Electrical circuit diagram of a homemade laboratory power supply. Laboratory power supply: master class on how to make a simple device with your own hands

For setting up, repairing automotive electronic and radio devices or charging batteries must have good source nutrition.

Use of modern circuit technology and element base allow you to make a power source at home, which in its basic technical characteristics is not inferior to the best industrial designs.

Basic requirements that such a power source must satisfy:

• voltage adjustment in the range 0 - 25 V;
• ability to provide load current up to 7 A with minimal ripple;
• adjustment of current protection operation. In addition, the operation of the current protection must be fast enough to prevent damage to the source itself in the event short circuit on the way out.

The ability to smoothly adjust the current limit in the power supply allows you to configure external devices prevent their damage. All these requirements are satisfied by the proposed universal power supply circuit. Besides, this block power supply allows you to use it as a source of stable current.

Basic technical specifications power supply:

• smooth voltage adjustment in the range from 0 to 25 V;
• ripple voltage, no more than 1 mV;
• smooth adjustment of current limitation (protection) from 0 to 7 A;
• voltage instability coefficient is not worse than 0.001%/V;
• current instability coefficient is not worse than 0.01%/V;
• The source efficiency is no worse than 0.6.

Schematic diagram

The electrical circuit of the power supply consists of a control circuit, a transformer (T1), a rectifier (VD4 - VD7), power control transistors VT3, VT4 and a switching unit for transformer windings.

The control circuit is assembled on two universal operational amplifiers (op-amps), located in one housing, and is powered by a separate transformer T2. This allows the output voltage to be adjusted from zero to more stable work the entire device.

For relief thermal regime For the operation of power control transistors, a transformer with a sectional secondary winding is used. The taps are automatically switched depending on the output voltage level using relays K1, K2. What allows, despite high current in load, apply heat sink for VT3 and VT4 comparatively small sizes, and also increase the efficiency of the stabilizer.

The switching unit is designed to, using only two relays, ensure switching of four taps of the transformer; it turns them on in the following sequence: when the output voltage exceeds the level of 6.2 V, K2 turns on; when the level of 15.3 V is exceeded, K1 is turned on (in this case, the maximum voltage is supplied from the transformer windings).

The specified thresholds are set by the zener diodes used (VD10, VD12). The relay is turned off when the voltage drops negative sequence, but with a hysteresis of approximately 0.3 V, i.e. when the voltage drops by this value lower than when turned on, which eliminates chatter when switching windings.

The control circuit consists of a voltage stabilizer and a current stabilizer. If necessary, the device can operate in any of these modes. The mode depends on the resistance of the regulators "I" (R21, R22). The voltage stabilizer is assembled on elements DA3, VT5, VT6.

Rice. 1. Schematic diagram laboratory source power supply with current limiting adjustment.

The stabilizer circuit works as follows. Necessary output voltage set by resistors “roughly” (R9) and “finely” (R10). In voltage stabilization mode, the signal feedback voltage (-Uoc) from the output (X2) through a divider of resistors R9, RIO, R11 is supplied to non-inverting input 2 operational amplifier DA3.

A reference voltage of +9 volts is supplied to the same input through resistors R3, R5, R7. When the circuit is turned on, the positive voltage at output 12 DA3.1 will increase (it comes to control VT4 through transistor VT5) until the voltage at the output terminals X1 and X2 reaches the level set by resistors R9, R10.

Due to the negative voltage feedback coming from output X2 to input 2 of the DA3.1 amplifier, the output voltage of the power supply is stabilized. In this case, the output voltage will be determined by the relationship:

where Uop = + 9 V.

Accordingly, by changing the resistance of resistors R9 “roughly” and R10 “precisely”, you can change the output voltage (Uout) from 0 to 25 V. When a load is connected to the output of the power source, a current begins to flow in its output circuit, creating a positive voltage drop across resistor R23 (relative to the common wire of the circuit).

This voltage is supplied through resistor R21, R22 to the connection point R8, R12. A reference negative voltage of 9 volts is supplied from the zener diode VD9 through R6, R8.

The DA3.2 operational amplifier amplifies the difference between them. While the difference is negative (i.e., the output current is less than the value set by resistors R23, R24), + 15 V operates at output 10 DA3.2. Transistor VT6 will be closed and this part of the circuit does not affect the operation of the voltage stabilizer.

When the load current increases to a value at which a positive voltage appears at input 7 of DA3.2, there will be a negative voltage at output 10 of DA3.2 and transistor VT6 opens slightly. A current will flow in circuits R16, R17, HL1, which will reduce the opening voltage based on the regulating power transistor VT4.

