lm723 current sensing

Discussion in 'The Projects Forum' started by m3vuv, Jul 4, 2014.

  1. m3vuv

    Thread Starter New Member

    Jul 4, 2014
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    Hi all,im in the process of building a 30a variable psu using ths ic,from what i can see the current limit is set by the input on pin 2 and 3 on the ic,all well and good,now the but!,every schematic i have seen gets the signal for pins 2 and 3 from the voltage drop accros a resistor in series with the output from the pass transistors,i dont like this idea due to the heat dissapated from the resistor,i have a couple of ideas i have been thinking about and would like some comments on them.
    idea 1, put the current sense resistor in series betwen pin 10 (ic output) and the base of the driver transistor for the pass transistors.
    idea 2,stuff one of the leads to the primary of the transformer thru the middle of a torroid ferrite transformer,rectifie the output from that and use that for the input of pin3.
    my aim is to cut down on using resistors in the psu output to sense the current for the ic,any ideas welcome,cheers Paul m3vuv.
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    how about a DC current transformer, or use a very low ohm nichrome wire shunt for the sense resistor.
     
    Last edited: Jul 4, 2014
  3. k7elp60

    Senior Member

    Nov 4, 2008
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    Doing the quick math 0.7V/30A = .0233 Ω with a power dissipation of over 20 watts. I am sure you already figured this. The idea of using the current sense resistor in series with the base of the series pass driver transistor sound like a workable idea.
    I have built a number of lead acid battery chargers and I have used a resistor from the base to emitter of a npn transistor to limit the current. Then used the same resistor to monitor the charge current, amplify the voltage drop and use a comparator to turn on an LED when the charge current drops to a calibrated level. Since the 723 regulator has an internal reference you could do similar thing as I did with a dual op amp. 1/2 to amplify the current and the other to trigger the 723 into current limit.
     
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
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    What was the part number of the IC again?? LM713 is an op amp.
     
  5. m3vuv

    Thread Starter New Member

    Jul 4, 2014
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    my mistake,its a 723.
     
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
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    You don't say what the voltage will be for your supply, but for 30 amps - assuming a linear supply will take several large pass transistors in parallel and probably another to drive all of them since the 723 is only good for 150 ma. What you might think about is lets say you have five 2N3055 in parallel. You will need an emitter resistor of about .1 ohm for each anyway and you can use one of them for the sense resistor.
     
  7. m3vuv

    Thread Starter New Member

    Jul 4, 2014
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    Why didnt i think of that!,it goes from about .8 ov a volt thru to 30v,next job is to sort out some over voltage protection for it,im planing on having a switch and resistor network so i can thro the switch and have a prefixwed 13.8 volts for my transceiver with over voltage protection,to have protection on the variable part of it wouldnt be simple i think,unless there is a way of using a dual gang pot for the voltage adjust and the over voltage protection,ive got a mc3423 chip,i will have to look at that.
     
  8. bertus

    Administrator

    Apr 5, 2008
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  9. ronv

    AAC Fanatic!

    Nov 12, 2008
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  10. m3vuv

    Thread Starter New Member

    Jul 4, 2014
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    I dont want it to blow a fuse,to me thats crude,i have a 50amp relay and was thinking to use that on the output from the psu to switch the load off,also use another relay on the primary of the trnsformer and use a signal inverter to switch off both relays from a +ve signal from the ver voltage sense ic,then have a push button to reset it.
     
  11. ronv

    AAC Fanatic!

    Nov 12, 2008
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    You could do that, but in the grand scheme of things a relay is pretty slow. Usually over voltage is to protect from a failure like a shorted pass transistor so a blown fuse is not an issue. If your looking for something to track where you set the adjustment that's another story.
     
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