POWER SUPPLY DESIGN

[darkatsuki12]

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.

what program do you use for simulation? Will the resistors in ladyphoebe's circuit be okay with the power capacity she assigned?

 

[audioguru]

I forgot about how to design a differential input. I remember that the "long tail" (my R2) should have a high resistance or be a very high resistance current source. So I increased the value of R2 and the resulting opamp has very good voltage regulation.

 

[Rolf Zetterberg]

Now it's beginning to look like a power supply,although there's still much to do.For a start could someone remove the dead short between T1 emitter and R3?

 

[ladyphoebe15]

I wanna ask if its possible if i can add a LED in series after with resistor 1k ohm . About the how much power each resistors can sustain, Is it fine that 0.33 ohm and o.22 should have 3W, the 1k ohm have 2W and the others are 0.5 W?

 

[audioguru]

The simple formula for power dissipation is the current squared times the resistor value so for an output of 3A then each 0.33 ohms resistor has 0.75A in it and the power dissipated is 0.186W. A 1/4W resistor will be fine.

The 0.22W resistor with 3A in it dissipates 1.98W so use a resistor size larger than 2W because a 2W resistor dissipating 1.98W is extremely hot.

If you add an LED in series with 1k at the output then it will not light when the output is less than 2V for a red LED or 3V for a blue LED. Also when the output voltage is low then the LED will be very dim.

One problem, all opamps have a frequency compensation capacitor to prevent them from oscillating at a high frequency when negative feedback is used because phase shifts in the amplifier change the negative feedback to positive feedback at high frequencies. Try a 100pF capacitor between the base and collector of your Q3.

 

[ladyphoebe15]

The simple formula for power dissipation is the current squared times the resistor value so for an output of 3A then each 0.33 ohms resistor has 0.75A in it and the power dissipated is 0.186W. A 1/4W resistor will be fine.

The 0.22W resistor with 3A in it dissipates 1.98W so use a resistor size larger than 2W because a 2W resistor dissipating 1.98W is extremely hot.

If you add an LED in series with 1k at the output then it will not light when the output is less than 2V for a red LED or 3V for a blue LED. Also when the output voltage is low then the LED will be very dim.

One problem, all opamps have a frequency compensation capacitor to prevent them from oscillating at a high frequency when negative feedback is used because phase shifts in the amplifier change the negative feedback to positive feedback at high frequencies. Try a 100pF capacitor between the base and collector of your Q3.

in that case, where can i put a LED?

 

[audioguru]

where can i put a LED?

Your first circuit had an LED. Its 5.6k resistor can be fed from 32VDC and its LED will be fairly dim.

 

[Rolf Zetterberg]

Audioguru,a 0.33 ohms 1/4W resistor mounted on a PCB and inside a warm case will almost act like a fuse if the power supply is subjected to a sustained short.Normally you see-even in the cheap Chinese power supplies-wirewound resistors of 3-5Watts.

 

[audioguru]

Audioguru,a 0.33 ohms 1/4W resistor mounted on a PCB and inside a warm case will almost act like a fuse if the power supply is subjected to a sustained short.Normally you see-even in the cheap Chinese power supplies-wirewound resistors of 3-5Watts.

Q4 in the latest schematic limits the output current to 0.68V/0.22 ohms= 3.1A so each 0.33 ohm resistor has a maximum current of 0.77A and a power dissipation of 195mw. If it is too warm (it won't be hot) then use 0.22 0hm resistors for only 130mW each.

 

[Rolf Zetterberg]

Yes,I know that and your calculations are correct,but how do you explain all the home-built and professional and less professional power supplies that I've seen through life almost all use oversized resistors here?In fact,I can't recall a single one using 1/4W resistors in this position.
Imagine a power supply shoved in under a shelf together with other instruments in a summerwarm room and running for hours at full power-it gets very warm inside.This puts strain on all components.A resistor should never be used at it's full rating(or even close to that) in this environment!Also,if one the resistors in question burns off,then all the load is placed upon the remaining three that could result in even more unnecessary problems.