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Linear Voltage Regulator Design


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Hi nearly old man Staigen,
You are absolutely correct! I was way off with my selection of high-value "turn-off" resistors for the output transistors in my "fixed" circuit. I have now chosen lower value resistors based on what is inside a TIP120 darlington.

I used a 1k resistor in series with the collector of the 2N3906 to limit its collector current and dissipation if the project's output is shorted. It will still provide enough drive to the output transistors to blow a fuse if the output is shorted.

I think the circuit can deliver 30V at 1.5A with a 34V supply to protect the 36V max-rated opamp.

It would be fun to try to stop this extremely high gain circuit from oscillating. ;D


Hi Alun and Cyw,
I don't think a single 2N3055 or power Mosfet can dissipate more than about 60W even on a pretty big heatsink. Therefore the max current is only 2A at low output voltages. That's why I proposed using 2 output transistors in our 0V-30V modified project for 3A output, and using 3 output transistors on our modified for 5A project.



Thank you for your help....

Can the cirucit deliver 30V at 2A??

On the other hand, I would like to use LM358 for testing, Is it OK??

any op-amp is prefectly on this cirucit??
As I know CA3140 is 4.5MHz, BiMOS Operational Amplifier with MOSFET
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Which U1, U2, U3, diode and P-channel MOSEFT is prefect in this circuit???Thanks

The MC34071 will be fine for U1, but U2 needs to swing down to below 0.6V and U3 needs to swing as low as possible, I was going to suggest the the CA3130 but it's only rated to 16V, :( perhaps audioguru can think of a suitable op-amp.
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Hi Alun,
It is difficult to select U3 for your circuit.
It needs inputs that operate near the positive supply, and an output to ground. Cmos opamps do this, but have limited supply voltage rating.
If its 10k input resistors were increased so that they have 1.8V or more voltage drop then an MC34071 opamp could be used if you can tolerate its output going only as low as about 0.1V. ;D

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Hi there again

Maybee the LM358 blow, its only good for 32 Volts, and you put 36 Volts on it!

Hey cyw1984, cant you read? The datasheet clearly says its max voltage is 32 Volts!

And Audioguru, why didn't you included some type of voltage regulation or something for the poor little IC? A zener and a resistor and a cap will do that!

//Staigen

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Hi Staigen,
Yeah, the LM358 is blown-up. It is supposed to have a gain of 3.0.

I didn't regulate the supply for the opamp because the MC34071 that I recommended and most other modern opamps have a + power supply rejection ratio of about 82dB at the rectifier ripple frequency. So 1V of ripple on the supply will cause only 0.1mV of ripple at its output. ;D

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If my Vin 30 or 32V..can the output achieve 30V?

Of course not. The max output voltage will be 3V to 4V lower than its supply voltage.

how to change the circuit...because I want to common the VCC and GND to supply voltage?

I don't know what you mean.

And what values of filter is suitable?

You'll have to try different values until its transient response isn't too slow, and that it doesn't oscillate.
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Of course not. The max output voltage will be 3V to 4V lower than its supply voltage.


I don't know what you mean.


You'll have to try different values until its transient response isn't too slow, and that it doesn't oscillate.


I want to common the Opamp VCC and GND to the Vin and GND
how to do that- -sorry fot my poor english
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I want to common the Opamp VCC and GND to the Vin and GND. How to do that?

The MC34071 opamp (or MC34072 dual or MC34074 quad) that I recommend has a max supply voltage of 44V. Its inputs can work fine down to ground. Its output can swing almost to ground.
Just connect its supply pins to Vin and ground. ;D

Hi Ante,
The MAX420 is a chopper stabilized opamp that is very slow. I have never used a thingy like that. I am amazed that its offset voltage is only a few uV. I wouldn't use it here.
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I get a problem here...
If my Vin 30 or 32V..can the output achieve 30V???
how to change the circuit...because I want to common the VCC and GND to supply voltage....

