### Author Topic: LM723 and Variable Current Control Technique????  (Read 3316 times)

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#### Gurumurthy

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##### LM723 and Variable Current Control Technique????
« on: June 17, 2017, 07:02:30 pm »
Hi to all!!

This was a circuit Discussed in our Forum Last year.

the doubt is first circuit , how does the current control knob operates that is 1Kohm 10 turns potentiometer in the first circuit?

the second circuit contain potentiometer  of 500ohm which control the current in output of regulator Circuit?

the third circuit contain potentiometer accross a Diode, How does it make Current variable regulator?

I have also Discussed this in my previous thread topic, current limiting is not happening in LM723???...

So i am totally confused with these circuit. Please make me to understand the Technique behind Current control transistor in LM723 for Variable current control in power supplies..

Help me in this regard.. I think i have conveyed my thoughts.if any thing is incomplete . Correct me if i am wrong..

#### Kleinstein

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##### Re: LM723 and Variable Current Control Technique????
« Reply #1 on: June 17, 2017, 07:32:35 pm »
The Current limit in the LM723 engages with more than about 0.6 V between pins 2 and 3. In Circuit 3 the ampmeter is used as a shunt for the current limit. So it would not work well without. The pot reduces the voltage of the diode and the shunt a little to set a higher current limit. Due to the diode the characteristic is a little nonlinear and can work with a relatively low drop on the shunt. The nonlinear curve is in a good sense more like a log scale for the current. So the current limit can be set to low values. However thermal stability sets a practical lower limit, as the diode and LM723 internal transistor will have different temperature.

The first circuit works rather similar. The difference is that the base to emitter drop of the output stage is used instead of the extra diode. With the two diode drops, the adjustment can go all the way to zero. However thermal drift can be a bigger problem than with the 3rd circuit.

The Circuit 2 is a little tricky. Here one a shunt is used to make up the drop. The additional pot as a divider increases the point where the current limit sets in even more. So only a moderate adjustment range is practical - with higher setting there will be a lot of power loss at the shunt. So this not a really practical circuit.

#### Gurumurthy

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##### Re: LM723 and Variable Current Control Technique????
« Reply #2 on: June 18, 2017, 01:39:51 am »
HI Kleinstein,

In circuit 3, the voltage accross the diode is say 0.7V . The current control transistor , base emitter pin is at 2 and 3 which has cut in potential of 0.65. hence a difference of 0.05 is going to do the current control with the help of pontentiometer.

whether my thinking is correct. Correct me if i am wrong..

#### Gurumurthy

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##### Re: LM723 and Variable Current Control Technique????
« Reply #3 on: June 20, 2017, 06:19:58 am »
Dear Sir,
What are the ways to monitor current and Design a Variable current control Power Supply. Teach me the way or give some references from books.

Smf