Electronics > Projects, Designs, and Technical Stuff
723 psu issue
coromonadalix:
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.
Kleinstein:
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.
coromonadalix:
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
Dzambaz:
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:
Navigation
[0] Message Index
[*] Previous page
Go to full version