power supply 0-30 volt stailized power supply

[shaknitboss]

Dont mean to resurrect a dead horse but I recently came across the old stabilized o-30 volt power supply.

I found the original magazine article from 1978. Seems like it lost it's original design over the years. I attached the

pdf of the issue. It starts on page 40.

Practical-Electronics-1978-10.pdf

[audioguru]

The circuit is the same as the old Greek kit here except a few part numbers are changed and the maximum output current is only 2A.

It also has the problem of an overloaded transformer since with a 2ADC output the 25VAC input is 2.83A, not 2A.

I ,saw some 741A opamps rated at 44V which might work but I do not think the maximum output voltage will be 30V, probably only 26VDC at 2A with lots of ripple. 

I wonder which circuit came first? 

[shaknitboss]

The higher voltage op amps i've see are pretty expensive.

Wonder if you could incorporate a circuit in it to bring the vcc down to 12v. and still have it work.

 

If the circuit works right I was wondering if you could  add some pass transistors and some

10 watt or so emitter resistors to the output.

 

[admin]

This power supply is also published on an old Czech electronic magazine attached here.

and here is also two nice builds of this PSU: https://translate.google.com/translate?sl=auto&tl=en&u=paja-trb.cz%2Fkonstrukce%2Fzdroj.html

and here: http://diyfan.blogspot.com/2012/02/adjustable-lab-power-supply.html

0-30V 3A PSU.pdf

[shaknitboss]

Every design seems to be a little bit different.

[Whizard]

On 4/18/2019 at 4:24 AM, audioguru said:

The circuit is the same as the old Greek kit here except a few part numbers are changed and the maximum output current is only 2A.

It also has the problem of an overloaded transformer since with a 2ADC output the 25VAC input is 2.83A, not 2A.

I ,saw some 741A opamps rated at 44V which might work but I do not think the maximum output voltage will be 30V, probably only 26VDC at 2A with lots of ripple. 

I wonder which circuit came first? 

What does 'overloaded transformer' means? We knew that current supplied from the windings of a transformer isn't 'pushed' to the load, it's 'pulled' by the load. So I think it's safe to use 2A rating transformer for producing 2A-DC ouput as long as the output isn't overloaded.

Can you explain please?

Edit: but is it safe to use 3A rating instead of 2A based on your statement above.

[audioguru]

If the transformer is rated at 24VAC/2A then it provides a maximum power of 24V x 2A= 48VA.

But if the output is 30V at 2.0A then the load uses 30V x 2A= 60VA plus heating power. The peak of 24VAC is 40V which feeds the bridge rectifier which charges the main filter capacitor to the peak voltage minus the rectifier forward voltage. So then the transformer must produce 40V at 2.0A= 80VA which is much more than the overloaded 48VA. 80VA/24V= 3.33A, not 2A.

For a regulated output of 30.0VDC at 3A then I recommended using a transformer rated at 28VAC/4.3A.

For an output current of 3A then you must look at how much heat is produced by the single output transistor which is why I recommended using two output transistors to share the heat.

Edited with strike-throughs.

[Whizard]

8 hours ago, audioguru said:

If the transformer is rated at 24VAC/2A then it provides a maximum power of 24V x 2A= 48VA.

But if the output is 30V at 2.0A then the load uses 30V x 2A= 60VA plus heating power. The peak of 24VAC is 40V which feeds the bridge rectifier which charges the main filter capacitor to the peak voltage minus the rectifier forward voltage. So then the transformer must produce 40V at 2.0A= 80VA which is much more than the overloaded 48VA. 80VA/24V= 3.33A, not 2A.

For a regulated output of 30.0VDC at 3A then I recommended using a transformer rated at 28VAC/4.3A.

For an output current of 3A then you must look at how much heat is produced by the single output transistor which is why I recommended using two output transistors to share the heat.

How did you know that the peak voltage of 24VAC is 40V? Are you measuring it with an oscilloscope or calculate it manually?

As far as I know that the formula for determining a peak voltage is by using Vpeak = Vrms x sq root 2. So based on this it must be 33.9V not 40V. isn't it?

I'm going to use 100VA/24V, 4.17A transformer for producing 30VDC at 2A safely. What do you think?

[audioguru]

Sorry, I was wrong, you are correct. The peak of 24V RMS is 33.9V not 40V. The peak of the 28V RMS transformer I recommended is 40V. Then the full load drops the rectified voltage to 38VDC with ripple and the losses in the circuit have enough extra voltage for the output to be well-regulated 30VDC at 3A. 

[Whizard]

Hi, again thanks for your answer. One more little things here, can you tell me where to add a secondary output transistor for this schematic. I'm really curious to know. Thanks in advance.

 

And also can I use a centre-tapped transformer for producing negative supply voltage? If yes how to place it in?

