I found this simple neat variable voltage/current power supply circuit which has all the features I have been searching for. http://electronicsproject.org/rippl...able-output-voltage-and-current-power-supply/
There are a number of points in the diagram that are not clear to me and so I have made modifications to the circuit and coloured them in red.
Are my modifications correct?
Also how can the output be increased to 3A. I am thinking to add a second 2N3055 in parallel with T2 but am not sure how to add the emitter resistors and how to calculate their values.
Any help will be much appreciated.
Many thanks to all.. Chris ...

Mods Edit:
Please don't post the Email, it could be bring the spammer to the forums and you, thank you.

 

Your changes look okay, but something looks funny in the area of Vr2 and Vr3. It's not often that much current is put thru a rheostat.
Usually .1 ohms is enough for the emitter resistors. They just need to compensate for the differences in Vbe of the 2 transistors.

I found this simple neat variable voltage/current power supply circuit which has all the features I have been searching for. http://electronicsproject.org/rippl...able-output-voltage-and-current-power-supply/
There are a number of points in the diagram that are not clear to me and so I have made modifications to the circuit and coloured them in red.
Are my modifications correct? View attachment 99464
Also how can the output be increased to 3A. I am thinking to add a second 2N3055 in parallel with T2 but am not sure how to add the emitter resistors and how to calculate their values.
Any help will be much appreciated.
Many thanks to all.. Chris ...

Mods Edit:
Please don't post the Email, it could be bring the spammer to the forums and you, thank you.

Your changes look okay, but something looks funny around Vr2 and Vr3. It is unusual to put that much current thru a rheostat. Maybe you can add some values to the schematic.
Usually .1 ohms is enough for the emitter resistors. They just need to make up for the difference in Vbe of the transistors.

 

Note that at 3A output the internal power dissipation into a low voltage load can be around 50W or more so you will need a good heat-sink (perhaps fan cooled) for the series pass transistor(s).

 

Your changes look okay, but something looks funny in the area of Vr2 and Vr3. It's not often that much current is put thru a rheostat.
Usually .1 ohms is enough for the emitter resistors. They just need to compensate for the differences in Vbe of the 2 transistors.

Your changes look okay, but something looks funny around Vr2 and Vr3. It is unusual to put that much current thru a rheostat. Maybe you can add some values to the schematic.
Usually .1 ohms is enough for the emitter resistors. They just need to make up for the difference in Vbe of the transistors.

Thank you very much for your help . I will add values and also second 2N3055 will post new mods tonight.

 

Note that at 3A output the internal power dissipation into a low voltage load can be around 50W or more so you will need a good heat-sink (perhaps fan cooled) for the series pass transistor(s).

Thanks for your reply. I plan to use big heatsinks for each 2N3055 and also to add a 80mm cooling fan directly over the transistors

 

What makes you think the output will be ripple free? Lower than some level I could believe eg. 10 mV P-P

 

What makes you think the output will be ripple free? Lower than some level I could believe eg. 10 mV P-P

Hi . I have not looked at that aspect yet, just copied the heading from the original article, maybe they meant "very low ripple"

 

Hi . I have not looked at that aspect yet, just copied the heading from the original article, maybe they meant "very low ripple"

That would be in line with my expectations, but as always the devil is in the details. How low is very low? BTW I would consider 10 mV P-P to be in the category of very low ripple. The next step would be a post regulation pi-filter consisting of one or more sections. If you succeed in building it, I would be interested to know the actual ripple. Maybe one of our ambitious simulation freaks (whom I hold in the highest regard) would provide a ballpark estimate.

NB. I consider questions and answers like this to be one of the benefits of simulation. Not for accuracy and precision you understand, but for ballpark estimates and understanding.

 

Your changes look okay, but something looks funny in the area of Vr2 and Vr3. It's not often that much current is put thru a rheostat.
Usually .1 ohms is enough for the emitter resistors. They just need to compensate for the differences in Vbe of the 2 transistors.

Your changes look okay, but something looks funny around Vr2 and Vr3. It is unusual to put that much current thru a rheostat. Maybe you can add some values to the schematic.
Usually .1 ohms is enough for the emitter resistors. They just need to make up for the difference in Vbe of the transistors.

