When you asked for 10KΩ pots that set one of the parameters. The required voltages to the various parts set the other, from there it was a simple matter of dropping the values and watching what dropped out.

 

When you asked for 10KΩ pots that set one of the parameters. The required voltages to the various parts set the other, from there it was a simple matter of dropping the values and watching what dropped out.

I have been doing some measurement and calculation on my board which i found out alot of values are not ideal. I understand that no circuit is able to get an ideal reading, however is there any component maybe like a capacitor that can be added for more ideal readings?

 

Define not ideal. Likely you can tweak whatever needs tweaked.

 

I have made a weird discovery, it is normal to have the resistances of R5 be decrease in it range of reisstance when it is connected in the circuitry? R5 when soldered onto a board, gives a 3.48Kohms and a voltage output reading of 5.16V. Here are the strange readings which amuse me.

3.48Kohms 5.160V
3.69kohms 2.146V (Doesn't make sense) wiper is in half knot pos.
2.93Kohms 2.112V
1.86Kohms 258mV
The range of voltage at the output is correct but why isn't the resistances?

However the readings play well for R2.
0.7Ohms 18.95mA
6.11KOhms 8.13mA
9.99kOhms 0.07mA

Is there any reason to why the resistance of R5 change, i have change my wiper with a new one and this happen. THE Circuit does work but i do not understand how such resistance ranges can change despite having a 10Kohm resistor.

 

If it is in the board then you aren't just looking at the resistor in question, but all the other resistors in the design also. All the other resistors going to the power supplies are effectively in parallel with R5, which will throw the readings off. If you take the schematic, figuratively remove all the solid state parts (diodes may be iffy), and calculate the resistor networks that are left you'll find they are right on.

Is this what was bothering you in post #82?

As a lead tech for Collins Radio we would take a schematic and calculate what the resistances would look like IN the board. One of the common causes of failure were resistors that had drifted in time, but you rarely get a direct reading of resistors in the circuit.

 

The only thing which i see it as a challenge now is the tolerance of the potentiometer. I have done the circuit on the board and have made use of 10KOhms PIHER potentiometer. http://www.piher-nacesa.com/pdf/14-PT15v03.pdf

These potentiometer has a tolerance of 20% making the values at the output offset a little to 5.16V and 18.95mA. Any idea to how it can be solved.

Testing the old circuitry on a test board with another kind of potentiometer (no idea on its brand or tolerance) has given me a exact 20mA and a 5V. Problems only occur after a change of potentiometer was done which i suspect is the tolerance differents.

 

You aren't going to get exact values most likely. You can tweak values with test select resistors, if it is important to you. Most times in electronics we judge what we will consider acceptable as far as offsets or tolerances. If you want exact values you can use on board pots to tweak some of the other components.

You could add pots for R4 and R3 to tweak the values to exactly what you want. Personally I don't think it is worth the bother. A smaller value for R3 and R4 would increase the ranges, and visa versa.

 

You aren't going to get exact values most likely. You can tweak values with test select resistors, if it is important to you. Most times in electronics we judge what we will consider acceptable as far as offsets or tolerances. If you want exact values you can use on board pots to tweak some of the other components.

You could add pots for R4 and R3 to tweak the values to exactly what you want. Personally I don't think it is worth the bother. A smaller value for R3 and R4 would increase the ranges, and visa versa.

Hi ok thanks for that info. Can i also ask for more information on the power supply course which u were talking about previously.. Power supply course 102 if i remembered.

 

That was a figure of speech. Basically op amps are fundimental power supplies. With no feedback, they aren't really that low impedance, but add negitive feedback and the suckers go to the milliohm level on the output (1/1000 Ω).

Most introductory courses in college are whatever 101. Electronics 101 for example. The next course would then be 102.

I was a bit suprised the sim for the earlier design oscillated, but I have major respect for Sgt Wookie's skill. I think I even understand it, looking at it after the fact (but I'm not going into it here). Electronics frequently does what you don't expect, but it is very logical about it.

 

I have tried playing around with the circuitry and have adjusted the voltage at the output to 0 to 10V with replacing the 7805 to a 7815. However the current output has a range of 50mA to 80mA.

Is there something i must change in my resistance setting as well, like a high resistance on a particular resistor?

 

Uh, there should have never been a 7805, did you mean the 7809?

Yes, the components were calculated around the parts specified.

 

Opps got it wrong. Yup meaning the 7809 changing to a 7815. Anyway the resistors that are affecting the current values are R2 and R3 right?

 

R2 and R3, and R4 and R5 are both voltage dividers that control the setting. Have you worked on figuring out voltage dividers?

 

R2 and R3, and R4 and R5 are both voltage dividers that control the setting. Have you worked on figuring out voltage dividers?

Well i am able to calculate them and on how they are supplying the voltages to the respective circuitry. To my understanding, R2 and R3 plays an important role for the current limiter which CR1 plays an important role which separates the voltage between R4 and R5 resistors which holds the voltage to the voltage regulator.

Am i correct on this??

 

CR1 compensates for the Base Emitter voltage drop of Q1. It could have been a resistor, or be eliminated altogether (there would be dead space in R2's adjustment).

I thought about putting a couple of diodes below R5 to compensate for Q2 and Q3 BE junctions, but decided to keep it simple.

CR1 is part of the R2 and R3 voltage divider, nothing more. It drops a constant .6 volts.

You are basically correct in the above statement.

 

So by changing the resistance values of R2 and R3, i will be able to change my current range and hence leaving the R4 and R5, the voltages can remain as it is.

 

You can increase the voltage output of a 78xx series regulator by adding resistance between it's ground pin and GND.

There is a nominal 5mA current flowing from the ground pin to GND, so for every 100 Ohms resistance you add between the ground terminal and GND, the output voltage will increase by 500mV.
E=IR, so since I=5mA and R=100, E=0.5V or 500mV.
Note that Vout cannot be greater than Vin-2.

 

You can increase the voltage output of a 78xx series regulator by adding resistance between it's ground pin and GND.

There is a nominal 5mA current flowing from the ground pin to GND, so for every 100 Ohms resistance you add between the ground terminal and GND, the output voltage will increase by 500mV.
E=IR, so since I=5mA and R=100, E=0.5V or 500mV.
Note that Vout cannot be greater than Vin-2.

But if the settings of this is done, the voltage will increase but still having the current slightly increase am i right.

Was thinking of how we can have a voltage and a current limit that can maintain at a range of 0 to 20mA but a voltage of 0 to 10v or 0 to 24V.

 

You will have to recalculate the values if you change the regulator. As I have said before, it is a reference for all the circuitry.

This is good practice for you I would think. Plan on the 10KΩ resistor, the new voltage, and work from there.

 

I have tried recalculating the circuitry and have found problem in obtain the desire output i will. At the moment, i am wanting a 10V output with a 20mA.

Correct me if i am work, with just changing the resistance values of the voltage divider, the desire result can be obtain. However since a 20mA is wanted, i would not need to change R2 and R3 but instead R4 and R5.

Am i correct?