# Voltage Divider with a Buffer

Discussion in 'General Electronics Chat' started by taharajabali, Jun 3, 2011.

1. ### taharajabali Thread Starter New Member

Jun 3, 2011
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Hello, I am attempting to build a circuit will basically allow me to vary the input voltage to a low impedence load. I tried using a simple 5Kohm potentiometer, and connected an opamp after to it, so as to be able to drive the low impedence load. The problem however is the power dissipation. The load needs about 0.3A at 24 V. My power supply can source this no problem, however i have about 36 of these loads and would like to control their input voltages individually.

I did some quick math to and found that the op-amp exceeds its junction temperature and shuts off. I am using a TCA0372BDP1G (from digikey) opamp, that is capable of sourcing upto 1A.

I read that adding a pull-up resistor will lower the power dissipation, however my calculations reveal that even with the pull-up, i will still be exceeding the juntion temperature of the opamp.

is there any other circuit that will allow me to use my power supplies current directly without doing through the opamp, but still use the opamp as the voltage source? Any help will be much appreciated.
Thanks.

Apr 20, 2004
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3. ### SgtWookie Expert

Jul 17, 2007
22,182
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If you use any linear regulation method, you will wind up with a power dissipation problem.

You have not mentioned what your main voltage supply output is (and if it is fixed or variable), and what range of voltages do you need to apply to your loads?

4. ### taharajabali Thread Starter New Member

Jun 3, 2011
2
0
Thanks for the replies.
@SgtWookie, My main voltage supply output is a 24V-DC, 10A variable power supply, and I need to apply voltages between 1V upto the max 24V to my load.

@beenthere,
I had not considered that option. I'll look into it. Thanks for the suggestion.

5. ### SgtWookie Expert

Jul 17, 2007
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1,728
The LM317 is a linear 1.5A regulator. It has a minimum dropout (measured from the IN to OUT terminal) of 1.7v, and a minimum output voltage of ~1.25v.

If you use 24v for the input, you will have a range of ~ 1.25v to 22.7v for the output. Power dissipation in the regulator will increase very significantly as the output voltage decreases.

You should use the variable control on the supply to change the output voltage; as that capability seems to already be in your power supply.