[audioguru]

The very old circuit you posted works poorly. TR3 is its output transistor and it gets too hot. Add a second 2N3055 transistor parallel with TR3 to share the heat and add a 0.33 ohms/2W resistor in series with each emitter to the output. The transistors collectors can be connected directly together by the heatsink if the heatsink is insulated from any other part of the circuit. 

If you want a 0V to -30V, 2mA to 3A power supply then make a second one of this project and use its +30V output as 0V and use its 0V output as 0V to -30V.

A center-tapped transformer will produce an AC output, not DC and not variable.

[Whizard]

On 10/31/2019 at 2:07 AM, audioguru said:

The very old circuit you posted works poorly. TR3 is its output transistor and it gets too hot. Add a second 2N3055 transistor parallel with TR3 to share the heat and add a 0.33 ohms/2W resistor in series with each emitter to the output. The transistors collectors can be connected directly together by the heatsink if the heatsink is insulated from any other part of the circuit. 

If you want a 0V to -30V, 2mA to 3A power supply then make a second one of this project and use its +30V output as 0V and use its 0V output as 0V to -30V.

A center-tapped transformer will produce an AC output, not DC and not variable.

I have no idea how the parallelized transistors look like. My friends told me to use darlington-pair for a power transistor. I have a schematic of power transistor parallel-configuration for an inveter, here's the picture:

Sorry for asking this, but can you show me an example of a parallelized power transistor. Thx in advance

[audioguru]

The parallel transistors in the circuit you posted do not have series emitter resistors so the ones with high gain take a high current and the ones with a low gain take a low current. Emitter resistors help to match the transistors.

This is how the output transistors are connected in parallel:

[Whizard]

On 11/5/2019 at 2:50 AM, audioguru said:

The parallel transistors in the circuit you posted do not have series emitter resistors so the ones with high gain take a high current and the ones with a low gain take a low current. Emitter resistors help to match the transistors.

This is how the output transistors are connected in parallel:

The old circuit I posted above have included the resistor for the emitter of TR3 (you can see the red marking I made). Should I add more emitter resistor for this output transistor and for the newly added transistor?

This is the new design I've made from the old previous circuit. Please take a look.

[audioguru]

R7 has nothing to do with the output transistors. Instead R7 senses the amount of output current and sends its sensing voltage to the current adjustment opamp.

Each of your output transistors need a series 0.33 ohm emitter resistor so that the two transistors are better matched to share the heating. 

[Whizard]

On 11/7/2019 at 6:16 AM, audioguru said:

R7 has nothing to do with the output transistors. Instead R7 senses the amount of output current and sends its sensing voltage to the current adjustment opamp.

Each of your output transistors need a series 0.33 ohm emitter resistor so that the two transistors are better matched to share the heating. 

 

Okay, how about the current sense resistor? For my "30V, 2A power supply" circuit it uses 0.33 ohm 2W resistor for sensing, but your circuit used 0,47 ohm 5W resistor for its sensing purposes. Is there any difference that may affect if I change my 'current sense' resistor with 0,47 ohm 5W like yours?

[audioguru]

Your 0.33 ohm resistor R7 with 2A in it has a voltage across it of 0.33 x 2= 0.66V. The current-setting circuit in the 30V/3A power supply has 0.47 ohms x 3A= 1.41V across it.

Your current-setting circuit needs to be calculated to show 2A when its pot is set near maximum. Only one R7 is used (not two in parallel).

[Whizard]

On 11/17/2019 at 12:25 AM, audioguru said:

Your 0.33 ohm resistor R7 with 2A in it has a voltage across it of 0.33 x 2= 0.66V. The current-setting circuit in the 30V/3A power supply has 0.47 ohms x 3A= 1.41V across it.

Your current-setting circuit needs to be calculated to show 2A when its pot is set near maximum. Only one R7 is used (not two in parallel).

 

Thanks for your help, I'll try to build this circuit wth 24V 3A transformer and see how it works.

[Whizard]

On 11/17/2019 at 12:25 AM, audioguru said:

Your 0.33 ohm resistor R7 with 2A in it has a voltage across it of 0.33 x 2= 0.66V. The current-setting circuit in the 30V/3A power supply has 0.47 ohms x 3A= 1.41V across it.

Your current-setting circuit needs to be calculated to show 2A when its pot is set near maximum. Only one R7 is used (not two in parallel).

Anyway, do you have any idea to deal with the thermal solution.
The transistors both will generate high power thus they need a heatsink with at least 1°C/W minimum of thermal resistance.

The problem now is in my country, it was so difficult to find a proper heatsink with that kind of thermal resistance because the manufacturers here don't know at all about it (or probably just don't care).

Do you have any solution for me?

[audioguru]

Many power supplies use a fan blowing past the heatsink so that the heatsink can be smaller.

[Whizard]

35 minutes ago, audioguru said:

Many power supplies use a fan blowing past the heatsink so that the heatsink can be smaller.

Should the fan blow the air pass through the heatsink or suck the air from inside the case where the heatsink located?