how to select lower voltage?

Hi
i'm designing a lab power supply with lm741
i have to cv and cc opamps and their output must applied to power stage input
each opamp should able to decrease voltage (lower output must be applied to power stage input)
how i can do this?
best regards

if the series pass transistor is a triple stage darlington, the the base resistor feeding the first transistor can be a high valur ( >100K), just feed the output put of each op amp into this point via a 10k resistor. The other way might be to design the PSU as a CV one then use the output from the CC amp to modify the reference voltage for the CV amp.
Frank

hi
your second proposal seems good
i find another ways from other sites called foldback circuit
have you an example of that circuit?
thanks

Hi
i'm designing a lab power supply with lm741
i have to cv and cc opamps and their output must applied to power stage input
each opamp should able to decrease voltage (lower output must be applied to power stage input)
how i can do this?
best regards

Click to expand...

Hi ali
you can easily achieve this . by using a high current gain series pass element e.g a darlington pair which it's collector has been tied into your input and it's emitter is connected to the out put .
then you'll need to take a feedback from the out put by adding two resistors and connect it to a simple error amplifier and then use a simple current sensor resistor in series with ground of your load then affect these samples into the comparator of current limiting section ( hiccup )
that's all you need to do .
By the way , 741 is not a good choice for a PSU . for a precise PSU !! however you can do it with that too
Any question ?
Best Wishes
Goldsmith

thanks dear friend
1-what opamp you recommend?(be found in the tehran)
2-would you draw a schematic?
best regards

Hi Ali

1-what opamp you recommend?(be found in the tehran)

Click to expand...

You can easily find gang of the op amp types that you're interested in , in Tehran . but my suggestion is TL072 for your purpose or perhaps better ones with higher gain and higher input resistance .
LF 356 is also good .

2-would you draw a schematic?

Click to expand...

For sure , but i need that you tell me about your exact requirements , then i'll design a circuit for you and put it's schematic here soon .

Best Wishes
Goldsmith

my input is 36v and i need voltage adjustable from 0 and max current is 5 amp
thanks

my input is 36v and i need voltage adjustable from 0 and max current is 5 amp
thanks

Click to expand...

Ok , how much is the red boundary for out put voltage ? my mean is i hope you don't want your our put be variable from zero up to 16 volts . because thus you shouldn't expect any regulation factor .
isn't it ? so how much is your expected out put voltage as vmax ?

i need min 30v in the output
and i have another question
in var psu when vout set into vx output capacitors charged at vx if we reduce output voltage and output capacitors enough large they can damage transistor?
what we can do?
thanks

i need min 30v in the output
and i have another question
in var psu when vout set into vx output capacitors charged at vx if we reduce output voltage and output capacitors enough large they can damage transistor?
what we can do?
thanks

Click to expand...

ok , tomorrow i'll send you the schematic , because now i've to go to bed because it is exactly 22 hours that i'm awake !
About variable PSU why you think they can suffer the transistor ? why it should be ?
By the way , in linear power supplies out put capacitor will have very low capacity so there won't be any relaxation problem !
Best Wishes and good night ( sorry , good morning !!! )
Goldsmith

Hi Ali
I've designed what you need . here it comes :

If you need to add an LED as overload indicator , with in red , you can ask me about how to do that !
Any question about the circuit ?

Good Luck
Goldsmith

1-can you introduce me a TO package transistor instead 2n3055?
2-and a simple circuit to create negative supply for op amp?(i think -2v is enough right?)
3-and if use any voltage below 5v can i remove d1 and d2?
4-what is input offset voltage of an opamp and how to null offset?
thanks a lot and sorry for asking so many questions

Hi Ali

1-can you introduce me a TO package transistor instead 2n3055?

Click to expand...

You'd better to use 2n3055 because it is easy to eliminate dissipation through a suitable heatsink with that . and also it's power boundaries are so good for a linear PSU . and i think it will be the best solution for you to use it !

2-and a simple circuit to create negative supply for op amp?(i think -2v is enough right?)

Click to expand...

Who told -2 volts is enough ? i can anticipate something ! perhaps you've saw some of the old circuits which have used some diodes in series with ground line to prepare 2 volts as negative supply . right ?
For my opinion it is not a good idea ! and of course it is not enough ! . you'll have two choices . because you'll need some other voltages like 15 and 5 too , you'd better to use a very low power transformer ( a center tapped transformer ) ( with three heads ) then you can easily create every positive and negative voltage that you're interested int !
Another way is using a simple voltage multiplier with two diodes and two capacitors and a zener diode . it will deliver a negative voltage instead of ground . but i still think the best way is that independent cheap transformer .

3-and if use any voltage below 5v can i remove d1 and d2?

Click to expand...

Do you really know why i've used those diodes ? in parallel with be of some of the transistors ? as you probable know opamps in that circuit , will deliver minus voltage in their out put ( in some of the specific times of operation ) and as i hope you know a bjt transistor ( npn one ) is not able to accept negative voltages above a red boundary ! then you'll have some breaking . so i've used them to prevent such a effect .

4-what is input offset voltage of an opamp and how to null offset?

Click to expand...

i think you mean how to optimize offset issue in op amps and what is it and where it is came from . i think you could google it before you ask . here some of the concepts regarding the offset are coming :
www.analog.com/static/imported-files/tutorials/MT-037.pdf
http://www.ecircuitcenter.com/Circuits/op_voff/op_voff.htm

Best Wishes
Goldsmith

Do you really know why i've used those diodes ? in parallel with be of some of the transistors ? as you probable know opamps in that circuit , will deliver minus voltage in their out put ( in some of the specific times of operation ) and as i hope you know a bjt transistor ( npn one ) is not able to accept negative voltages above a red boundary ! then you'll have some breaking . so i've used them to prevent such a effect .

