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POWER SUPPLY DESIGN

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i tried using that schematic but it won't go down to zero so i tried implementing that error amplifier with mine.
 

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Your new circuit will not work because the added transistor turns off when the output voltage is less than about 27.5V. Also the voltage-setting pot turns down the amount of negative feedback when its voltage is turned down.

I made an extremely simple opamp with 3 of the transistors of an LM358 opamp. It has a non-inverting input for your adjustable reference voltage (a pot from the 27V zener diode) and it has an inverting input for negative feedback from the output. Its inputs should work all the way down to ground.

Can you calculate its resistor values?
 

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Your new circuit will not work because the added transistor turns off when the output voltage is less than about 27.5V. Also the voltage-setting pot turns down the amount of negative feedback when its voltage is turned down.

I made an extremely simple opamp with 3 of the transistors of an LM358 opamp. It has a non-inverting input for your adjustable reference voltage (a pot from the 27V zener diode) and it has an inverting input for negative feedback from the output. Its inputs should work all the way down to ground.

Can you calculate its resistor values?
This is somehow close to my project so I want to ask what transistors were those 3 and how will this be connected to the circuit? I don't have the knowledge of what you meant where the negative feedback from output is and which are the output transistors. Is the 4.7k pot the one you are referring as the pot from the zener diode?
 
Maybe you have not learned enough about basic electronics to understand this very simple circuit.

The BC547B transistor that you used in your schematic is fine for all the low current transistors in my new circuit.
Negative feedback from the output causes the error amplifier to correct any difference between the reference voltage from the voltage-setting pot and the output voltage.
The output transistors are the three transistors in your circuit.
 

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Check your circuit again (transistor types).
And the circuit needs some more components and more 2N3055s to cope with 3 Amps,but I guess you'll come to that eventually.
 
Maybe you have not learned enough about basic electronics to understand this very simple circuit.

The BC547B transistor that you used in your schematic is fine for all the low current transistors in my new circuit.
Negative feedback from the output causes the error amplifier to correct any difference between the reference voltage from the voltage-setting pot and the output voltage.
The output transistors are the three transistors in your circuit.
i'm quite confuse to the 2 BC547B since the circuit shows an PNP transistor but BC547B is an NPN transistor.


Check your circuit again (transistor types).
And the circuit needs some more components and more 2N3055s to cope with 3 Amps,but I guess you'll come to that eventually.
whose circuit? ladyphoebe15's circuit or audioguru's circuit?
 
i'm quite confuse to the 2 BC547B since the circuit shows an PNP transistor but BC547B is an NPN transistor.

Good catch! You're right, the two transistors just to the right of the adjustment pots are PNPs but have the BC547B labels. This is clearly incorrect.
 
Check your circuit again (transistor types).
And the circuit needs some more components and more 2N3055s to cope with 3 Amps.
I wanted young lady Phoebe to notice my errors on purpose. If the output is shorted then I hope she is not burned by the very high temperature of the 2N3055.
 
I wanted young lady Phoebe to notice my errors on purpose. If the output is shorted then I hope she is not burned by the very high temperature of the 2N3055.
Can the 2 PNP transistors be any PNP transistors? Is changing the 2N3055 to another transistors or adding a heatsink solve the problem for being burned?
 
Can the 2 PNP transistors be any PNP transistors? Is changing the 2N3055 to another transistors or adding a heatsink solve the problem for being burned?
BC557B or 2N3906 PNP transistors have a low range of hFE and should be used for the input transistors.

Do a simple heating calculation:
The transformer is 24VAC so its peak voltage is 24V x 1.414= 33.9V. The full-wave rectifier drops about 2V to 31.9V. If the output is shorted then the current is 0.7V/0.22 ohms= 3.2A. Then the amount of heat in the 2N3055 output transistor is 31.9V x 3.2A= 102.1W which is a lot of heat.

The aluminum heatsink must be enormous with many fins and a high speed fan must be used.
Two 2N3055 transistors with series emitter resistors should be used to share the heat on a pretty big heatsink.
 
Hi,

Does it have to be a linear regulator or can it be a switcher? Higher current and voltage works out better with a switcher in many cases.
 
Your new circuit will not work because the added transistor turns off when the output voltage is less than about 27.5V. Also the voltage-setting pot turns down the amount of negative feedback when its voltage is turned down.

I made an extremely simple opamp with 3 of the transistors of an LM358 opamp. It has a non-inverting input for your adjustable reference voltage (a pot from the 27V zener diode) and it has an inverting input for negative feedback from the output. Its inputs should work all the way down to ground.

Can you calculate its resistor values?
How do you calculate the values for the resistors?
 
How do you calculate the values for the resistors?

Hello,

Make one with an op amp first, then once you get THAT working, replace the op amp with transistors.
 
I've configured my circuits using the op amp negative feedback of audioguru. I have simulated with Proteus and I assigned the two resistors value about the PNP transistor =1.2k while the one below=820. Not sure of how to know what values of resistors is really needed.
 
I've configured my circuits using the op amp negative feedback of audioguru. I have simulated with Proteus and I assigned the two resistors value about the PNP transistor =1.2k while the one below=820. Not sure of how to know what values of resistors is really needed.
I would begin with R4. You want its value to be low enough so that it turns on the output transistors well but does not overheat Q3. The datasheet shows that it is very hot at 625mW so let us select 180mW.
If its collector voltage is half the supply at 16V then its current will be 180mW/16V= 11.3mA. Then R4 is 16V/11.3mA= 1.4k ohms which is not a standard value. Use 1.5k ohms/1W.

For an output of 25V at 3A then the maximum base current of Q5 is 115mA which will melt Q4 when the output voltage is low at 3A because then Q4 must dissipate 115mA x 30.5V= 3.5W so Q4 must be a power transistor like a TIP31 on a small heatsink. Its maximum base current is 2.5mA which creates a minimum voltage drop of 3.75V across R4 which is fine.

The maximum collector current of Q3 is 32V/1.5k= 21.3mA so its maximum base current is about 70uA. Let the maximum current in R3 be 10 times more so R3 is 0.7V/0.7mA= 1k ohms.
Try 100 ohms for R2.

I simulated it to see the voltages. I used two resistors across the 32V to be the zener diode and the voltage setting pot. it works fine.
 

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You have four 2N3055 transistors so each one will have a maximum current of 3A/4= 0.75A and their emitter resistors will each waste 0.75A x 100 ohms= 75V which is ridiculous. Use 0.33 ohms for the emitter resistors. Your little BC547B for T4 will burn up driving the output transistors to 3A at a low output voltage. Replace T4 with a TIP31 power transistor like I did.

Good for you, your simple opamp works better than mine and will work even better if R5 is 2.7k, R6 is 10k and R2 is 1.5k.
 
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