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723 psu issue

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m3vuv:
Hi all,im putting together a psu using a 723,im using this schematic,the iss use a lower value sense resistorue i have is,i want the current feed back to work from about 100ma upwards,the issue i have is to get 600mv from the current sense resistor,it has to be a high value to produce 600mv across it for 723,this lowers the output to much overall,is it possible to amplify  the sense voltage to feed the 723 cl and cl pins,therefor use a lower value sense resistor,ive thought of using a sg3532 but they are expensive!,cheers m3vuv.

stj:
why parallel drive?

i have stuff i repair that can do atleast 7A that just uses a single 2n6057 darlington.

m3vuv:
what has that got to do with what im asking??

stj:
it's relevent to the circuit being over-complex.

duak:
Back in the 70's I built a 723 based lab supply that still works to this day.  The downside is that there's no elegant way of having a wide current limit range without switching.  If you have a variable sense resistor, it has to have a large enough value for the low current setting while being able to carry the supply's maximum current.

m3vuv's comment made me think that it should be possible to amplify the developed current sense voltage to feed the internal current limiter transistor.  It seems like an obvious solution, but I've never seen it.  A 1X to 70X variable gain amplifier should be a piece of cake to design.  One of the nice things about the 723 current limiter is that it's very fast in that there is only one transistor's delay in reducing the current.  An amplifier will add some delay.  During high current transients (short circuits, charging capacitors, etc.)  I could see the amplifier winding up and applying too much drive current to the current limiter transistor and then possibly damaging it.  I would add a few parts to limit the output span and drive of the amplifier.

I could also see using the existing current limiter to limit the maximum current, while adding an amplifier and second current limiting transistor in parallel to implement a precision variable current limiter.

Hope this makes sense,

Cheers,

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