Hi really thanks alot and don't need to apologise=)

I would like to check with you the uses of R2 which i believe is to adjust the current to a limit right while R5 on the other hand is to adjust the voltage at the output. Am i right to state that?

According to the test, I have made use of a 10Kohms potentiometer at the output and with a 12V input, i am able to produce a 23mA output (able to adjust) and a 9V output. However i am unable to make use of R2 and R5 to make any adjustment.

 

First, let's establish while schematic you are using. Look at the corner and tell me the post you are using.

Something worth mentioning. Both R2 and R5 adjust the VOLTAGE going into the transistors, which in turn causes the transistors to vary either current or voltage.

R2 is current, R5 is voltage. You should see the voltage vary at the wiper of both.

I'm currently assuming post #40 is the current schematic.

 

Yup i am using Post 40. I have checked the connections of the circuit and there is no problems. Adjust some potentiometer till i got a Output voltage produce is 7.67V and a 23mA.

Trying to adjust the volatge to at least 5V to see the max current at that volatge range..

 

Pot R2 should go between 5.6V to 8.3V, and Pot R5 should vary between 0V to 7.2. Are you getting this?

 

Regarding the test which i have did, byadjusting the potentiometer to the minimum and the maximum range it can provide. The circuit output performs in a way that the Maximum output that it can go is 9.74V at 22.82mA and Minimum output values of 7.67V at 21.45mA. Output meansurement at R7. This happens when i exclude the last 10K ohms R7.

However when it is included (R7), the problem occurs when adjusting R2 and R7, the voltages will decrease from 8v to 0v and then increase again to 2V. This happen by just adjusting the knot of R5 in 1 direction.

 

Pot R2 should go between 5.6V to 8.3V, and Pot R5 should vary between 0V to 7.2. Are you getting this?

Yup i got the R2 Values exactly the same to yours, however for for R5 a range of 0v to 9.6v.

Anyway from the theory of operartion, kindly explain to me what does this part mean? If the current pulled from the load is less than the current programed into the constant current source then the transistor Q1 is turned fully on.

 

http://cache.national.com/ds/LM/LM723JAN.pdf

Just looking at another alternaitve. From the regulator above,under page 11, can the
Foldback Current Limiting application be used. The 20mA Short circuit current placed with a potentiometer to adjust the current?

 

I thought about current foldback, it isn't applicable for your application (I think). Current foldback is a short circuit protection, where if you pull too much current the voltage drops, and so does the current. The circuit trickles just enough current to detect whether the short is still present or not.

The constant current source will try to output as much voltage as is required to feed the load you have with the current setting. The regulator following it is to prevent it from going over it's set voltage. Basically if you are pulling less current than it is set for the transistor is no longer in its linear region, it is saturated (fully on). A constant current source is good for LED's, laser diodes, and other applications that need a fixed current source.

You definately have a wireing error around R5. R4 and R5 are a simple voltage divider, connected directly to the 9V regulator U1 output. You might have the wiper and outside pins switched, or a simple short somewhere.

Just in case, here is the schematic convention.

 

Thanks. I will give it a check. But do tell me what are the expected output voltage range and current range of this schematic so that i will roughly know whether i am on track or not.

 

Get the voltage of the pots right, this is critical, and completely predictable.

Q1 should follow the voltage at the base on the emitter + 0.6 Volts. This voltage is set by R2.

Q2/3 should follow the voltage at the base of Q2 on the emitter of Q3 - 1.2 Volts. This voltage is set by R5. R6 should be the same voltage on both sides, or very, very close.

R8 is there to keep the 7809 (U1) happy with 10ma current. C1 and C2 keep U1 from oscillating.

The op amps turn what is a high resistance voltage into a low resistance output. The outputs should match the pots.

R7 biases Q2/3 if there is no load, which is also critical.

The output is extemely low current to 20 ma, and 0 to 5.1 Volts.

Pot R2 should go between 5.6V to 8.3V, and Pot R5 should vary between 0V to 7.2. Are you getting this?

I mis-spoke for the pot R5, it should get between 0V to 6.2V.

 

Ok thanks.. I will give it a check on the circuit i have done and feedback to u the results.anyway some available components which i have made use due the unavailability of school's component are the LM358P and the normal diode(1N4001) which i am using 1N4007. Same goes with the PNP and the NPN transistors which are 558 and 550. I believe these will not be much of an effect to the circuit right? Darlington pair is also made up of 2 550 NPN transistors.


Most importantly what is the input volatge which u set?

 

ok i have checked the circuits. Here is what i have observed.