Hi ronv. Here are the values and second power transistor added. Are the emitter resistors correctly positioned? I am not sure of the role of R2. Should this be reduced to increase the out put to 3A. The current values are for 1.5A output (Transformer secondary out put 24v 4A)

 

That would be in line with my expectations, but as always the devil is in the details. How low is very low? BTW I would consider 10 mV P-P to be in the category of very low ripple. The next step would be a post regulation pi-filter consisting of one or more sections. If you succeed in building it, I would be interested to know the actual ripple. Maybe one of our ambitious simulation freaks (whom I hold in the highest regard) would provide a ballpark estimate.

NB. I consider questions and answers like this to be one of the benefits of simulation. Not for accuracy and precision you understand, but for ballpark estimates and understanding.

Hi Papabravo, I am going to start this project in about 2 weeks time (busy laying out pcb and collecting parts etc). Will let you know if successful (or failure). Will check ripple when finished.

 

Hi Papabravo, I am going to start this project in about 2 weeks time (busy laying out pcb and collecting parts etc). Will let you know if successful (or failure). Will check ripple when finished.

Thank you, and good luck with the project.

 

Hi Papabravo, I am going to start this project in about 2 weeks time (busy laying out pcb and collecting parts etc). Will let you know if successful (or failure). Will check ripple when finished.

I had a chance to look a little closer at this. There are some bad circuits on the internet and I would like to congratulate you on finding one.
Let me see what else we might come up with.

 

The current of transformer need to be as:
Ii_ac = Io/0.6 = 3A_dc/0.6 = 5A_ac

Why choose 5A:
Adding 20% as backup current.
Ii_ac = (3A_dc*1.414)*(1+20%) = 4.24V_ac * 1.2 = 5.08A_ac.
So if used the dc current divided by 0.6 is more easier.

 

The current of transformer need to be as:
Ii_ac = Io/0.6 = 3A_dc/0.6 = 5A_ac

Why choose 5A:
Adding 20% as backup current.
Ii_ac = (3A_dc*1.414)*(1+20%) = 4.24V_ac * 1.2 = 5.08A_ac.
So if used the dc current divided by 0.6 is more easier.

Thanks for the very useful info.

 

I had a chance to look a little closer at this. There are some bad circuits on the internet and I would like to congratulate you on finding one.
Let me see what else we might come up with.

Hello ronv, do you mean - I have found a bad one - or - I have found a good one among all the bad ones?

 

Its a BAD circuit, i would use a regulator chip with a pass transistor to boost the current, several available Lm317, Lm2596, LM338...

 

Hello,

You might want to have a look at the following PDF:
http://educypedia.karadimov.info/library/DC Power Supply Handbook.pdf

This was found on this page of the EDUCYPEDIA:
http://educypedia.karadimov.info/electronics/powerelectronicssupplies.htm

Bertus

 

Hello ronv, do you mean - I have found a bad one - or - I have found a good one among all the bad ones?

Sorry, It's a bad one.
Let me ask you a couple of questions to see how complicated we need to make it.
It gets easier (and better usually) if you can live with a minimum voltage of 1.25 volts. For most circuits this works okay.
The same goes for the current limit. We can build one that goes down to 0 amps, but it is more complex than one that drops the voltage down to 1.25 volts under over current conditions.
The easy ones are built around an IC like the LM317.
http://www.ti.com/lit/ds/symlink/lm317hv.pdf
Here is a first pass at one that matches your specs.

 

Sorry, It's a bad one.
Let me ask you a couple of questions to see how complicated we need to make it.
It gets easier (and better usually) if you can live with a minimum voltage of 1.25 volts. For most circuits this works okay.
The same goes for the current limit. We can build one that goes down to 0 amps, but it is more complex than one that drops the voltage down to 1.25 volts under over current conditions.
The easy ones are built around an IC like the LM317.
http://www.ti.com/lit/ds/symlink/lm317hv.pdf
Here is a first pass at one that matches your specs.

O sh one t. OK will check it out, back to the drawing board.

 

Hello,

You might want to have a look at the following PDF:
http://educypedia.karadimov.info/library/DC Power Supply Handbook.pdf

This was found on this page of the EDUCYPEDIA:
http://educypedia.karadimov.info/electronics/powerelectronicssupplies.htm

Bertus

Thanks Bertus. will check them out