Author Topic: 10 Amps power supply with LM723  (Read 3070 times)

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Offline mike_mike

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10 Amps power supply with LM723
« on: November 03, 2018, 10:10:13 pm »
Hello,

I am planning to build a new power supply using LM723 and NPN power transistors. I saw that there are a lot of schematics on the internet, but I am wondering which of them is a good one ?
So my questions are:
1. Can you recommend a good 10 Amps power supply with LM723 ?
2. Is there the possibility to appear oscillation in this power supply that you recommended ? How can I prevent the oscillations ?
3. What are the layout guidelines for such a power supply ?
4. What other problems can appear in the circuit ?
« Last Edit: November 03, 2018, 10:14:49 pm by mike_mike »
 

Offline Wolfgang

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Re: 10 Amps power supply with LM723
« Reply #1 on: November 04, 2018, 12:40:02 am »
The ARRL (American Radio Relay League) has some 30V/10A supply suggestions.

Important things are:
- adequate cooling (fan)
- bridge rectifier with sufficient current loading capability (20A min)
- 4 x 2N3055
- frequency compensation (see ARRL schematics)
- overtemp protection
- output crowbar to protect your load if something goes wrong.

http://www.dd1us.de/Downloads/an%20rf%20proof%2030%20amps%20power%20supply%201_0.pdf
 

Offline mike_mike

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Re: 10 Amps power supply with LM723
« Reply #2 on: November 05, 2018, 03:16:22 pm »
Thanks for the document. It is very interesting.
I built the power supply from the link: http://www.electronics-lab.com/project/3-30-v2-5-a-stabilized-power-supply/
The single difference was that I used 0.33 R in parallel with 0.18 R for R5.
I tested the power supply using a 24V ac / 100VA transformer.
The output voltage was 19.97Vdc without load and with load it was 19.94Vdc. The load was a resistive load of 2.90A.
It is normal the difference of 0.03V between the output voltage with and without load ?
The output voltage was measured on the divider formed by R4 and R3.
« Last Edit: November 05, 2018, 03:17:55 pm by mike_mike »
 

Offline imo

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Re: 10 Amps power supply with LM723
« Reply #3 on: November 05, 2018, 03:36:03 pm »
I would add a small resistor, like 100ohm, into TR1's base.
30mV change with 2.90A load? -> the 723 NS datasheet says the load regulation with an external NPN pass transistor is 15mV per 1A change. With thin wires and larger currents it could be even more..
PS:
Quote
10 Amps power supply with LM723
with your 0.18ohm || 0.33ohm the over-current protection trips at something like 5.5Amperes..
« Last Edit: November 05, 2018, 03:56:27 pm by imo »
 

Offline mike_mike

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Re: 10 Amps power supply with LM723
« Reply #4 on: November 05, 2018, 07:22:14 pm »
Can you be more explicit ?
I saw in the datasheet that load regulation is maximum 0.6% of Vout, so in my case it is about 0.11V (0.6% * 19.97V). Is that correct ?
« Last Edit: November 05, 2018, 07:35:14 pm by mike_mike »
 

Offline imo

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Re: 10 Amps power supply with LM723
« Reply #5 on: November 05, 2018, 07:36:05 pm »
Read above datasheet. Figure n4.
 

Offline mike_mike

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Re: 10 Amps power supply with LM723
« Reply #6 on: November 05, 2018, 07:58:47 pm »
I don't know which one is fig n4. The figures does not have a number.
Are you talking about the attached one ?

Thank you.
« Last Edit: November 05, 2018, 08:41:47 pm by mike_mike »
 

Offline imo

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Re: 10 Amps power supply with LM723
« Reply #7 on: November 05, 2018, 11:52:01 pm »
Figure 4 means Figure 4 :)
 

Offline mike_mike

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Re: 10 Amps power supply with LM723
« Reply #8 on: November 14, 2018, 09:34:50 am »
I have a question regarding the LM723 power supply. I built this power supply on a PCB that was made in China.
I verified the output voltage drop with different load currents at different output voltages. BUT, before making this verification, somebody tried to take out the PCB from the power supply case. The PCB was glued inside the power supply case using hot glue. He tried and managed to take out the PCB using a screw driver that he put underneath the power supply PCB and then he lifted up the PCB. I checked the PCB visually, with the naked eye and I did not found any problems.