Setting the voltage input to 15.45V, and having a chnage of R3 to 20K ohms due to lack of school components.(will change it tml when the lab stuff comes)

These are my measurement on the transistor:
Q2 VBE 0.613V
Q3 VBE 0.607V
Q1 VBE is 4.98V

Output voltage and current measured at R7 is 5.67v and 38.7mA (Current Falls from 40mA to stable 38.7mA).

However measurement on the potentiometer readings on R2 and R5 are going no where as i do not see a change of values while adjusting them. Am i so far correct and having the readings to what u are expecting?

 

R3 is OK, it will increase your range slightly, which isn't a bad thing. Have you got the wiper of R5 to go to the ranges we needed? You MUST have 0V to 6.2V, or very close. Until we get that we are spinning our wheels.

R2 should also be variable voltage. Again, this has to happen. If you don't get the variation expected there then nothing else will work, 6.3V to 8.3 Volts.

Until you figure this out, nothing else matters. You might even disconnect the other devices on the wipers to isolate the cause.

You are using a 7809 for voltage regulation?

 

R3 is OK, it will increase your range slightly, which isn't a bad thing. Have you got the wiper of R5 to go to the ranges we needed? You MUST have 0V to 6.2V, or very close. Until we get that we are spinning our wheels.

R2 should also be variable voltage. Again, this has to happen. If you don't get the variation expected there then nothing else will work, 6.3V to 8.3 Volts.

Until you figure this out, nothing else matters. You might even disconnect the other devices on the wipers to isolate the cause.

You are using a 7809 for voltage regulation?

I am using a 7809CV regulator. Ok i will focus on the potentiometer i am using or maybe have a change of them. I got chased out of school for staying in the lab to late so i will post something early in the morning of the results i have obtained.

But just to check,the readings for the Transistor are correct right? They seem to have perform to what i have read from the operation theory you have provided.

 

Q1 is off, Q2/3 is about right. A forward biased junction should be .6 V, or a bit more.

 

Your BC550 and BC558 transistors - do they have a suffix letter? (A, B, C)
That makes a difference as to their rated gain (hFE).

 

I've tried to move it away from gain as much as possible, though with Q2 and Q3 it still matters.

 

Your BC550 and BC558 transistors - do they have a suffix letter? (A, B, C)
That makes a difference as to their rated gain (hFE).

they are both C. c558c and cbc550c.

 

I have made changes to both potentiometer and these are my readings...

R2:
5.71V to 8.63V
R5:
0V to 13V ( +INV of op-amp 2)
5.71V to 13.47V (R2 adjusted to 5.71V),(+INV of op-amp 1)
8.63V to 13.47V (R2 adjusted to 8.63V) ,(+INV of op-amp 1)

Which part of the circuitry did u measure to get 0v to 7.2v for R5?

Is there a procedure to how the circuit must be adjusted? Like first R2 then R5?

In order to also measure the output, is there any addition components that must be added to the common collector.

My output readings @ R7 is 1.25V to 5.6V (Adjustable by R5) while the current measured when sorted R7 to gnd is 38mA.(Cannot be adjusted by R2)

 

Sent as a PM:
Hi i would just like to understand why was it that in your first circuitry that u have designed, u have made use of a zener diode of breakdown 5.1V besdie the wiper R5.

Is it to for saftey purposes or to apply a 5V range over to the next circuit?

Because my original drawing was a basic voltage regulator schematic. If the source had not been regulated a zener or some other voltage reference would have been necessary.

As I thought about the design I realized we already had a voltage reference in the form of U1, which made the zener redundent.

By adding a simple voltage divider off U1 whatever comes off the wiper of R5 is also regulated, though sensitive to loading. U2 forms a very good high impedance buffer, which allow us to use resistors we want as opposed to letting the transistors gain force us to use low values.

I have made changes to both potentiometer and these are my readings...

R2:
5.71V to 8.63V
R5:
0V to 13V ( +INV of op-amp 2)
5.71V to 13.47V (R2 adjusted to 5.71V),(+INV of op-amp 1)
8.63V to 13.47V (R2 adjusted to 8.63V) ,(+INV of op-amp 1)

Which part of the circuitry did u measure to get 0v to 7.2v for R5?

Is there a procedure to how the circuit must be adjusted? Like first R2 then R5?

It appears you have wired R4 of the voltage divider to the wrong side of U1. The ranges I named are accurate, you need to examine the schematic versus your setup until they match. R2 looks about right, the transistor emitter should be .7 volts higher than the wiper of R2, just like the voltage on the emitter of Q3 should be 1.2 volts lower.

Is CR1 in this circuit? It is there for a reason. The reason I ask is your high is 8.63 volts. What is the regulator U1 putting out?

R2 wiper : 6.3 - 8.3 volts
R5 wiper : 0 - 6.2 volts

U2 is used entirely as a voltage follower, voltage in is voltage out.