LM723 power supply

I made ​​a regulated power suply with the "classic lm723" .In the original circuit the courent source was a 2n3055 transistor. I had a spare MJ3000 and i used this instead of 2N3055.

The problem is that in idle mode the maximum voltage is 6.5 V instead of 12V. I checked the Out pin of the 723 and the voltage is changing according with the Vref variation on the non inverting input on pin no. 5 on the LM.
I will attach the circuit here.

Please help me.

Thank you!

Check the voltage at pin 4 and see if it is the same as pin 5.

Hi,

Did you check for oscillations with a scope or something? That would show up as reduced voltage on a DC voltmeter.

Also, the ammeter shown in the schematic is not in the best location because it drops some voltage. A better location is before the voltage set divider.

Thank you all.
1. I dont understeand why the voltage on inverting input should be the same with the one on the non inverting input. Sorry - this is the ideal summing point condition.
2.Unfortunately i dont have a scope, also i didnt place the ammeter on my board. Are u taliking about the oscilations from the transformer ? If thats the case than i tested it with a 12 V regulated power supply.
Now correct me if i am wrong: (i am a beginner and i try to combine practice with theory)

In this circuit on pin six i must have a potential of 7.15 V (from datasheet). By placing a variable resistor between pin 6 and 5 i modify the potential of pin 5 from 0 to 100% of pin 6 potential. Now, this is an noninverting op. amplifier (this circuit) which means the voltage meassured between ground and the emitor is :voltage on pin 5 multiplied by (1+(R4/R5)).

Melfior_Ra said:

Thank you all.
1. I dont understeand why the voltage on inverting input should be the same with the one on the non inverting input. Sorry - this is the ideal summing point condition.
2.Unfortunately i dont have a scope, also i didnt place the ammeter on my board. Are u taliking about the oscilations from the transformer ? If thats the case than i tested it with a 12 V regulated power supply.
Now correct me if i am wrong: (i am a beginner and i try to combine practice with theory)

In this circuit on pin six i must have a potential of 7.15 V (from datasheet). By placing a variable resistor between pin 6 and 5 i modify the potential of pin 5 from 0 to 100% of pin 6 potential. Now, this is an noninverting op. amplifier (this circuit) which means the voltage meassured between ground and the emitor is :voltage on pin 5 multiplied by (1+(R4/R5)).

Click to expand...

What you say may be true, however the 723 regulator works like this: Pin 10 is the output of the opamp with pin 5 the + and pin 4 as -. Pin 5 determines the output voltage and the op amp will drive the output until the two input voltages are the same unless there is something wrong. Oscillations could give a false reading or the circuit could be miswired.

The reason the voltages at the inverting and non-inverting inputs should be the same (within a few mV) is because of the large gain that the regulator op-amp has. Suppose that the amp output voltage is 5V and the open-loop amp gain is 1000. That implies that the voltage difference between the two inputs is 5V/1000 = 5mV.
Can you detect any significant voltage across the current-sense resistor R3 ? That might indicate the current limiter is active because of a short circuit somewhere.
BTW, it would help if you told us the component values.

Alec

Melfior_Ra said:

Thank you all.
1. I dont understeand why the voltage on inverting input should be the same with the one on the non inverting input. Sorry - this is the ideal summing point condition.
2.Unfortunately i dont have a scope, also i didnt place the ammeter on my board. Are u taliking about the oscilations from the transformer ? If thats the case than i tested it with a 12 V regulated power supply.
Now correct me if i am wrong: (i am a beginner and i try to combine practice with theory)

In this circuit on pin six i must have a potential of 7.15 V (from datasheet). By placing a variable resistor between pin 6 and 5 i modify the potential of pin 5 from 0 to 100% of pin 6 potential. Now, this is an noninverting op. amplifier (this circuit) which means the voltage meassured between ground and the emitor is :voltage on pin 5 multiplied by (1+(R4/R5)).

Click to expand...

Hi again,

Well no not from the line transformer from the circuit itself. The circuit has feedback and with feedback (even negative feedback) there is always the possibility of oscillation. Thus, a quick check for oscillation is a good idea even if the circuit appears to be working normally. Since your circuit doesnt always operate normally, a good time to check is under the conditions where it does not work normally.
When a regulator oscillates it could force the output to be a pulsed wave rather than a continuous wave. The reasons this could happen are numerous so without going into detail you should check to see if the output pulses and if so try to correct it. One reason could be the compensation capacitor is not the right value or is not functional for some reason. Another reason is some component values are too high or too low. It would also be a good idea to indicate the value of your components such as the capacitor values and resistors R2 through R5.

One way to check for oscillations with a meter is to connect a small value cap like 0.1uf in series with your meter set to read AC voltage. You should read 100mvac or less approximately on the meter. The small cap prevents the DC component from reaching the meter which sometimes causes cheaper AC meters to read some value even though it's AC and not DC.

Thank u all!

I will check the oscilations with Mr.Al method but the output on pin 10 of the 723 seems to be ok: gain*potential on pin nr. 5. I meassured the voltage over colector and emitor and it is allmost always half of the voltage that i might have on the output. Lets say that i must have 10V between emitor and ground, when i measure this i have only 5. The other 5 volts are voltage drop between colector and emitor. I changed the MJ with a BD677 and nothing changed.
This are the values:


Specification
Output (approximate values):

Vmin = (R4 + R5) / (R5*1.3)
Vmax = (7.15 / R5) * (R4 + R5)

Imax = 0.65/R3

Max. Power on R3: 0.42/R3

Min. Input DC Voltage (pin 12 to pin 7): Vmax + 5

Parts List
B1 40V/2.5A
C1 2200uF (3300uF even better)
C2 4.7uF
C3 100nF
C4 1nF
C5 330nF
C6 100uF
D1 Green LED
D2 1N4003
F1 0.2A F
F2 2A M
IC1 LM723 (in a DIL14 plastic package)
R1 1k
R2 Pot. 5k
R3 0.56R/2W

R4 3.3k
R5 4.7k
S1 250V/1A
T1 2N3055 on a heatsink 5K/W
TR1 220V/17V/1.5

Are you sure the transistor is connected correctly?

It was the transistor!! Not one but 3 were burnt )))))))