The glow of the red LED (HL1) indicates that the circuit has entered current limiting mode. In this case, the output voltage of the power supply will decrease to a value at which the output current will have a value sufficient for the current feedback voltage (Uoc) taken from resistor R10 and the reference voltage at the connection point R8, R12, R22 to mutually were compensated, i.e., zero potential appeared.

As a result, the output current of the source will be limited at the level specified by the position of the slider of resistors R21, R22. In this case, the current in the output circuit will be determined by the relation:

where Uop = - 9 V.

Diodes (VD11) at the inputs of operational amplifiers protect the microcircuit from damage if it is turned on without feedback or if the power transistor is damaged. In operating mode, the voltage at the inputs of the op-amp is close to zero and the diodes do not affect the operation of the device.

Capacitor C8 limits the band of amplified op-amp frequencies, which prevents self-excitation and increases the stability of the circuit.

Settings

With error-free installation in the control unit circuit, you will only need to adjust the maximum output voltage adjustment range 0: 25 V with resistor R7 and the maximum protection current of 7 A with resistor R8.

The switching unit does not need to be configured. It is only necessary to check the switching thresholds of relays K1, K2 and the corresponding increase in voltage on capacitor C3.

When the circuit is operating in voltage stabilization mode, the green LED (HL2) lights up, and when switching to current stabilization mode, the red LED (HL1) lights up.

Details

Trimmer resistors R7 and R8 are SPZ-19a type; variable resistors R9, R10, R21, R22 - type SPZ-4a or PPB-1 A; fixed resistors R23 - type C5-16MV for 5 W, the rest are from the MLT or C2-23 series of appropriate power.

Capacitors C6, C7, C8, SYU type KIO-17, electrolytic capacitors C1 - C5, C9 type K50-35 (K50-32). The DA1 chip can be replaced with an imported analogue 78L15; DA2 - on 79L15; DA3 on rA747 or two 140UD7 microcircuits.

LEDs HL1, HL2 are suitable for any different colors glow. Power transistors are installed on a radiator with an area of ​​about 1000 cm^2.

Two power transistors are installed in parallel to ensure reliable operation of the device in the event of a short circuit at the output terminals.

In the worst case, power transistors must briefly withstand power overload P = Ubx*I = 25x7 = 175 W. And one KT827A transistor can dissipate power no more than 125 W. Diodes VD4 - VD7 must be installed on a small radiator.

Relays K1, K2 are used in standard size R-15 (Polish production) with a winding on operating voltage 24 V (winding resistance 430 Ohm) - due to their unframed design, they have small dimensions and sufficiently powerful switching contacts. You can also use domestic relays such as REN29 (0001), REN32 (0201).

Switching voltage from transformer T1, relays K1 and K2 are inertial and do not provide an instantaneous reduction in the voltage coming from the secondary winding of T1, but they will reduce the thermal dissipation of power on the power transistors when long work source.

Microammeter RA1 is a small-sized type M42303 or similar with an internal shunt for a current of up to 10 A. For ease of operation of the power source, the circuit can be supplemented with a voltmeter indicating the output voltage.

An industrial transformer type TPPZ19-127/220-50 is used as a network transformer T1. T2 - type TPP259-127/220-50. The transformer can be made independently on the basis of an industrial transformer with a power of 200 W, winding all the windings (T1 and T2) on one transformer.

About once a year, an inexorable desire awakens in me to make a laboratory power supply (for example, I described my last laboratory). And then they offered to review something - well, I couldn’t resist, because I’ve wanted to try this module for a very long time. Unfortunately, there will be no dismemberment, because the structure is extremely difficult to disassemble, and I was afraid not to assemble it properly in reverse. :)

There was already a similar module, but this one attracted me with its display. Still, large numbers are much more convenient than small ones.

I will start, however, not with the main character of the review, but with the second, no less important one (also provided for review), without which this module is useless.

The power supply is somewhat different from the original version, and, unfortunately, not for the better. External differences consist in the inscription ac-dc 24v instead of 2412DC on the original version, and the presence of a certain website address on the bottom side of the board. The “internal” differences are much more interesting. But first, the appearance.

The main problem of this copy (or rather the entire batch) is the low-quality output connector. it is completely disgustingly soldered, and naturally poorly soldered. You need to solder it right away, because it barely holds on. However, as I wrote, this is a problem of an instance or a batch, and in general, the likelihood of this problem repeating among other buyers after some time is not so great.

