Author Topic: 2x LM723 max safe voltage for this circuit ?  (Read 376 times)

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Offline MathWizard

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2x LM723 max safe voltage for this circuit ?
« on: April 18, 2021, 07:51:23 pm »
I found someone on here point at this LM723 PSU w/ 1 723 for current limiting and another for the voltage control.

http://electronics-diy.com/electronic_schematic.php?id=964


I have a transformer here that has a 12V rail, for the control, but the other winding is more like +54V unloaded, and is rated 36V @1.5A . I have TIP142t so they are rated for 100V, but what about the LM723, is there any way for it to get exposed to too much voltage ?

I found an LM723 model for ltspice and 1 for the TIP142, so far it seems it should be fine, but I can miss a lot.

http://ltwiki.org/files/LTspiceIV/lib/sub/Lm723.sub
http://ltwiki.org/images/4/49/TIP142.sub
 

Offline dietert1

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Re: 2x LM723 max safe voltage for this circuit ?
« Reply #1 on: April 18, 2021, 08:27:22 pm »
Only in transients overvoltage may enter from the high voltage part of the circuit. To be sure, you could protect the two IN+ pins of the LM723s with zeners. Also i'd guess the circuit will oscillate while current limiting. For stability you may need a larger output cap - which contradicts current limiting..

Regards, Dieter
 

Offline MathWizard

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Re: 2x LM723 max safe voltage for this circuit ?
« Reply #2 on: April 18, 2021, 11:14:11 pm »
Ok thanks, for now I only need a few 100mA, so I'll try it on the breadboard next. I'm wasn't planning on a 36V 1.5 PSU, so I can always get another transformer later.
 

Offline coromonadalix

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Re: 2x LM723 max safe voltage for this circuit ?
« Reply #3 on: April 19, 2021, 12:10:01 am »
@ dieter1   it will not oscillate, the 2x lm723  fc (grequency compensation) pins are occupied with capacitors  their job is to cut the oscillations

The 2x lm723 operate as a floating regulator section over/superposed the main one (red lines)   there's a limit to the output capacitor, a much bigger value may cause problems,  stick to the schematic, no need to add zenners to the +IN  pins

The 2x lm723 operate in a safe range, since they float over the main line,  if you change the main line voltage the voltage (vsense) potentiometers may need to be recalculated ??

The only thing i would add is a fast switching diode on the emitter collector of Q01  to add reverse voltage protection.
« Last Edit: April 19, 2021, 12:12:37 am by coromonadalix »
 
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Offline dietert1

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Re: 2x LM723 max safe voltage for this circuit ?
« Reply #4 on: April 19, 2021, 05:32:28 am »
That's exactly the problem: In many cases the load includes buffer capacitors, so there you are with oscillation. This problem is inherent in all circuits with the main pass transistor in emitter mode (like this one). You may want to take a look at the typical output circuit of an audio amplifier, with an inductor, a snubber cap and two resistors. Often enough circuits proposed by hobbyists are half ready..
By the way, not all LM723 do have that zener output pin. If i remember correctly it does not exist on the hermetic metal can version.

Regards, Dieter
 

Offline m3vuv

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Re: 2x LM723 max safe voltage for this circuit ?
« Reply #5 on: April 19, 2021, 08:45:32 am »
i was going to build this but found the pcb artwork crap with the stupid colors for toner transfer
 

Offline Kleinstein

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Re: 2x LM723 max safe voltage for this circuit ?
« Reply #6 on: April 19, 2021, 10:24:46 am »
The circuit with the floating regulator usually needs some capacitance at the output. How much depends on the details - one may get away with some 100 µF if really tuned well maybe 10 µF. Fine tuning may want an extra resistor in series to the compensation cap for the voltage mode.  Usually one wants a combination of some 100 nF as a film cap (low ESR) and than the larger cap with some more ESR (e.g. normal electrolytic, not a special super low ESR one).

The way the voltage is set also effects the loop gain. So the response would be most crtical if set to a low voltage. It may be better to change the voltage set point to be on the reference side and not chaning the feedback divider, like it would be the natural choice if using a DAC. The way as shown was popular in the old times, but it is the more problematic choice.

It is hard to tell if the current mode part may oscillate - it has the added transistor from the CV part and for this reason may be a little more tricky. Normalle the current regulation is not that critical in the configuration with the floating regulator.
 
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Offline coromonadalix

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Re: 2x LM723 max safe voltage for this circuit ?
« Reply #7 on: April 19, 2021, 10:34:05 am »
The "can" doesn't have the zenner, it could not be used in high voltage output circuits unless you added it externally

An amplifier is not an power supply, if you add snubber circuits, inductors etc .. it will affect the regulator response and efficiency


Many professionals / brands psu's are based on the lm723, and i never saw any oscillations unless the device under test has/have big isssues

Build many psu with the "723"  and they have passed the test of time with many capacitives and inductive loads.

Every psu design has it's limit,  even professional brands have them,  i have many models at home, my old 723 design has enough sensitivity it can power up a led with no current limiting resistor, can go from a mere 5ma to 10 amps, but at the time i did not put a switched secondary x-former taps circuit, it can dissipate a lot since i operate the 723 at almost their vcc limit.

The 2x 723 circuit is fine with an minor adjustment to my eyes, it will depend how the OP want to use it.

 

Offline MathWizard

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Re: 2x LM723 max safe voltage for this circuit ?
« Reply #8 on: April 20, 2021, 08:14:04 pm »
So far I built a board for the 2x recitifer's and bulk filtering, and then I was playing around with the spice models, and I've had that oscillating, so I started to breadboard it anyways.

But then I started a L200 PSU, to power the above, because lately I've been running my stereo off my store bought bench PSU. So the L200 voltage regulation is working great, but not the current limiting yet.
 


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