0-30 Vdc Stabilized Power Supply

[phaethom]

i change zener diodes and the problem is the same (voltage on output is between 21.7 and 31 )

 

[Thomas1]

1.if I use an ammeter for it ...it will show the ampere that the load use or the ampere that was set ?

The stupid thing will only show the current flowing trough it, but this hasn't always to be the current flowing trough the load
In plain: You can connect a switch across the output terminals of the project and produce a shortcut by pushing it. Then you can adjust the current limit, flick the switch back and use the project the normal way with the ammeter showing the current flowing trough the load.

 

[audioguru2]

An ammeter in series with the load causes a voltage drop that ruins the excellent voltage regulation of the project.

Instead, connect the ammeter in series with the output transistors then its voltage drop is made-up by the circuit since it is inside the negative feedback loop. But maybe the voltage drop is high enough to reduce the max voltage output of the project at high current.

You can use a voltmerter to measure the voltage across R7 which is the current-sensing resistor, then calibrate it for amps.

You can also use a voltmeter to measure the voltage produced by the current-setting pot then calibrate it for the amps setting.

 

[arman_linkin]

The stupid thing will only show the current flowing trough it, but this hasn't always to be the current flowing trough the load
In plain: You can connect a switch across the output terminals of the project and produce a shortcut by pushing it. Then you can adjust the current limit, flick the switch back and use the project the normal way with the ammeter showing the current flowing trough the load.

beacause of my bad english...Icant understand your answer. i am also an amature in electronic.so please explain it more clear

Instead, connect the ammeter in series with the output transistors then its voltage drop is made-up by the circuit since it is inside the negative feedback loop. But maybe the voltage drop is high enough to reduce the max voltage output of the project at high current.

can you draw a diagram (or mybe a shematic) for me (about out put transistors)??????? :-*
i cant understand it.

You can use a voltmerter to measure the voltage across R7 which is the current-sensing resistor, then calibrate it for amps.

You can also use a voltmeter to measure the voltage produced by the current-setting pot then calibrate it for the amps setting.

and what is the range of each voltmeter????which of them show the current flowing trough the load.and which of them show the amps that was set with p2.
note : its not NOT NECESSARY for me to set current at special place.but it IS important for me to show the current flowing trough the load...

so help me please with diagram not with written message. :-*

I hope you apologize me for my gossips :-[ :'(

 

[audioguru2]

Hi Arman,
An Amp-meter shows the current in the load.

Here is the schematic modified with an amp-meter in series with the output transistors so its voltage drop is made-up bu the circuit.

Or you can use Ohm's Law to calculate the voltage across R7. R7 is 0.47 ohms. 3A through 0.47 ohms produces a voltage across it of 3A x 0.47 ohms= 1.41V. Calibrate a voltmeter to show 0.3V when a voltage divider has 1.41V across it then the meter will show the load's current.

P2 is the current-setting pot. It is in series with R17 and R18 and they are fed from pin 6 of U1 which is 11.2V. When P2 is set for 3A it has 0.17V across it which can be amplified and fed to a voltmeter to indicate the current setting of P2.

View attachment 40203

 

[arman_linkin]

One more thing....I built this project and I tested it with 1A 24V trans(beacause I didnt have 3A Trans)but it doesnt worked correct.LED was always On and the out put voltagewas 0.41 V evev when I turn p1 and p2

 

[audioguru2]

The original project might work with a 24V/1A transformer but its ordinary opamps will be operating with a supply voltage higher than their absolute max rating.

If the opamps survive then the low output voltage indicates that maybe a transistor or diode is connected backwards.

 

[AN920]

Shouldn't the ammeter be placed after point 3? That way all the current will go through the meter including transients from the output cap.

 

[audioguru2]

I put the ammeter inside the negative feedback loop so its resistance doesn't ruin the very good voltage regulation of this circuit.

If the meter's resistance is 0.1 ohms then it would reduce the voltage 300mV at 3A if it was placed after point 3.

 

[AN920]

Good point.

 

[arman_linkin]

The original project might work with a 24V/1A transformer but its ordinary opamps will be operating with a supply voltage higher than their absolute max rating.

