0-30V Stabilized Power Supply

[fikic]

Hi Fikic,
With no load on the project, the output pin 6 of U3 should always be high at about +24V to +28V. So it does not reduce the output voltage through D9 and it does not turn on Q3 to light the LED.

The slider of the current adjusting pot P2 applies a small positive voltage to the (+) input pin 3 of U3. The voltage should be +0.005V when P2 is turned to minimum and should be +1.414V when P2 is turned to maximum. Without a load, there is no voltage across R7 so the (-) input pin 2 is 0V. Then the output pin 6 of U3 should be always be high.
What range of voltage at pin 3 of U3 do you measure when P2 is turned from minimum to maximum?

Maybe the terminals of P2 are connected wrong.
Maybe U3 is defective. Replace it.

P2 is connected ok. When P2 is at it's minimum, the voltage on pin 3 of U3 is 15.05mV, at it's maximum it's 2.07V. Is that normal? At P2 minimum, the voltage on pin 2 of U3 is 45.5mV, at it's maximum is 95.5mV. So there is voltage drop on R7 in any case. Why is that?

PS.: Is normal, that when I connect a load on PSU, just Q4 and Q5 get warm, Q2 is like nothing is on it..

 

[audioguru2]

When P2 is at it's minimum, the voltage on pin 3 of U3 is 15.05mV, at it's maximum it's 2.07V. Is that normal? At P2 minimum, the voltage on pin 2 of U3 is 45.5mV, at it's maximum is 95.5mV. So there is voltage drop on R7 in any case. Why is that?

R7 is 0.47 ohms so with 95.5mV across it the load current of the project is 203mA which is 28 times too high. The current should be only 5.6mA in R4 plus 1.2mA in P1 plus 0.56mA in R6.
Check the few connections along the output of the project for a connection to something that is wrong.

Is normal, that when I connect a load on PSU, just Q4 and Q5 get warm, Q2 is like nothing is on it?

The heating in Q2 depends on the voltage across it and the current in Q4 and Q5 and on their amount of current gain that could be only 35 or could be more than 100. When the load current is 3.0A and the output voltage is low then Q2, Q4 and Q5 heat the most.

 

[fikic]

R7 is 0.47 ohms so with 95.5mV across it the load current of the project is 203mA which is 28 times too high. The current should be only 5.6mA in R4 plus 1.2mA in P1 plus 0.56mA in R6.
Check the few connections along the output of the project for a connection to something that is wrong.


The heating in Q2 depends on the voltage across it and the current in Q4 and Q5 and on their amount of current gain that could be only 35 or could be more than 100. When the load current is 3.0A and the output voltage is low then Q2, Q4 and Q5 heat the most.

The voltage drop across R7 is just 5mV. 95mV voltage drop is just on pin 2 of U3.

 

[audioguru2]

The voltage drop across R7 is just 5mV. 95mV voltage drop is just on pin 2 of U3.

But R21 is supposed to be 10k and it connects pin 2 of U3 to R7. When current-setting pot P2 is minimum then R21 has a max of 0.5uA (for an MC34071 opamp) to 2uA (for a TLE2141 opamp) of input bias current which makes pin2 a max of 5mV to 20mV, not 95mV.
This input bias current in R21 turns off the current regulation when the load current is low.

Yesterday I explained why R7 has a small voltage drop when there is no load current. 

 

[fikic]

But R21 is supposed to be 10k and it connects pin 2 of U3 to R7. When current-setting pot P2 is minimum then R21 has a max of 0.5uA (for an MC34071 opamp) to 2uA (for a TLE2141 opamp) of input bias current which makes pin2 a max of 5mV to 20mV, not 95mV.
This input bias current in R21 turns off the current regulation when the load current is low.

Yesterday I explained why R7 has a small voltage drop when there is no load current. 

You didn't understand me. At P2 minimum pin 2 of U3 is 45.5mV, at its maximum pin 2 of U3 is 95.5mV. Anyway, Too high...

 

[stelsing]

Good evening. Sorry for my broken English. I have a transformer with a winding 24 volts 5 amps. Can I use the original scheme, but with TLE2141A can use the original scheme? Or that I get with this transformer, if you make schema changes? I want to output current 5 A, you need to change in the scheme?
And yet. Can I make my own scheme? does it work?
Thank you for your reply.

