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Re: 0-24V/0-5A Lab Power Supply


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Hi all,

Some time ago I was watching the "0-30 Vdc Stabilized Power Supply" in this forum and because I needed a similar PS to that described finally I decided to launch the project, taking as its starting point the original design and making the necessary changes to suit my needs.

Would be possibile for you to measure and document some critical voltage in your design, so it would be possible for whoever is building to check that everything is correct?


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Guest wat69

Hi sfabris... while the electronic is part of my life for almost 40 years since I have not practiced too much literature and very little of English, so they try to explain as precisely as possible. If something is not well understood I would tell me and I will try again. Thank you.

Technical Specifications

Input Voltage: 12 VAC, 10 A / 24 VAC, 5 A
Output Voltage: 0-12 VDC / 12-24 VDC, continuously adjustable
Output Current: 2 mA-10 A (0-12V) / 1 mA-5 A (12-24V), continuously adjustable
Operation Modes: Constant Voltage / Constant Current
Output Ripple Voltage: < 1 mV p.p.
Load regulation: < 0.05 %

Circuit Description

This design uses a 120VA transformer with double secondary (2 x 12V, 5A) which is switched by a DPDT power relay to get a parallel connection (12V/10A) or a serial connection (24V/5A), allowing maximum power transformer and keep the input/output voltage difference within reasonable limits, especially for low output voltages. For this task a simple comparator OpAmp is used which activates the relay when set output to 12 VDC or more.
Also uses a small SPST relay to put a resistor in parallel with the current adjust. pot. (P1) to set the limit to 5A or 10A, as appropriate.
This circuit may be dispensed with and use a 24V/5A transformer which results in an adjustable output voltage from 0-24V and 5A of maximum current.

Note: If the PSU is intended to be used to its maximum power is recommended to use a slightly high power transformer of 140~150VA to be possible.

The AC output voltage of the transformer is rectified by a 15A diode bridge that can be placed on the same heatsink that the output transistor.
This voltage is filtered by capacitors C1/C2 and C3/C4 for high-frequency components, getting around unregulated 33VDC, with which it feeds the whole circuit. It uses a 5V precision micropower voltage reference (U2-LT4050) with an constant amplifier gain of 2 (U3-LT1637) obtaining exactly 10.0V output, which is used as a reference voltage across the circuit.

By means the potentiometer P2 the reference voltage is applied to the non-inverter input of U4, which has a gain of approximately 2.5. The inverter input is connected to output through the resistive divider R13-R12-VR3 taking a sample and comparing it with the voltage applied to the non-inverter input through P2, to get this voltage multiplied by 2.5 in its output: 10 x 2.5 = 25V (VR3 allows to accurately adjust the maximum output voltage to 24VDC. With VR4 to adjust the OpAmp offset to 0V).
This OpAmp output controls the NPN Darlington transistor BDV67, connected in series with the positive branch of power.

In series with the negative branch is interlayered R16 of 0.1 ohms, used as a current sensor, which produces a voltage drop of 500mV with an output current of 5A or 1000mV for 10A, which is applied through R3 at non-inverter input of OpAmp U1, comparing with the voltage applied to their inverter input through P1, VR2 and R6, connected to Vref. The voltage between terminals of P1 is 0~500mV when JP1 is closed, or 0~1000mV when J1 is open.

When the voltage at R16 reaches the value selected by P1 the output of U1 actives transistors Q1 and Q2, growing to control of the inverter input of U4 and ligths the LR led, indicating that the circuit is operating in constant current mode. The reason for the Q2 is used is to minimize the voltage drop if output overload and to avoid interactions with the led. If instead Q2 we use a diode the voltage drop are 0.6V, in the worst case, we would produces a minimum output voltage of 1.5V (0.6 x 2.5 = 1.5V) due to the U4 amplification factor of 2.5.
U1 offset is adjusted with VR1 to obtain a maximum current output of about 1mA when the P1 cursor is at 0V potential.

Only uses a single power transistor, although several can be used in parallel if desired. In the tests the BDV67A was perfectly capable, but need a generous heatsink to maintain its maximum temperature below the limit. (You can also use the Bourns BDW83A Darlington).

The LT1637 OpAmp has been selected to have some extraordinary features, which also include a shutdown input (uses in U3), which allows to put out the PSU to 0V at any time without disconnecting the source, or use for other purposes.

This is all about this project. Be happy that someone suggest improvements or changes and I will try to answer any questions related to it.
If someone decides to build this project I have the PCB design, which I will give him gladly.

Sorry for my bad English, but I have help from Google. :)

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  • 2 weeks later...

Hi Wat69,

thanks for posting your useful modifications to this project.  I'd like to try to build this circuit and I was wondering whether you could post the PCB layout and some more details on how you actually built yours.

Thanks so much,

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  • 2 weeks later...
Guest wat69

The darlington has not survived a shortcut.  :'(

The BDV67A transistors supplied by Farnell are weak MOSPEC with a maximum power dissipation of 125W (Magnatec are 200W), so I decided to replace it with a pair of discrete transistors in darlington configuration, failing to find another model of higher power.
I used the D44-H8 as driver and the Motorolo MJL3281A (ON Semiconductor) as an element of power, with excellent results.  :D

The MJL3281A is a robust 200W (260V/15A) and the D44-H8 (60V/8A), although serious enough a smaller transistor, is very fast and cheap.
The D44 can be mounted on a small heatsink or a small piece of aluminum which dissipates very little power.

This is the mod:


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  • 3 weeks later...

First question, "It uses a 5V precision micropower voltage reference (U2-LT4050)" . This LT4050 have disapear?? Not nedded anymore?..
A seconde question, I pretend make a Supply like yours, but to use with one transformer of (2x15.5V 3A) or (31V 1.5A). I need to make modification in the circuit or not?  Thanks,


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  • 2 weeks later...

Hi winterpt,

I do not understand your first question. The LT4050-5.0 (U2) is connected to the (U3) LT1637 (view schematic) and I know that is gone :-\

With a 31 VAC transformer the no stabilized voltage is close to 44 VDC which is the maximum operating voltage of the LT1637, so you have to trust that it can operate without problems. The filter capacitors C1 & C2 should be 50V or more and also the value of R12 may have to tweak it for 30V (or 31V) output and the LEDs resistors (R1 & R2) could be of 3K9 or 4K3. The rectifier bridge B1 (15A/200V) may also be less powerful.

For 1.5 Amp. the resistor R16 should be replaced by 0.33 ohms, since the maximum voltage drop is 500mV at 1.5A (JP1 closed) or 1V at 3A (JP1 open).

Apart from the above considerations do not think it necessary to modify anything else.

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  • 3 weeks later...

Hi msr2009, my intention is not to compete with the other PSU project, I've adapted this project only to my needs.

For indicators have used cheap chinese 1V DVM's (I think using the GC7139/GC7140 chips).

I include the files of the PCB (I use the ExpressPCB/SCH program); I expect them to help.
In the main circuit are included the SMD components but also must take into account that are mounted upside down on the other side of the board (this program does not allow the light reversed). I separated the files of the components of the top and bottom for clarity.

Thanks and good luck.


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