Power supply

Thread Starter

Chucksue

Joined Jul 2, 2018
7
I’ve built this power supply but I can’t get the output to go to zero volts. The lowest I can get it to is 2.8 volts. Any suggestions????
 

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AnalogKid

Joined Aug 1, 2013
11,157
Please rotate and expand the schematic so it is readable.

The datasheet states that the minimum regulated output is 2.0 V, so while there probably is a reason your circuit cannot go that low, the chip just can't regulate down to 0 V without a negative power supply. The reason is the error amplifier input stage.

The datasheet has an internal schematic. It shows that the error amplifier input stage is an NPN differential pair and an NPN current source. Thus, the inputs cannot go to GND without these transistors going into cutoff.

The reason so many linear power supplies use the LM358, LM324, or other similar opamps is that their input stages are designed specifically to function down to ( and below - ! ) 0 V without a negative power supply. The genius of Robert C. Dobkin.

ak
 

Thread Starter

Chucksue

Joined Jul 2, 2018
7
Please rotate and expand the schematic so it is readable.

The datasheet states that the minimum regulated output is 2.0 V, so while there probably is a reason your circuit cannot go that low, the chip just can't regulate down to 0 V without a negative power supply. The reason is the error amplifier input stage.

The datasheet has an internal schematic. It shows that the error amplifier input stage is an NPN differential pair and an NPN current source. Thus, the inputs cannot go to GND without these transistors going into cutoff.

The reason so many linear power supplies use the LM358, LM324, or other similar opamps is that their input stages are designed specifically to function down to ( and below - ! ) 0 V without a negative power supply. The genius of Robert C. Dobkin.

ak
IMG_0495.png
 

AnalogKid

Joined Aug 1, 2013
11,157
Much better.

R9 creates an offset at the low end of the the adjustment range. The minimum reference value at the error amplifier non-inverting input is 0.52 V, not 0.0 V. When multiplied by the circuit gain of 3.75, this means that even if the 723 could regulate down to 0.0 V, the minimum output voltage would be 1.95 V. So for two different reasons, this circuit cannot do what you want.

ak
 
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