Author Topic: 723 psu issue  (Read 6658 times)

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Offline m3vuvTopic starter

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Re: 723 psu issue
« Reply #25 on: December 13, 2018, 12:51:23 am »
hi any schematics of an example i can try?,cheers m3uv.
 

Offline xavier60

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Re: 723 psu issue
« Reply #26 on: December 13, 2018, 01:58:19 am »
hi any schematics of an example i can try?,cheers m3uv.
I have been submitting the results of a design that is being tested on another thread.
The big test will be when I commit the design to a PCB. I won't say much more until it's done.
The original , https://dangerfromdeer.com/tag/power-supply/ , doesn't work properly.
HP 54645A dso, Fluke 87V dmm,  Agilent U8002A psu,  FY6600 function gen,  Brymen BM857S, HAKKO FM-204, New! HAKKO FX-971.
 

Offline coromonadalix

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Re: 723 psu issue
« Reply #27 on: December 14, 2018, 08:13:38 am »
normally  if you add an power diode in serial or the sensing resistors,  you'll get more voltage to control your current limitter, but you will lose more voltage at the output

To negate this,  you have to put the current control circuit before the power outputs collector.  That way the lm723 is managing / regulating the voltages fluctuations better, and you have less voltages drop at the output.

See the attached photos for an example, i think the l146 can be subbed by the lm723

I have an very old lm723 design similar to this, but have more parts in the current limitter section,  i cant find my schematics anymore, it use the foldback regulation technique.

It can drive leds down to 10-20 ma ... up to many amps ...

You have the same applicable principle to the 3 legged variables regulators too ..
« Last Edit: December 14, 2018, 08:22:33 am by coromonadalix »
 

Offline coromonadalix

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Re: 723 psu issue
« Reply #28 on: December 14, 2018, 08:36:44 am »
oh and check this attached photo  for negating the minimal 1.2 volt minimum at the output,   the lm723 internal design will never get to 0 volts at the output.
 

Offline Kleinstein

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Re: 723 psu issue
« Reply #29 on: December 14, 2018, 11:13:02 am »
The circuit with the negative supply to get an output voltage down to zero has 2 serious drawbacks:
1) the reference voltage is now to a large part the external zener diode to stabilize the negative supply - this would likely be rather poor quality compared to the usually rather good 723 internal reference. In the shown circuit there would also be quite some ripple from the negative supply.

2) The negative voltage adds to the supply of the 723 - so the maximum voltage would be lower by some 5 V.  The voltage to the 723 is already too high without it, if 30 V transformer is used.
 

Offline coromonadalix

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Re: 723 psu issue
« Reply #30 on: December 14, 2018, 11:48:07 pm »
The - 1.2 volts is only applied to the voltage potentiometer, the 723 will maintain its stability,  this montage has been proven effective, if not  the other schematic is good too, i dont know many projects who need less than 1.2 volts ??  and for the - 1.2 volts you can use Ldo regulators too,  better stability.

If you tweak the pin 5 and 6 on the 723  you may get lower than 1.2volts,  i could go down to almost 1 volts.

Normally there is no other way to counter act the minimal output voltage unless you do something like this.  If not  you can build an power supply with op amps
like some mastech psus who goes down to a real zero volts, the circuitry is not very complicated to replicate.


You can have or build this circuit   Velleman k7200 psu, its a very well build circuit,  1x 723 as the voltage regulator and 1x 723 for the current regulator.
https://www.velleman.eu/downloads/0/manual_k7200.pdf

Having a relais on a tapped transformer will help to minimize the disspipated temperature if you set low voltages.

your 2) assumption is wrong in part,  the -1.2 volt will be substracted in the potentiometer voltage (and 723 feedback circuit) if set at the minimum, you will get to zero volts, and it will be -1.2v  less than any voltage set for sure.

The important thing is never to get past 40vdc for the 723 supply pin,  25vac is going near that 40vdc (Full bridge rectifier) if you want to get past the 35-37volts output, you have to use an floating lm723 design. There is some 723 circuits on google who goes at 70vdc.

You have the rare 3 legged pin tl783 who does 1.2 to 125 vdc at the output, i managed to get a few of them, and build some psu, but the dissipated heat is enormous.

Thats why for now i use already build Ebay snatched  psu's like Sorensen, Kepco Ate 75-8, an my newest Kikusui Pad70-5, they use ac phase scr controlled bridge rectifiers,  they will dissipate only the required heat (power) no more.

Still my old 723 based psu is rock solid, but if i compare them for the same voltage and current settings, the 723 based one dissipate a lot more heat, i have active variable speed cooling on it snatched from an atx psu, and all my 2n3773 are matched in the HFe (gain)  to have equilibrium in their dissipation on the heatsink.
 

Offline Kleinstein

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Re: 723 psu issue
« Reply #31 on: December 15, 2018, 10:09:12 am »
The last circuit diagram shows a -4.7 V supply to the LM723. This would reduce the maximum output voltage by about these 4.7 V.  There is a much more elegant way to make the 723 regulate down to 0 V (actually some 100 mV open circuit, depending on the minimum load): just add a a small fraction (about 1.5 V at the divider) of the reference voltage to the feedback divider.  The voltage is than adjusted at the non inverting input. As a positive side effect this avoids a loop gain that depends on the set voltage. This happens if the voltage is adjusted with a variable resistor in the feedback.

The 40 V maximum voltage of the 723 normally (no extra measures to limit the maximum voltage) means the transformer should not be higher than about 25 V. Under low grid conditions this gives a raw voltage of close to 30 V at the positive end. With some 10% ripple and about 3 V dropout for the 723 the maximum output voltage would thus be at around 24 V under load. To get a little more one could use something like separate filtering for less ripple at the 723 without using excessive filter caps for the main load current. Another point would than be using voltage limiting for the 723 so that a 27-30 V transformer could be used.
 

Offline coromonadalix

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Re: 723 psu issue
« Reply #32 on: December 15, 2018, 01:30:01 pm »
here's a classic project :  with op amps and goes down to zero

http://electronics-diy.com/50v-bench-power-supply.php


here's an 70v 2amp psu with an 723

https://www.eleccircuit.com/0-70-volt-2-amp-dc-variable-power-supply/

The current limitter is before the collector output transistor.


My psu is somewhat similar to this one:  with a few tweaks,  variable current adjustment and an added power diode in serie with the sense resistor to go down at 10-20 ma, enough to power a led without blowing it.
https://www.eleccircuit.com/wp-content/uploads/2014/06/The-circuit-diagram-of-0-50V-3A-Variable-DC-power-supply.jpg
« Last Edit: December 15, 2018, 01:31:55 pm by coromonadalix »
 

Offline Dzambaz

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Re: 723 psu issue
« Reply #33 on: June 13, 2019, 06:10:39 pm »
The power supplies with 723 that use an auxiliary negative power supply with a Zener-diode to achieve 0V (as in the proposed schematic) have a fundamental design flaw, because any deviation of this auxiliary voltage will be seen as a deviation in the output voltage.
Since this schematic is still suggested as a solution in many forums for electronics, I recently made a video with several practical demonstrations and voltage measurements to provide better understanding of the issue:

« Last Edit: June 13, 2019, 08:06:26 pm by Dzambaz »
 
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