Author Topic: How do make a current limiting knob using LM723 for Linear Reg power supplies?  (Read 25842 times)

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Offline GurumurthyTopic starter

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Hi to all!!!

Currently I am trying to construct Laboratory type Linear Power Supply with LM723 IC. How do make a current limiting knob using LM723IC and my Specification Were 0-30V DC and 0-5A Current ? I am much concern about Current Adjust knob !!!!
« Last Edit: May 17, 2017, 04:16:53 pm by Gurumurthy »
 

Online Kleinstein

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The current adjustment on the 723 circuit is usually not that great, but might be still acceptable. There are several options, depending on how accurate and how fast the current limiting should be. Depending on the circuit used, the adjustment range can be rather limited (e.g. 100 mA to 1 A). This is because the original circuit inside the 723 uses just the transistor base-emitter voltage as its reference. One can reduce the current setting by adding a part of a diode drop to the voltage, but this is limited, as the drift will be relatively high.
So the current limit regulation is more like an over current limit for protection, adjustable by choosing the right shunt.

There are a few circuits that use a shunt at the low side and an extra OP for the current limit - usually more accurate, but also slower.
 

Offline GurumurthyTopic starter

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Thank you sir. I need schematics to understand sir.
 

Online Gyro

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You should find the original uA723 Application notes useful:

http://www.ve6aqo.com/Manuals/uA723_Application_Note.pdf
Best Regards, Chris
 
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Online David Hess

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A wide range variable current limit is best done by using an external error amplifier which pulls the frequency compensation node down; the output of the internal 723 error amplifier is designed to accommodate this.  I would keep the existing Vbe based current limiter and set it for maximum overload current but that is up to you.
 

Offline GurumurthyTopic starter

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Sir, It would be meaning full and I will get an insight if circuit is given.... Sorry sir i am new to power supply design.
 


Offline oldway

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Look at the solution choosed by Philips in his triple power supply PE1542...(schematic fig 1001).

http://www.analogdesign.be/files/gear/Philips_PE1535-PE1545_Operating_Manual.pdf
 
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Offline GurumurthyTopic starter

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Thank you sir. I need some practical circuit to build my power supply.
« Last Edit: May 18, 2017, 05:14:43 pm by Gurumurthy »
 

Offline oldway

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Thank you sir. I need some practical circuit to build my power supply.
Did you look the operating manual of the link I gave to you ?
At the end of this operating manual, you have the schematics with the values of all the components.
Is that not a practical circuit to build your power supply ?  |O

 

Online Kleinstein

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The Phillips circuit looks very reasonably, and it is a tested circuit.

Just keep in mind that the pin numbers on the 723 are for the metal can. Today one will likely get the DIP version. So a little work finding the corresponding pins is still to do. The BDY20 used can be replaced by a more common 2N3055 or similar. The only thing a little confusing is finding out which pots are for setting the voltage / current by the user and which are just trimmers to set the voltage / current range.
 
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Offline oldway

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The trimpots are indicated by a sign that looks like a hammer, the potentiometers do not have this mark.
 
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Offline GurumurthyTopic starter

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Hi Sir,
        Can you please explain me this circuit. I cant understant Current Limiting Circuit.
 

Offline oldway

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Off topic....See videos of Dave about power supplies....
 

Offline BravoV

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Is that not a practical circuit to build your power supply ?  |O

Off topic....See videos of Dave about power supplies....

The OP is well known to spam the whole forum again and again with new thread on the same subject, when he didn't find any schematic that he likes, or no one answer him within a day. Looks like he doesn't like circuit that has too many components, or circuit that uses components that are not in his drawer.

Just read his past created threads.  :palm:

Also I did ask him a simple yet very important question on his 1st thread, but it seems like he ignored me.  :-//

I'm guessing that what he want is only a circuit that consist of max 2 transistors and few capacitors and a LM317 to build an adjustable 0-30V and 0-5A complete with with CC and CV feature.

Watch for his upcoming newly created thread as he doesn't like to use the 723 anymore.  :-DD
« Last Edit: May 19, 2017, 06:25:12 am by BravoV »
 

Offline GurumurthyTopic starter

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 Hi bravo,
            Yes bravo , I want to  build power supply with integrated circuits than discrete devices. But I know that power handling capability will be small.

        I want the circuit which will give understanding , so that I can trouble shoot the circuit when problem arises.


         I need the simple circuit which will control the current. The circuit should teach me how to control current. I don't want exact circuit with lm317 or lm723. So that I will be knowing what is happening and what I am doing.

       Mr. Old way has referred nice manual. Sure I will go through it.

      When the problem is small, we can find the solution easily rather than blinking with huge chunk. Hope u will understand sir. Correct me if I am wrong...


     Lot of thanks to mr.bravo , mr old ways and all those good heart who replied to my post.

     
 
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Offline oldway

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I think that Gurumurthy tries to do the squaring of the circle!  |O

Already, a linear supply of 0 to 30V with a max current of 5A, this represents a dissipation of 35V x 5a, that is 175W, it is totally unrealistic.   :palm:

You will have to use a way to reduce dissipation as a pre-regulator or switchover of transformer taps.