My questions are:
1. It is possible for this circuit to work correctly if it was damaged by the screwdriver action ?
2. It is possible that if I will build another power supply (using exactly same schematic and a exact copy of the PCB) the circuit to work bad ?
3. It is possible for this circuit to work good because if it was damaged and to work bad if it was not damaged ?
« Last Edit: November 14, 2018, 09:37:30 am by mike_mike »
 

Offline imo

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Re: 10 Amps power supply with LM723
« Reply #9 on: November 14, 2018, 09:46:42 am »
Specify your voltage drops at different loads.
 

Offline mike_mike

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Re: 10 Amps power supply with LM723
« Reply #10 on: November 14, 2018, 09:52:23 am »
V without load = 19.84V, V with load = 19.80V, load = 2.90A
V without load = 25.0V, V with load = 25.0V, load = 2.20A.
for the tests, I used a 0.33R resistor in parallel with the 0.18R.

So the circuit PCB was forced out of the power supply case, using the screwdriver, but on the PCB I did not found any problems while I was inspecting the PCB using the naked eye.
« Last Edit: November 14, 2018, 10:31:22 am by mike_mike »
 

Offline imo

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Re: 10 Amps power supply with LM723
« Reply #11 on: November 14, 2018, 02:09:41 pm »
What is your problem then? It seems it works fine.
 

Offline mike_mike

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Re: 10 Amps power supply with LM723
« Reply #12 on: March 18, 2019, 06:23:18 am »
Hello, quick question about the attached schematic: somebody said to me that the current limit (current sense) is temperature dependent. Can you explain why is this happening ?
 

Offline imo

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Re: 10 Amps power supply with LM723
« Reply #13 on: March 18, 2019, 07:56:28 am »
In 723 the current sense is done via an NPN transistor on the chip (like your Fast CC). With temperature the trigger shifts -2mV/degC. Calculate what current do 2mVolts represent on the shunt resistor and you get the dependency in mA/degC.
 
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Offline lycralout

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Re: 10 Amps power supply with LM723
« Reply #14 on: March 18, 2019, 08:12:27 pm »
 
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Offline mike_mike

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Re: 10 Amps power supply with LM723
« Reply #15 on: April 10, 2019, 02:11:35 pm »
Hello. I build the power supply from the attached schematic, but I found that there is a problem regarding the current limit control.
If I short between the pins 2 and 3, the output voltage varies from 1V to about 30V. But if I connect pins 2 and 3 to the circuit, the voltage varies up to about 12V. I also tried to use an external NPN small power transistor (BC548) and I found the same problem is happening again. I used only POT1, POT2 and POT3 from the original schematic. I did not used any Ammeter, I just connected a wire. The BE voltage of the small power NPN transistor is always about 0.55-0.6V.
I am uploading the original schematic: original_schematic.png
And I am uploading the schematic when I used the external small power NPN transistor: schematic_img.png.

More details about the problem:
1. Input voltage 33Vdc, from 24V/160VA transformer.
2. Voltage on pin 12: about 33Vdc.
3. Voltage on pin 11: about 32.5Vdc.

I tried the following solutions:
1. I changed the potentiometers with some new ones, but the problem is the same with the new pots.
2. I checked the PCB for bridges and it is OK.

What should I do to make this circuit work correctly ?
« Last Edit: April 10, 2019, 02:13:25 pm by mike_mike »
 

Offline imo

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Re: 10 Amps power supply with LM723
« Reply #16 on: April 10, 2019, 03:28:26 pm »
In the original schematics if you short 2 and 3 the current limit is off.
If 2 and 3 is wired into the circuit and it triggers at 12V it means at 12V output the current through the shunt 0.15ohm is high enough to create 0.6V voltage drop on the shunt and open the the current limit transistor.
Probably a problem with your wiring or shunt value.

You do not use 723's pins 2 and 3 in your schematics (eagle). The external transistor T4 is doing the limiting. So let 2 and 3 shorted (and connect to COMP maybe).

