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39v 8a Laboratory Power supply circuit, Need Help

FilipSebik

Sep 7, 2017
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Hey, this is my first post here, so I need help with this circuit so I won't burn out my transformer or the parts, because they can be quite expensive for me :D
Here is the scheme
regpsu.png
So I want to know if all those transistors would survive, the voltage control through R5 would work and current control through R6 would work. This circuit is based of this video
D1 is full bridge rectifier that converts AC to DC
C1/2/3 are for filtering
R1 is for discharging capacitors after the PSU is shut down
R2 limits the current through the zener diode
D2 sets the regulating voltage - I am unsure about that
R5 controls the voltage
D3 does something which I don't know what
R4 and R6 control the output current - Which I don't know if it works
R3 is for quick discharging of C4 after the voltage has been controled and C4 helps keep the output voltage at the level it is supposed to be
Tell me what to improve and what to fix so I can have working power supply. Thanks
 
Last edited:

kellys_eye

Jun 25, 2010
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Where to start? Probably by recommending you don't build it and try a more efficient way of doing it!

What do you need such a supply for? Is your requirement specifically for high voltage/high current? Sure you actually 'need' this design?

However....you should have some balancing resistors in the emitter of the pass transistors to equalise volt drop across them - otherwise that's a pretty basic and very inefficient circuit that will require a sh1tload of heatsinking!

Have you looked at the data sheet for the BD912? It's power derating is abysmal - down to only 35W at a junction temperature of 100degC. Draw some heavy amps at low volts (say 5A at 12V) and the 'rest' of the volts/amps (about 35V at 5A or 175 watts of heat!) will be dissipated in those pass transistors. That's 45W per transistor........ not counting the junction-to-case and case-to-heatsink losses so you'll be cooking chickens on the heatsink!

Seriously? Reconsider.......
 

duke37

Jan 9, 2011
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I do not see any current control. R6 and R4 control current through D3 but this is not dependent on the current being supplied. The normal way is to measure the voltage across a low value series resistor and shut down the output voltage when the current limit is reached.
 

Audioguru

Sep 24, 2016
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It does not regulate the voltage much because it is missing an opamp as an error amplifier with feedback.
The base-emitter voltage of the transistors change as they heat and change with load current. The 39V zener diode's voltage changes as it warms up and also changes a little when its load current changes.

I agree that you and the output transistors will have 323W of heat to remove (not possible).
 

AnalogKid

Jun 10, 2015
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I do not see any current control. R6 and R4 control current through D3 but this is not dependent on the current being supplied.
The current control is based on dangle-biasing/starving the output transistors. R6 and R4 control the current through D3 *and Q1*. As the output current increases, the Q2-Q5 base current requirement increases, increasing the voltage at the Q1 emitter. At some point that lifts the emitter of Q1, decreasing the total base current available.

The current control is very "soft". That is, the transition from a constant voltage output to a constant current output occurs over a relatively large range of output current. Also, the crossover point will drift a lot with temperature and component aging.

ak
 

dorke

Jun 20, 2015
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So recommend me another circuit for this until I buy the parts :D

You should specify your needs first ,i.e. define the specs of the power supply.
I very much doubt you need a Variable 30V/8A linear power supply.
You see ,that is considered a beast in size/weight and price.

240W power supplies are better made in switching mode not linear. (except for the noise issue)
If you indeed want low noise(how low?),
then the combination of SM pre-regulator and "tracking" linear P.S is the way to go.
 

dorke

Jun 20, 2015
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It does not regulate the voltage much because it is missing an opamp as an error amplifier with feedback.
The base-emitter voltage of the transistors change as they heat and change with load current. The 39V zener diode's voltage changes as it warms up and also changes a little when its load current changes.

I agree that you and the output transistors will have 323W of heat to remove (not possible).

It regulates.
It isn't a precision regulator but it will do well.
The issue of the heat stability in the zener reference is always present: op.amp or not!.

