How to design what amounts to a variable load resistor

Discussion in 'The Projects Forum' started by CaryInVancouver, Apr 15, 2011.

  1. CaryInVancouver

    Thread Starter New Member

    Jan 25, 2011
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    I am interested in using a power supply that provides 12A at 5V. It requires a minimum 3A load, however. I understand that could be achieved with a (5V/3A=) 1.7 Ohm load resistor placed between +5V and ground (possibly achieved by using larger-Ohm resistors wired in parallel). My question is whether there is a more efficient way of maintaining a load of at least 3A, so that the max load is not effectively reduced to 9A.

    Let X be the load in amps. I would like to add some parts to the circuit that would create an additional load of 3-X amps (or 3A) when X < 3A, but add no load when X >= 3A. Any ideas?

    If it's not obvious, I'm pretty new to this hobby.

    Cary
     
  2. wayneh

    Expert

    Sep 9, 2010
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    There are definitely ways to accomplish this. The question is which is easiest.

    I'd consider a constant current supply, where the current level is controlled by a measurement of the total load. So if total load is greater than 3A, the circuit supplies zero to its load. At any lower total load, the circuit adds current to make up the difference.

    If that all makes sense, look at the constant current supply here for the general idea. It would need to be modified to vary the current depending on the total being driven by the power supply. I think you'd need a second op-amp on the LM358 to accomplish that - to convert the total current into a reference voltage to apply to the pin of the op-amp in the circuit shown. This would replace the reference voltage set with the potentiometer in the diagram.
    Picture 1.png
     
    Last edited: Apr 15, 2011
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  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    I don't get it.. Why buy a 12A power supply only to have to put a dummy load on it..
     
  4. CaryInVancouver

    Thread Starter New Member

    Jan 25, 2011
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    I more interested in the question than in the particular power supply (i.e., the educational aspect). Cary
     
  5. wayneh

    Expert

    Sep 9, 2010
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    Computer PSUs need some loading in order to be drawn into regulation. Not sure that's relevant here.
     
  6. retched

    AAC Fanatic!

    Dec 5, 2009
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    It may be the exact reason.

    If metered with no load, the power supply may appear to be 7v or higher.

    The manufacturer may only guarantee 5v @ 3A.
     
  7. CaryInVancouver

    Thread Starter New Member

    Jan 25, 2011
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    The circuit powers about 150 RGB LEDs and an Arduino. When all the lights are off, I need to make sure the Arduino gets 5V.
     
  8. CDRIVE

    Senior Member

    Jul 1, 2008
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    Gee, 36 Watts of waste when we're supposed to be looking for ways to save it. Seems counter productive.
     
  9. k7elp60

    Senior Member

    Nov 4, 2008
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    I have a load bank which essentially the same as the posted schematic, except I am using a 20A NPN darlington transistor mounted on a large heatsink. It is real handy for checking current limit of power supplies, discharging batteries for load tests etc. The beauty of the circuit the load current remains constant when discharging batteries. Using resistors the load current decreases as the battery discharges.
     
    Last edited: Apr 15, 2011
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  10. CaryInVancouver

    Thread Starter New Member

    Jan 25, 2011
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    I'm just reading about op-amps now in my electronics self-study regime. This should be a good exercise to improve my understanding of those devices.

    Thanks again, Wayne.

    Cary
     
  11. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    I have simulated something similar to what wayneh proposed.

    Seems to do the job in principle [keeping the minimum current to 3A] with only the 5V rail supply supplying the whole circuit.
     
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  12. wayneh

    Expert

    Sep 9, 2010
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    Nice. I was having trouble with the logic and thought a 2nd op-amp might be needed, but your solution makes perfect sense - a current "mixer" if you will.

    Are those "10m" resistors 10mΩ resistors? I'm not familiar. Ten is too much.
     
  13. CDRIVE

    Senior Member

    Jul 1, 2008
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    Those resistors are 10 milliohms which equals .01 Ohms.
     
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  14. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Sorry Wayneh - I thought the thread had sunk to the bottom. CDRIVE pulled it up correctly - 10milli-ohms was what I intended.
     
  15. Wendy

    Moderator

    Mar 24, 2008
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    Another way...

    Electronic Loads

    I have the parts, just need to get them together. If it works out I'll be posting it in the completed projects.
     
  16. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Good on you Bill.

    The OP seems to have wandered off. Some of us are actually still interested.:)
     
  17. Wendy

    Moderator

    Mar 24, 2008
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    Most of them come back. If they don't, oh well...

    But if he does it will still be here.
     
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