# 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
12,379
3,232
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.

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.

• ###### Min Current Controller.jpg
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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...

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.