LM723 voltage regulator

Discussion in 'General Electronics Chat' started by gehan_s, Sep 20, 2012.

  1. gehan_s

    Thread Starter Member

    Sep 17, 2012
    dear all,

    i am trying to make a voltage regulator using LM723. i understand the voltage regulation part. what i need is the current limiting. i have designed a opamp circuit to limit the current. it works as follows. i add a series 1ohm resistor to the load and measure the voltage difference across it. then i amplify the output (with a gain of 3.3) of it and give it to a comparator which compares it with a reference value set by me. if the output of the in-amp is larger than the set value it will turn on the internal current limiting transistor. i have connected the emitter of that transistor to the output of the power supply via a 10ohm resistor (C) to prevent the output shorting with the error amplifier.

    the maximum current i'm hoping to take out is 1.5A with a Vout max of 28V. the reason i added the 3.3 gain is because i want it to be converted to a digital value for displaying purposes. hence at maximum current my in-amp will give about 4.9V.

    my questions are,

    1. will this opamp circuit work? (specially because i am trying to measure very small difference voltages and will the in-amp withstand these high voltages at its +,- terminals)

    2. do i need the 10ohm resistor at pin 3 OR is there a better way to do it ????

    3. can i connect the in-amp output straight away to pin 2 via a resistor ad get rid of the LM358

    4. is there a better way to do all of these things?????

    i have added a picture of my design herewith

    thanks in advance
  2. t06afre

    AAC Fanatic!

    May 11, 2009
    It is time to go to bed for me now. But I will come back to you question tomorrow. I think your circuit have some snag in the current limit part. I know the LM723 quite well
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    Yeah I think you're violating the input common mode range (same as the power supply voltages) with 28V in on a 15V powered device.

    Additionally, a current limit circuit needs a current to limit. Seems silly, but once your comparator trips the output current goes to zero,so the comparator untrips and sends the full current out again, so the thing burps at a high current.

    Depending on the speed of things this current may be significantly higher then what you thing the limit is set for.

    I see no need for it.

    If you need to keep that amp I would reccomend a direct connection without the comparator. This needs teh gain adjusted for a voltage out of .65 volts for the max current.

    I would use one of the circuits straight out of the app notes (such as Figure 22. Positive floating regulator, then use a high side current monitor IC to convert the current to a ground referenced voltage to measure the current.

    To pick the current limit resistor (R6) all you need do is solve R = E / I where I is your desired current limit, and E = .65 volts. This "E" is implied in the spec sheets, and it's just a transistor Vbe so it does vary with temperature.

    Here's something I just used: I needed a 28V switched signal with current limiting, and I also wanted to measure the current down into some components.


    28ON is a logic signal from a PIC. SEN3 also goes to the PIC to an analog input. Had I thought of it at the time the voltage sense point (junction of R19 and R21) probably would be best placed at U5-8, though for this low current circuit it's fine as is.

    Note my circuit is just a suggestion and it limited to 130mA at 28V. It's intended to show you how to measure the current. The MAX4080 is half the price of the AD622 anyway, and there are many similar devices from several manufactures now. Several allow you to adjust the current to voltage gain to just where you need it.
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  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
    Same basic idea used by just about every linear P/S designer including me. Except, you need to loop compensate the LM358 because you need an error amplifier, not a comparator.
    See U1A in the attached drawing. It amplfies the voltage drop across the 0.25 Ohm resistor. The .o1 feedback cap and 10k input resistor are the compensation.

    Better to just use an external NPN transistor to steal the base drive of the pass transistor as shown in my design rather than messing with the 723 input.

    Using the 723, you might also try: use the current limit error amp to pull down on pin 5 of the 723. Since that sets the output voltage, that would program the regulator output to 0V, so it would reduce the current as far as needed to limit it at the design value.

    I have used a similar method in other protection circuits.
    Last edited: Sep 20, 2012
  6. t06afre

    AAC Fanatic!

    May 11, 2009
  7. gehan_s

    Thread Starter Member

    Sep 17, 2012
    dear all,

    thank you all for replying and i'm so sorry for my late reply for i'm quite busy these days. from what all you are saying what i should do is take the voltage difference between the 1ohm resistor (using difference amplifier) give it some gain and then feed it to pin 2 directly (without a resistor) and then ground in 3 via a resistor. by controlling the gain of the diff-amp i should be able to control the maximum current. can you give me an semi accurate low cost op-amp for this. my O/P current range is from 0-1.5A, so the op-amp must be able to detect mV changes. also it should be able to handle up to about 30V at its +,-terminals. (i tried to do it with lm339 and lm358 but it didn't work)

    finally i would also like to know is there a better way to limit the maximum current (i should be able to choose the current limit via a pot).

    thank you !!!!!!!!
  8. t06afre

    AAC Fanatic!

    May 11, 2009
    In this case you can use a LM324, as it can handle up to 32 volt supply voltage. See the datasheet for circuits ideas. http://www.ti.com/lit/ds/symlink/lm124-n.pdf se the "Ground Referencing a Differential Input Signal" or the "High Input Z Adjustable-Gain DC Instrumentation Amplifier" normally I would not recommend building your own difference amplifier. But in this case it will do more than fine. What you can do is to try to match the resistors used as good as you can.
    Did you take a look at simulation result I pointed you to. It will give you an insight in how the current limiter work. Last thing I would perhaps have reduced the current sense resistor to around 0.2 ohm.
    Last edited: Sep 23, 2012
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