Double check the voltage drop at the 0.15ohm shunt while varying the output voltage.
Doublecheck the value of the 0.15ohm shunt!
« Last Edit: April 10, 2019, 03:51:11 pm by imo »
 

Offline mike_mike

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Re: 10 Amps power supply with LM723
« Reply #17 on: April 10, 2019, 04:20:43 pm »
I changed the 0.15R resistor with a new one and I checked the voltage on the 0.15R resistor and it is about 2.8 miliV, when the output voltage is 12V.
The BE voltage of the transistor is still 0.55-0.6V.
I connected the pin 2 and pin 3 together but the situation is the same, the output voltage is still about 12V, and sometimes it is varying around 12V.

Edit: When I powered again the power supply, the voltage was about 17V and not 12V as it was when I powered it initially.
When I set the output voltage at about 2.1V, the voltage on the 0.15R resistor is 2.1mV.
I also changed LM723 with a new one, but without succes.
« Last Edit: April 10, 2019, 04:26:48 pm by mike_mike »
 

Offline imo

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Re: 10 Amps power supply with LM723
« Reply #18 on: April 10, 2019, 04:41:03 pm »
I changed the 0.15R resistor with a new one and I checked the voltage on the 0.15R resistor and it is about 2.8 miliV, when the output voltage is 12V.
The BE voltage of the transistor is still 0.55-0.6V.
How you can have BE=0.6V when the voltage at the shunt is 2.8mV??
It has to be the same when CL(base) and CS(emitter) are wired to the shunt..

« Last Edit: April 10, 2019, 04:47:50 pm by imo »
 

Offline mike_mike

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Re: 10 Amps power supply with LM723
« Reply #19 on: April 10, 2019, 05:05:34 pm »
I found that the 0.5-0.6V voltage drop at BE junction is because there is 0.5-0.6V voltage drop on the 120R resistor ...

I changed the 120R resistor with a new one, but it did not changed the situation.
Somewhere on the PCB is a component that makes the BE jonction to be at 0.6V... i think.
« Last Edit: April 10, 2019, 05:11:07 pm by mike_mike »
 

Offline imo

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Re: 10 Amps power supply with LM723
« Reply #20 on: April 10, 2019, 05:20:51 pm »
When the pot's wiper is at the lower side the BE voltage cannot be higher than 0.16V.
The Vref is say 7.2V and the divider is 120/(4k7+500+120)*7.2=0.16V.
 

Offline mike_mike

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Re: 10 Amps power supply with LM723
« Reply #21 on: April 10, 2019, 05:40:24 pm »
Yes. When the pot is at lower side and the output voltage is at minimum, then the BE voltage is 0.165V.
But if I rotate the Voltage pot, then the BE voltage increases slowly from 0.165V to 0.595V, even if the current pot is at the lower side.
 

Offline imo

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Re: 10 Amps power supply with LM723
« Reply #22 on: April 10, 2019, 05:55:36 pm »
There is a wiring problem then.
When increasing the output voltage, AND the current through the shunt is low (say 10mA), the BE voltage should not change much.
With set constant 0.165V at the current limit pot you would need 3Amps through the 0.15ohm shunt to get 0.6V at the base-emitter.
« Last Edit: April 11, 2019, 01:33:20 pm by imo »
 

Offline mike_mike

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Re: 10 Amps power supply with LM723
« Reply #23 on: April 11, 2019, 03:28:54 pm »
I built another schematic, and I hoped that it will work. But it doesn't and I do not know why.
When I power up the circuit, the 500R trimmer gets hot and the output voltage is always about 32.5-33Vdc. I changed the LM723 with a new one, I changed the 10K pot with a new one, I changed the power transistor (for the tests I used only one power transistor) with a new one. I also checked the layout and the PCB and there are no problems.  :palm: :palm: :palm:
This schematic is 100% functional, it was build by someone from my country and it works.
What should I do ?
« Last Edit: April 11, 2019, 03:48:05 pm by mike_mike »
 

Offline imo

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Re: 10 Amps power supply with LM723
« Reply #24 on: April 11, 2019, 03:40:03 pm »
I've done a simulation of your original schematics above and it works.
FYI:
CL POT           Current LIMIT (14.6V Vout)
5ohm             3.17A
250ohm         1.00A
300ohm         0.55A
330ohm         0.29A
350ohm         0.12A
               
« Last Edit: April 11, 2019, 04:17:59 pm by imo »
 
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