With your circuit?
No.

But with my MOSFET circuit, yes but is depends on the on resistance of the MOSFET and Rsence.
Vdropout  = current drawn * (Rsence + RMOSFET)
For example if you want 3A @ 30V and the MOSFET's on reistance is 0.4ohms and Rsence is 0.1ohms:
Vdropout = 3 * (0.1 + 0.4) = 1.5V
So the input voltage must be at least 31.5V for the output to be 30V when 3A is drawn.


Hi Alun,
It is difficult to select U3 for your circuit.
It needs inputs that operate near the positive supply, and an output to ground. Cmos opamps do this, but have limited supply voltage rating.
If its 10k input resistors were increased so that they have 1.8V or more voltage drop then an MC34071 opamp could be used if you can tolerate its output going only as low as about 0.1V. ;D

Then (I know it's a pain) but build a little negitive power supply for it, I don't know why I didn't think of this before. ;D

post-0-14279142414626_thumb.gif

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Hi Alun,
Now that you have a negative supply, you can use a TL071 or TL081 opamp for U3 because their inputs operate up to the positive supply. Their supply voltage is only 36V max so will probably be exceeded when the load is low.

TI probably has an opamp with inputs that operate at the positive supply and a 44V max supply, but I couldn't find one.

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audioguru,
You could use the MC34071, the inputs to U3 are held at the output voltage not the supply so as long as the ouput's 2.2V lower it'll be fine.

You could even use the MC34072 (dual) for U1 and U2 to save space but U3 needs to be separate as it needs the extra -3.6V supply.

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  • 2 weeks later...

Hi nearly old man Staigen,
You are absolutely correct! I was way off with my selection of high-value "turn-off" resistors for the output transistors in my "fixed" circuit. I have now chosen lower value resistors based on what is inside a TIP120 darlington.

I used a 1k resistor in series with the collector of the 2N3906 to limit its collector current and dissipation if the project's output is shorted. It will still provide enough drive to the output transistors to blow a fuse if the output is shorted.

I think the circuit can deliver 30V at 1.5A with a 34V supply to protect the 36V max-rated opamp.

It would be fun to try to stop this extremely high gain circuit from oscillating. ;D


Hi Alun and Cyw,
I don't think a single 2N3055 or power Mosfet can dissipate more than about 60W even on a pretty big heatsink. Therefore the max current is only 2A at low output voltages. That's why I proposed using 2 output transistors in our 0V-30V modified project for 3A output, and using 3 output transistors on our modified for 5A project.



I am sorry..i have try this circuit...but i don't know it 's not work..
the output is not linear and the output is disorder

how to shoot it?
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I am sorry..i have try this circuit...but i don't know it 's not work..
the output is not linear and the output is disorder

how to shoot it?


Revolver, air pistol, shotgun - take your pick. ;D

I'm joking of course, I know English isn't your first language, I'll try to correct your post for you, I think you must've meant:



I am sorry.. I have tried this circuit... but it's not working, the output isn't linear and is the voltage is wrong.

How can I troubleshoot it?


It might be oscillating, you should attach a 100uf capacitor followed by a 100nf ceramic disc capacitor on the output of the regulator, if this doesn't work then please answer the following questions:

What are you powering this circuit from?
Have you looked at the output on an oscilloscope?
What output voltage are you getting at what input voltage?
Have you checked the output from the DAC?
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Revolver, air pistol, shotgun - take your pick. ;D

I'm joking of course, I know English isn't your first language, I'll try to correct your post for you, I think you must've meant:



It might be oscillating, you should attach a 100uf capacitor followed by a 100nf ceramic disc capacitor on the output of the regulator, if this doesn't work then please answer the following questions:

What are you powering this circuit from?
Have you looked at the output on an oscilloscope?
What output voltage are you getting at what input voltage?
Have you checked the output from the DAC?



Vin=40V
I used the power supply to represent DAC
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