loads only about
    2 Amps, and the power dissipation is 180W. Shouldn't it be problem?

    Thanks
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I usually like to use resistors rated for 1.6 times (or greater) the expected power dissipation in the resistor. Your 45 Ohm 250 Watt resistors are only 1.38 times the expected wattage, so you don't have a lot of margin.

    Consider mounting them vertically so that convection will aid in cooling them.

    Those are the only resistors I see that are marginally rated for your project.

    If you want to use a rheostat, you should use a fixed resistor in series with it. Otherwise, it'll be all too easy to turn the rheostat down low, and fry it.
     
  5. superway

    Thread Starter Active Member

    Oct 19, 2009
    128
    0
    Thanks for your help, SgtWookie. I redraw the load please see my attachment. As your sugesstion, I add a series Rheostat 250W ( I don't know the value of rheostats yet) to the fixed resistors. Each fixed resistor rates 500W. Can you look it?

    thanks
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Well, I've been looking at various suppliers, but haven't found the resistance and wattage values I've been looking for.

    Are you planning on using resistors that you already have, or are you looking to re-build with new resistors?

    If you want to use resistors you already have, please list the count of how many you have of each resistance and wattage rating.

    Rheostats will be problematic no matter what. It may be easier and more reliable to switch in some fixed smaller resistors instead.
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    In the old days it was common to use Jug elements, the type with exposed wire coil around a ceramic block.

    They are good for many hundreds of watts, especially if fan cooled.

    For adjustability you can use an alligator clip onto any turn of the exposed filament (like a crude rheostat).
     
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