Any ways,who says an op.amp is needed?
Any "error Amp" will work fine(need to specify the % regulation needed):
single transistor(like in this case),
dual differential transistor etc. .
 

AnalogKid

Jun 10, 2015
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I agree with dorke. It is not a great, or even good circuit, but there is nothing explicitly wrong with it.

Note - any linear supply will have all of the thermal issues. Moving over 300 W of heat is entirely possible, but you will need at least 200 CFM of air flow.

ak
 

FilipSebik

Sep 7, 2017
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So I have transformer that is 230VAC to 2x 30V 125VA (250VA total) and I want to have Lab Bench Power supply from it. I will use it for tesla coils, amplifiers and power circuits, so stop telling me that I don't need 30V 8A. Just tell me if a buck regulator based of circuit that is not expensive as hell would work and if so, then I want the circuit or a more efficient linear power supply regulator circuit. Heat??? I have huge ass heatsink that I can use, so many things that I can use to dissapate the heat and heat is not my problem. Anyways in the upper circuit, someone said that the zener diode would heat up, but how when it has current limiting resistor that allows 8.43mA to flow through 1.3W zener diode, to heat it up so much that it would loose effiency?
 

dorke

Jun 20, 2015
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Well,
If you intend to fry your stubborn and non-educated big ass,
you can start anytime...
Good luck.
 

duke37

Jan 9, 2011
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I have a dual supply which has a pair of L200C control chips to set current and voltage. These with suitable extra pass transistors may do what you want. The box was bought many years ago and was well used then, there may be more modern devices.

Edit: I do not know the maker.
 

FilipSebik

Sep 7, 2017
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This
30V-10A-Variable-Bench-Power-Supply-circuit.jpg

or this
LM317-30V-variable-high-power-supply-6-20A-max.jpg

This is my last resort :D
 

AnalogKid

Jun 10, 2015
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The LM723 circuit will give much better voltage and current regulation than your original circuit. However, the 723 has a max operating voltage of 40 V.

The LM317 circuit has no current regulation. And, it has poor voltage regulation because it is not the circuit recommended on the LM317 datasheet. That circuit uses power PNP transistors, such as a 2N2955. There are many schematics on the internet that show how to add multiple PNP pass transistors to an LM317, but adding current regulation is difficult.

ak
 

FilipSebik

Sep 7, 2017
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The LM723 circuit will give much better voltage and current regulation than your original circuit. However, the 723 has a max operating voltage of 40 V.

The LM317 circuit has no current regulation. And, it has poor voltage regulation because it is not the circuit recommended on the LM317 datasheet. That circuit uses power PNP transistors, such as a 2N2955. There are many schematics on the internet that show how to add multiple PNP pass transistors to an LM317, but adding current regulation is difficult.

ak
So the LM723 is the way to go? I have 5 2n3773 which can be used instead of 2n3055 and I just need the regulator and some resistors to complete the circuit. Thanks
 

Audioguru

Sep 24, 2016
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A laboratory power supply does not have this very simple circuit with a few transistors hanging on a high voltage zener diode with no voltage sensing and no corrections.

A zener diode voltage rating higher than about 5V increases its voltage as it heats up. The 39V zener in the schematic increases its voltage 35mV for each rise of 1 degree C. If a 5.1V or 5.6V zener diode is used its voltage does not change when it heats and an opamp can amplify it to 39V.

How much does the output voltage of this simple circuit drop when it is loaded? A few volts? Then 5V drops to almost 2V? A laboratory power supply has voltage regulation of a few mV.

There is a lab power supply project at www.electronicslab.com that was a kit in Greece. I helped fix its problems 13 years ago. Its corrections are on two very long threads in their forum. It uses 3 opamps.
Recently Chinese companies copied the original defective circuit and were selling the kit at a very cheap price. It was sold for less than 1 year then went on clearance, probably when people found out that it had many parts overloaded causing it to be unreliable.
 
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