And in addition, he wants a simple circuit ....:-DD

I think he should open a topic in the "beginner" section and start by studying a little bit of electronics first.

It is clear that he does not know what he wants, nor if what he wants is possible or not ...   :scared:

There is no shame in being a beginner, we are here to help them, but at a minimum, the person should realize that he is a beginner and to have enough modesty to recognize it.
« Last Edit: May 19, 2017, 08:17:19 am by oldway »
 

Online Kleinstein

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Adding current limiting to a regulator like LM317 or LM350, that is not made for it is usually not a good idea. So explaining the function of those circuits does not make much sense as they don't work well anyway. Dave's video about building a supply around an LT3081 (or similar) has the same problem to start with. So if you want to build a lab supply, forget about LM317 and similar chips with integrated power stage.

The advantage with the 723 circuit is that understanding of the circuit is relatively easy, as there is no magic black box with not so well defined properties. The Phillips circuit is still relatively simple (not too many parts), though it has quite a lot of background behind it - so a good example to learn from. So it is really worth trying to understand that circuit.

A supply circuit to learn from should be more like 1 A - the parts a cheaper and less smoke to escape. For learning, the small one is just as good as a larger one.
 

Online David Hess

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Can you please explain me this circuit. I cant understant Current Limiting Circuit.

The LM317/LM338/LM350 wants to maintain 1.25 volts between its output and adjustment terminals.  R1 could be made to be adjustable however potentiometers which will support 1.5 amps and more through their wiper connection and with the necessary power rating are very rare.  So instead, R2 is used to add a voltage to the voltage across R1 and the LM350 acts to keep the sum of the voltages equal to 1.25 volts.

The 2N5640 operates as a constant current sink holding the voltage across R2 at 1.25 volts.  As the potentiometer is adjusted downwards, the voltage across R2 replaces the voltage across R1 and the output current decreases.
 

Offline oldway

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It is not possible to use a LM317/LM338/LM350 for a 0-30V 0-5A labo power supply because it exceed the absolute max ratings of the device.
input-output differential voltage: +35V max.
Dropout voltage: 2.5V at 3A (-25°C)
Min input voltage at max current and min mains voltage: 32.5V
Mains voltage variation: +/-10%
At max mains voltage (+10%) full load: 32.5 +20% = 39V
At max mains voltage no load: above 40V....exceed the absolute max ratings of 35V

Max power dissipation at full load and max mains voltage with output in short circuit : 39V x 5A = 195W  :palm:

It far too high, you have to use a tap switching by relays or a preregulator.....

The Philips power supply is only 0-20V 0-1A.....
« Last Edit: May 19, 2017, 09:33:20 am by oldway »
 
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Online Kleinstein

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The Phillips instructions include a view slightly different versions: one for 0-7 V and higher current and also one that is for 20 V and likely about 2 A. But it is relatively straight forward to change that.

For a high power at least transformer tap switching should be used - but high power should only be version 2 or 3.
 

Offline oldway

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Many engineers have been working on the project of a 0-30V 0-5A linear lab power supply and the result of their work is what is found on the market ....

Are they all fools or stupids ?
Do you think you can do simpler, cheaper, more reliable ?
 

Offline Ian.M

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The Fig. 1000 circuit (page 20 of Philips PSU manual, Oldway's Reply #7 link)  is about as simple as it gets fror a real PSU that wont let the holy smoke out the first time you connect it to a difficult load.

The specs on page 5 say: PE 1535-00  0V - 40V 0A - 0.5A.

Beefing it up to a 5A supply would be possible with a better pair of pass transistors and a preregulator feeding the pass transistors.  However that increases the complexity again . . . . .
« Last Edit: May 19, 2017, 10:57:55 am by Ian.M »
 

Offline oldway

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These Philips power supplies were highly appreciated for their robustness and ease of use.

However, this kind of circuit only applies to limited output power.

For larger power (400W and 1000W), Philips used a thyristors pre-regulator, but their schematics had major design shortcomings and errors, so I do not recomend to use their circuits.

https://elektrotanya.com/philips_pe-1642_1644_1646_1648_0-20-40-75-150v%2C0-20-10-6-3a_400w_bench_psu_1987_sm.pdf/download.html

 

Online Kleinstein

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The circuit diagram for the HY3002 has one obvious mistake in the drawing: the emitter of V26A should be at ground level.

To make it cheaper one could use an LM324 for all the OPs and maybe leave out the extra TL431 reference and use the 78xx instead. Chances are one could eliminate the -6V or -12 V internal supply. Still this would be only marginal savings.

The voltage regulation looks a little odd. The way of adjusting the feedback divider usually means the loop gain depends on the voltage setting and thus a compromise type performance. It might lead to instability for the combination of low voltage setting and highly capacitive load.

The phillips circuit for the 7 V/3 A version with two transistors in parallel could be a problem as show - just add emitter resistors and it should be fine. A slightly higher power version (like 30 V 2 A) should be OK -it just needs a lot of heat sink.
 


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