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Old 18th February 2012, 10:49 AM   #1
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Cool Voltage independent power load

Not sure this is the right place for such questions but here goes nothing.

For a power supply i need a power load that is independent of the voltage.
I'm not talking about a current source, the power load of a current source will rise if the voltage gets higher.
So i need a load that will draw a load of 1,5 watts from 4-65 volts all the way.
Can any one help me ? i have done some thinking but I'm a bit stuck.
There are a few restrictions, i can not use a negative voltage because the supply does not have one. i have a 5v supply for powering the components.
The 1,5watt should be dissipated in a to-220 transistor e.g. bd139.
No more then 3 opamps of 324 type(these opamps should be power from the 5v supply).

hope some one can help me out
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Old 18th February 2012, 11:05 AM   #2
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I assume this is for testing? You want an artifical load that keeps V*I = P = constant

If so I think you are looking at it from the wrong direction as its a load that _is_ dependent on V you need. Such as a current source controlled by the inverse of the voltage over it.
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Old 18th February 2012, 11:40 AM   #3
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Its not for testing, i want to use it to load a variable switching supply.
If i just use a plain resistor then it will only load the supply in the higher voltage region.
If i use a current source then it will draw excessive power in the higher voltage region, but will load the supply all the way down to 4v.
The thing is i want to waste as less power as possible.
I was mebay thinking of using a AD633 Analog Multiplier but is this not a bit overkill ?
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Old 18th February 2012, 11:57 AM   #4
wwenze is offline wwenze  Singapore
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Something that draws the same (or similar) power regardless of voltage...

Another switching power supply, loaded by a resistor or something.
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Old 18th February 2012, 12:17 PM   #5
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Quote:
Originally Posted by wwenze View Post
Something that draws the same (or similar) power regardless of voltage...

Another switching power supply, loaded by a resistor or something.
Great idea, but a bit overkill to i think
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Old 18th February 2012, 12:50 PM   #6
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A plot of the V versus I of such a load shows it has a negative dynamic resistance.

V = 1.5 / I ( from V = 4 to 65 volts ).

Loading a Power Supply with such a load, cannot give a stable operating point..
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Last edited by mchambin; 18th February 2012 at 12:53 PM.
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Old 18th February 2012, 03:42 PM   #7
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A constant power load itself is stable; however, due to the negative incremental resistance the power supply may become unstable. It depends on the supply.

egbert, you want to make P=V I a constant, so so you need to masure the VI product and make it constant. Obviously, this requires some type of multiplier. Also note that you need to limit the current at low voltages; otherwise it will become very large.
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Old 18th February 2012, 04:16 PM   #8
DF96 is offline DF96  England
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The voltage across a diode is proportional to the logarithm of the current. If you use Vbe of a BJT then this is quite accurate. Put a current proportional to the PSU voltage through one diode (just use a resistor). Put the load current (or a fraction of it) through another diode. Add the two voltages together with a summer. Arrange a feedback loop to keep this sum constant. If the sum is constant, then the product of I and V will be constant.
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Old 18th February 2012, 06:18 PM   #9
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Quote:
Originally Posted by DF96 View Post
The voltage across a diode is proportional to the logarithm of the current. If you use Vbe of a BJT then this is quite accurate. Put a current proportional to the PSU voltage through one diode (just use a resistor). Put the load current (or a fraction of it) through another diode. Add the two voltages together with a summer. Arrange a feedback loop to keep this sum constant. If the sum is constant, then the product of I and V will be constant.
This seems to work but I'm not getting really accurate results in my simulation model. tomorrow i will overview it.

Thank you all for al your help so far.
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Old 19th February 2012, 05:54 AM   #10
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I'm working on a constant power load for burn testing battery packs using an AD633 and an LM359. The AD633 is in the feedback loop of the LM359. Current is sampled via a resistor and voltage is sampled via a resistive divider. The voltages representative of current and voltage are sent to the AD633 and scaled. The output of the AD633 is a voltage representative of power consumption and is converted to a current and fed to the LM359 where it is compared to a reference current that defines the total consumption of the circuit.
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