 

[audioguru2]

The original circuit has many overloaded parts. Its transformer voltage is too low so its output at 3A is 25VDC, not 30VDC.

We modified this project so that it is reliable and produces 30VDC at 3A. We showed a simple change that allows its output current to be 5A.

 

[stelsing]

Well, I gather the schema changes and put your transformer. I get the output to 25 volts and a current of 5 amps, right?
If I'm right is going to modify the schema, and then maybe a transformer replaced if need be. Transformer will be needed with a winding 30 volts and 5 amp, right?

 

[audioguru2]

For a 5A output at 30V, the transformer must supply a peak DC voltage of about 39.6V to the rectifiers at 5A which is 198VA.
You can use a 28VAC transformer or a 30VAC transformer at 7.07A.

The modification to the latest 3A schematic for a project with an output of 5A is to add a 3rd 2N3055 output transistor with a 0.33 ohm emitter resistor, change C1 to 20,000uF and change R7 to 0.27 ohms at 10W. 

 

[stelsing]

Thank you for your help. Make a new scheme with the existing transformer, the voltage is what will already be the case. Transformer, while I will not. Maybe later, if necessary.
A transformer 24 volts, 5 amps maximum voltage which can be obtained with the new scheme?

 

[audioguru2]

A transformer 24 volts, 5 amps maximum voltage which can be obtained with the new scheme?

The 24VAC transformer at 5A is 125VA. Its peak voltage to the rectifiers is 40VDC and it is overloaded when the DC current exceeds 125/34= 3.7A. The max output voltage at 3.7A will be about 25VDC.

 

[lysium]

hi all specially Audioguru,

i build with mc34071, 24VAC transformer and picmaster PCB layout... yesterday i blew two op-amp U1 and U2. i trace from u3 when i turn P2 until 3.4-4.0V on R18 u3 and ZD 10V become very hot. and i think when those op amp blew i set the P2 more than 3.4V.... what's wrong?


regards,
ali

 

[audioguru2]

hi all specially Audioguru,

i build with mc34071, 24VAC transformer and picmaster PCB layout... yesterday i blew two op-amp U1 and U2. i trace from u3 when i turn P2 until 3.4-4.0V on R18 u3 and ZD 10V become very hot. and i think when those op amp blew i set the P2 more than 3.4V.... what's wrong?

U1 has a light load so it should be cool.
U2 has a little more work so it should be warm.
U3 has a light load so it should also just be cool.

Since U3 has a light load then the 10V zener diode that powers it should also be cool.

P2 is the current-setting pot. Its voltage is simply a reference voltage to opamp U3. U3 reduces the output voltage of the project when the load current is higher than the setting of P2.

 

[lysium]

what should i do? when P2 set to 3.8V the ZD 10V and op-amp is impossible to touch i think it will blow too if i set to long.

 

[audioguru2]

what should i do? when P2 set to 3.8V the ZD 10V and op-amp is impossible to touch i think it will blow too if i set to long.

Your project probably has the output of U3 shorted to ground or to the negative supply.
Maybe the U3 opamp is already blown because it is not supposed to get hot.

 

[lysium]

ok... i have to wait the new op-amp from digikey. will inform you later.

btw can i use opa445? i have few in the drawer.

thanks

 

[audioguru2]

can i use opa445?

Not in the new circuit. It was used 5 years ago in the first fix of this project.

 

[lysium]

after browsing old posting. i found that picmaster pcb drawing has a fault on C10 or C3 if using audioguru schematic it is + and - of the capacitor is backward in the pcb. and i found mine was backward and it is now shorted maybe this is the cause of the U1 and u2 blew?

 

[audioguru2]

after browsing old posting. i found that picmaster pcb drawing has a fault on C10 or C3 if using audioguru schematic it is + and - of the capacitor is backward in the pcb. and i found mine was backward and it is now shorted maybe this is the cause of the U1 and u2 blew?

C3 on my schematic filters the -1.3V supply which has nothing to do with opamps blowing up.
With C3 shorted then the current regulation will not work if the output of the project has a very heavy load at a very low voltage or if the output is shorted. Then U2 might blow up and Q2, Q4 and Q5 also might blow up.

 

[lysium]

i see.... right now the LED is light up after i change the capacitor but without U2 and U1 in place,  still looking in local store here.