Click to expand...

1-yes , but Vebo of bd135 is 5v and if we use -5v for opamp supply we have above -5v in output of opamp , voltages below Vebo can breakdown transistor?
2-and why we need -15 v for opamp supply? -2 or -5 v is not enough for opamp output be able to swing to zero?or that is for other reasons?
3-can we apply u3a output to u1a input?does it cause problems?

4-and can we use a diode instead q7 ?(and invert u3a inputs)

5-and what is c1 exactly function?(i know it's for stability ... but i want an analysis of its work)
6- and how to determine c1 value?
7-how to determine r6 and r10 values?
8-i want to use a big capacitor in output so i designed this circuit to discharge it faster when i reducing output voltage and no load connected , please see that and check it

sorry for asking many questions; because i don't want just to build one, i want to learn designing
thanks a lot

1-yes , but Vebo of bd135 is 5v and if we use -5v for opamp supply we have above -5v in output of opamp , voltages below Vebo can breakdown transistor?

Click to expand...

Hi Ali
Unfortunately sometimes things are not as easiest as you wish to be !
Some notices : to guarantee the best operation of that circuit you'll need higher negative voltage than 5 volts . and another problem is you shouldn't work nearby red boundaries . 5 volts is also nearby the red boundary . a good designer won't design nearby red boundaries . ok ?

2-and why we need -15 v for opamp supply? -2 or -5 v is not enough for opamp output be able to swing to zero?or that is for other reasons?

Click to expand...

There are gang of reasons behind of that issue . which needs more experience to be analyzed . my suggestion needs more time to you but here it comes : you understand how an op amp does work and what are the working regions of an op amp . and why we use op amp . an basics behind linear regulators . it means you'll need to read some books . some micro electronics and some PSU design books . and furthermore , earning many practical experiences .
So i prefer to teach you how to take fish from the sea instead of giving you one of them ! isn't it better ?

4-and can we use a diode instead q7 ?(and invert u3a inputs)

Click to expand...

Of course you can but the best behavior won't be harvested . this way is much faster than a diode . and of course more suitable than a diode ! by the way the diode not like what you've used . the arrangement that you've used is wrong ! you are making much of wasting power there !!
By the way why you've connected the collector of q7 into v4 ? do you know what a terrible event will be happened if you do that ?

5-and what is c1 exactly function?(i know it's for stability ... but i want an analysis of its work)

Click to expand...

You need to know more about control systems and it's theory . it is used to make a stable pole there to make the circuit stable . do you know about pole and zero ? if we try to justify it's role with simple words ( of course it is not the perfect way to justify it ) but for now you can justify it as : it is used to transfer fast changes through the feedback line as fast as possible !!!! but i really don't like this justification .

6- and how to determine c1 value?

Click to expand...

You need to know about compensation networks and criteria of stability .

7-how to determine r6 and r10 values?

Click to expand...

you'll need to know more about transistor and it's saturation mode ! my suggestion is as the same as past ! start reading !

8-i want to use a big capacitor in output so i designed this circuit to discharge it faster when i reducing output voltage and no load connected , please see that and check it

Click to expand...

Why you've used such a configuration ? it is absolutely wrong ! can you tell me why that is the worst idea ?! try to answer it please ! ok ?

sorry for asking many questions; because i don't want just to build one, i want to learn designing

Click to expand...

I don't have any problem with asking but sometimes it is impossible to teach all of the electronics in a thread !
Best Wishes
Goldsmith

9-how many 2n3055 i should use if i want to edit circuit to 50v 10amp
10-switching is good for variable power supply? what's your view? (it's my next goal)
thanks

9-how many 2n3055 i should use if i want to edit circuit to 50v 10amp

Click to expand...

for 10 amperes and 50 volts you'll need to use at least 6 2n3055 in parallel together . o fcourse with a suitable heatsink .

10-switching is good for variable power supply? what's your view? (it's my next goal)

Click to expand...

Of course it is good . but sometimes linear will be better it depends on your application and that is important to use by using a supply .
I recognize it is your next goal but i need to warn you about something : if you really want to learn and be a designer you need to start from somewhere . a right point . when you didn't learn linear systems as perfect as enough you can't go through more advanced supplies like switching supplies . SMPS needs more experience and more knowledge . and if you try to go trhough it with this weak background then you'll have to waste most of your time and money and then you'll be disappointed . so i think you can get what i'm advising you .

Good Luck
Goldsmith

7-max current passing through q5 is about 3.5ma and its hfe in this Ic is about 130 so q5 need about 30 uA to bias but you used partly low resistor for it
i think you trying to keep opamp output voltage low, right?
8-because i think it's wasting power when output voltage is fixed
thanks

7-max current passing through q5 is about 3.5ma and its hfe in this Ic is about 130 so q5 need about 30 uA to bias but you used partly low resistor for it
i think you trying to keep opamp output voltage low, right?

Click to expand...

Do you know beta of a transistor will be changed instead of different quiescent points ? and a question , why you think you'll need 3.5 ma there ? don't try to analyze this circuit with that view . it's not correct to analyze it like that .

8-because i think it's wasting power when output voltage is fixed

Click to expand...

Wasting the power across what transistor ? q5 ? it wasted power when vout is 30 volts will be around 5 mili watts !!

8-no , i mean wasted power in output cap dis charger resistor
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?
11- can i use 0.250W resistors for r1 to r4?
12-what is best way to turn on an led when output current limited?
13-what is the best way to make accurate voltages for opamps inputs? zener or tl431 or ...?
thanks