If the opamps survive then the low output voltage indicates that maybe a transistor or diode is connected backwards.

i check all of the diodes and transistors and ... but it steel doesnt work

 

[audioguru2]

Hi Arman,
Did you substitute any parts?

The circuit is easy to fix if you list voltage measurements and have it turned on without a load and with the voltage and current pots turned up to max.
Connect the negative wire of a DC voltmeter to the negative wire of C1. Measure the voltages:
1) The positive wire of C1.
2) Pin 4 of U3. (It should be a negative voltage)
3) Pin 6 of U3.
4) Pin 4 of U1.
5) Pin 6 of U1.
6) Pin 3 of U2.
7) Pin 6 of U2.

 

[PeterG]

Hi all,

I need your help.

What hfe should the BC548 (or BC547) and the BC557 (or BC327) have?

A search in this thread didn't turn up any hits, which I find surprising.
Hasn't anyone else wondered whether a BC548A, BC548B or BC548C
is the right one? Or doesn't that matter?

Thanks,
Peter

 

[audioguru2]

Hi Peter,
Q1 and Q3 have 10k base resistors that supply a base current of 3.2ma. Their collector current is less than 32mA so they will work fine if ther beta is only 10 or more.
The BC547 and BC557 have a minimum beta of 110 so any of them will work fine.

 

[enaB]

Can I use a 28V transformer?

 

[audioguru2]

Can I use a 28V transformer?

Yes if you upgrade certain parts.
A 28VAC transformer will be about 29VAC without a load. Its peak voltage is 41V and it will be rectified and smoothed into 39.6V. To have a 3A output then the transformer must be rated at 41V x 3A= 123VA which is 28VAC/4.4A.

The opamps use the smoothed 39.6V for their positive supply and also use a negative 5.6V supply, so their total supply voltage is 39.6V+5.6V= 45.2V. The TL081 opamps spec'd for this project have an absolute max allowed voltage rating of only 36V so they will probably fail. A few other opamps are rated for a max 44V supply but their rating also too low.

The 28VAC transformer will make probably 37VDC at max current. Then if the transistors have low current gain the max output voltage from this project will be about 28VDC.

The output transistor must dissipate 37V x 3A= 111W if the output is set for 3A and is shorted or is set to a low voltage. It can dissipate only about 70W if it has a huge heatsink or a high velocity fan, so two output transistors should be used to share the heat. The driver transistor will also dissipate far more power than the original little transistor can and should be upgraded.
The rectifier diodes will become extremely hot and should be upgraded to a full-wave bridge rectifier module bolted to the chassis.

A few resistors will become too hot and should be bigger. The main filter capacitor should have a higher value.

A recommended upgraded list of parts is here:View attachment 40309

 

[enaB]

Thanks

 

[phaethom]

my source already  work perfectly, now the second part, the digital panel. did anybody already make him
?

 

[subumann]

Just wondering if these could be replacements for TL081 :

OP07C
NE5523
MC1558
LM118-218-318
RM4558
LM301
UA748

And if yes what are the adjustments (trimmer, bypass caps etc.)
By the way, with the original specs the power suply work great.
Thanks in advance!

op-amps.zip

 

[audioguru2]

Hi Subumann,
It is good to hear that your original circuit works great. Other people have found that its max output is about only 25VDC and about only 2.5A at the same time. They also found that the transistors and rectifier diodes get very hot when the voltage is set low and the current is high.

The 24VAC original transformer has an output of about 25V or more without a load, so its rectified output is 35.35V peak and its unregulated positive voltage is 34V. Two opamps use that 34V as their positive supply voltage and -5.6V as their negative supply voltage and therefore the TL081 opamps are operating with a total supply voltage that is higher than their absolute max allowed of only 36V.

Two of your substitute opamps need a compensation capacitor. All of the dual opamps don't have an offset voltage adjustment needed for U2. Some of the opamps have diodes between their inputs. Most of the opamps are much slower than the original TL081 opamp and therefore might cause the circuit to oscillate.

I recommend using a 30VAC/4.3A transformer, high voltage and wideband OPA445AP opamps, two 2N3055 transistors and a few other parts changes for this project to meet its specs, stay cooler and be reliable.