how to select lower voltage?

[goldsmith]

8-no , i mean wasted power in output cap dis charger resistor

Hi Ali
A question , why you want to use a capacitor in out put ? to guarantee that ripple is as low as you want ? but this is not the good way because it will make your supply response and loop , relax . and it won't be able to affect fast changes .
and when you want to use a high capacity capacitor in out put , it is not a good idea to use a resistor or something else to discharge it , so what will be the benefit f it ? you can prevent to using a cap in out put then after that you discharge it !!! it is not a good way as i've mentioned before .

7-becuase r5 is 10k and if we except q6 Veb and q5 Vce we have about 3.5 ma in q5 collector , it isn't right?

I think i couldn't get your exact meaning . i think it is a good idea to explain your mean as clear as possible .
How you've calculated 3.5 ma for collector of q5 ? no that's not correct . did you get what i've told you before ? i told you try to read about principles that i've mentioned . if you don't do what i've told you , it is impossible that you understand how this circuit does work . and you will never be able to design a simple circuit like that . i've showed you the way .

11- can i use 0.250W resistors for r1 to r4?

if you need 5 amperes at out put then current through each resistor will be around 5/4=1.25 amperes . loss power across the each resistor will be (1.25)^2*0.1= 0.15 watt ( approximately )
So it means you can use that .

12-what is best way to turn on an led when output current limited?

When limiter is working it means Q7 is conducting so you can use another resistor and transistor exactly like that and use a red LED in it's collector . if you can't catch the idea i can draw it for you .

13-what is the best way to make accurate voltages for opamps inputs? zener or tl431 or ...?

What do you mean by op amps inputs ? is your mean those five volts and 0.3 volts supply ?

Good Luck
Goldsmith

 

[alipoor90]

hi
13-yes
thanks a lot

 

[goldsmith]

Hi and good morning in time of IRAN !
about those voltages , for example for 5 volt you can easily use a zener diode and resistor and take it from the 15 volts of op amp or perhaps using a 78L05 which is so small .
about 0.3 or perhaps 0.5 volts you can easily use a voltage divider network and obtain it from 5 volt

Best Luck
Goldsmith

 

[alipoor90]

hi again!
14-pn2222 is good for cc led?
15-how to select its base resistor when i don't have u3a output voltage?
thanks a lot

 

[goldsmith]

Hi Ali

14-pn2222 is good for cc led?

I prefer to say how to select a transistor for LED ! you need to know about wasted power across the transistor which it's duty is turning on the overload LED . so it should be able to tolerate the required current of LED ( and you need to consider a safety factor in your design , which can be around 30 or 40 percent ) . about the voltage it should be able to accept VLED-VS

15-how to select its base resistor when i don't have u3a output voltage?

Why you think that we don't have out put of U3 ? of course we have it . that op amp works as a comparator so it's out put will be zero volts ( approximately ) or it's VCC . so you have it's out put voltage and then you can easily calculate the resistor .
Best Wishes
Goldsmith

 

[alipoor90]

please check final schematic
thanks

 

[goldsmith]

Hi Ali
It seems fine . you can build it . after you build it let me know about it's behavior ok ?
Good Luck
Goldsmith