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Old 1st April 2014, 08:40 PM   #1
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Default lm723 variable supply schematic question

I keep seeing this schematic when looking for a variable supply. I was told on another thread (that i cant seem to find now) that i should look into lm723 for a variable bench supply. so as one of my wishes is to power universal heaters for a bunch of valves i plundered, i would like to give this circuit a go as it is 0-70v. But looking at the comments at the bottom of there page i wonder if the only issue is that there is an error with the pin numbering 10 and 11. Please can someone look this over and give thoughts there thoughts on this design

http://www.eleccircuit.com/0-70-volt...-power-supply/
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Old 1st April 2014, 09:40 PM   #2
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Hello jimmiegin
Funny, I am right now in the same process as You, regarding making a 0 - 30 Volt 3 Amp. powersupply, and I have looked a lot of places to find the best circuit. A lot of people, including those, that has made the application notes, has omittet the drawbacks of the old, but fine uA723.
Let us make a brainstorm on this topic!
First: 2 Amp, and a raw supply of more than 70 Volt, that gives more than 150 Watt dissipating power, if the output is short-circuitet. I would use no less than 6 pcs. 2N3055 as pass transistors.
Regards Soeren Poulsen Denmark
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Old 1st April 2014, 11:34 PM   #3
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Now i have looked i can see there is only a couple of changes to the 70v from the 30v supply. A couple of minor resistor values and the 50v transformer =70vdc as you pointed out. Is a transistor power disipation based on using no heatsink? And are you saying you would use 6 times 2n3055?. I think the power disp is 90w for 2n3055. Is this circuit even worth looking at?
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Old 1st April 2014, 11:41 PM   #4
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What about this one
0-30 VDC Stabilized Power Supply with Current Control 0.002
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Old 2nd April 2014, 02:21 PM   #5
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Hello jimmiegin.
I know that circuit too. it is good, but it has one minor issue: The operational amplifiers can only cope with +/- 18 Volt at the maximum, and that is 36 Volt from + to -.If the - pin is held at -6 Volt, the + pin may at max be 30 Volt.
As the pass transistors is configured as NPN darlington transistors, the output of the integrated circuit must go 2 Ube voltages higher, and that limits the output voltqge to 30 Volt - (2X0,65Volt)! If instead the constructor had used a PNP driver, the circuit would not have this limitation, but then there comes other issues! Still, let us go for the old uA723, because when we know all the limitations, we can come around them, and get a great powersupply!
The power dissipation in the pass transistor has nothing to do, with the power dissipation the datasheet says as the maximum. If You want 70 Volt out, You got to have at least 75 Volt raw supply, and shorting the output, pulls the maximum current of 2 Ampere, that gives 150 Watt.
That would cause one single power transistor to be glowing red!
An other thing, I forgot is, that the good old 2N3055 has a maximum Uce of 70 Volt, so we can not use it for that high a voltage. It calls for a 2N3773. - and more of them!
Regards Soeren
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Old 2nd April 2014, 02:29 PM   #6
Elvee is offline Elvee  Belgium
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The circuit p12 of this AN is much more serious: http://www.jaycar.com.au/images_uploaded/FN817.PDF

It can be scaled to basically any current/voltage with suitable components
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Old 2nd April 2014, 02:46 PM   #7
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Hello Elvee.
That circuit looks fine, and You are right, it can be configured at almost any high voltage, because it has the Q4 on the output of the IC. That is exactly, how I am intending to use the old uA723. NPN pass transistors, and a PNP driver for the pass transistor, then the uA723 must pull low, when more output current is needed. Then again, we must respect the maximum Uce of the output transistor in the uA723. Regards Soeren Poulsen
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Old 2nd April 2014, 08:03 PM   #8
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Soeren Poulsen and Elvee you input is very much appreciated. Please could you/someone tell me in dummy terms if....1. The 723 circuit could remain basic the same but with the replacement and addition of exe times 2n3773 and how this may be configured to make the thing workable. And 2. If the tl081 circuit would function as desired if higher voltage op amps where used instead?. And lastly, if I where to go for the fig 14 option in datasheet link Elvee posted, what additional transistors would be required to give 2 amperes max current please? Thank you guys.

Last edited by jimmiegin; 2nd April 2014 at 08:16 PM.
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Old 3rd April 2014, 05:07 AM   #9
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I took a look at the Intersil CA3130 design for .1 to 50vdc output Elvee posted. Does anyone know what is the limiting factor for preventing the output to going to zero volts.
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Old 3rd April 2014, 05:42 AM   #10
WSJ is offline WSJ  United States
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Quote:
Originally Posted by powerbob View Post
I took a look at the Intersil CA3130 design for .1 to 50vdc output Elvee posted. Does anyone know what is the limiting factor for preventing the output to going to zero volts.
It looks like Vsat of Q4. A FET in place of Q4 could bring Vout to 0.
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