In general, soldering is not very neat, and it is advisable to inspect the board and solder suspicious areas

The famous capacitor is sealed as usual as before, and it is also advisable to replace it, as the respected Kirich wrote. He also recommends hanging ceramics along the outlet and parallel to the output electrolytes.

The snubber diode, however, is soldered correctly:

The board is well washed, and in general everything is fine with it, if not for one small BUT. It seems that the manufacturer of the PWM controller on which this power supply is assembled decided to improve the “green” mode, and instead of reducing the frequency by light load- produces packets of pulses at the standard 62-64 kHz to the gate of the power transistor. It looks on the oscillator as a short burst of control pulses and a long pause - about 30 mS (when working without load), and with increasing load these pauses decrease. And everything would be fine, if not for that very small BUT - as a result, we end up with a fair amount of “saw”:

In the photo - operation without load and with a one-amp load seems to be the case. AC 0.2V/div and 5mS/div.

It seems that my thoughts above are correct, and this is such an interesting “feature” of new versions of the power supply. The old ones, as they said, significantly reduced the frequency - down to 14-15 kHz, but these ones begin to work “pulse” and produce a saw output. It’s not entirely clear to me how to deal with this - I also tried installing capacitors of larger capacity - but it didn’t work.

Naturally, advice on improvements is welcome in the comments, because now it seems that all power supplies come with such a “feature,” at least in the comments to Kirich’s review I came across similar oscillators.

However, oddly enough, in the end everything works quite normally.

Well, let's move on to the main character?

Supplied in a clear plastic box, wrapped in instructions. The instructions are large, on good paper, in Chinese and quite sane English.

As you can see, the stated accuracy is 0.5%, and I must say that it fully provides it, although at very low currents it lies, which, however, is natural - but this is lower.

The module itself is compact (window dimensions in the housing for installation are 39x71.5, plus samples up to 75.5, depth 35.5), display 28x27, digit height 5mm (on a “regular” ampere-voltmeter 7.5mm). The display itself is bright, contrasty, with good viewing angles. The only thing I don’t really like is the rather slow update (the readings are probably updated twice a second). But I think this is not a problem with the display, but with the firmware, and it doesn’t bother me at all.

Additional information

On the 8-legged microchip it says XL7005A - PWM controller 150 kHz 0.4A

Unfortunately, to disassemble it - non-trivial task, because three boards are soldered together in a “sandwich”, three connectors with 8 contacts, which are quite tight, and you can easily touch something and ruin it. so sorry. Above the encoder you can see the inscriptions rx gnd tx - apparently the module supports data transfer, and above that there is clearly a connector for flashing. In general, the build quality left a pleasant impression. The flux is not washed off in the places where the transition contacts are soldered, which is natural and understandable, and the flux is clearly such that it does not require washing off.

It is clear that such a module is purchased not for disassembly, but for assembly, and not for what, but the power supply. For those who do not know what a laboratory power supply is and why it is needed, I will briefly write what it is adjustable block power supply, with output current limitation and output voltage regulation. It is needed to power devices “on the table”, for example during repair or development. It prevents you from accidentally burning something;) You can also charge batteries with it, for example.

Let's move on to assembling the power supply. Perhaps I’ll hide it under a spoiler, otherwise there will be a lot of photos.

power supply assembly

We will assemble it in the Kradex Z-3 case. all the components fit into it so well that it seems that they were simply created for each other. ;)

Kradex cases are distinguished by the idiotic design of the connecting posts - they are too far from the side walls and too close to the front and rear. Therefore, we mercilessly bite them out and move them to the middle of the building, where they will not disturb anyone. We fix it with dichloroethane. Similarly, we make racks for attaching the power supply.

Next, we mill the front and back panel, as well as holes for the fan. in principle, it’s not really needed, but I decided to install it right away so as not to have to get up twice. Unfortunately, there was only enough space for a 50mm fan.

Since there will be a USB connector on the “face,” we solder textolite “ears” to it, and glued pieces of plastic with pre-cut M3 threads to the body. the shortest screws “from the computer” are perfect for attaching the connector to the front panel.

I don’t know that you need to clamp the cutter into the chuck, and there is a fan chuck, and the collets are good, but I’m a slob, and the material here is soft, so I’m too lazy to put in another chuck and I mill small things like that.

For USB power supply and the fan, I used converters from my previous review, gluing them to a radiator made of an 8x15 w-shaped profile. greatly improves cooling. The fan is powered from 6.5V - at 5V it blows very weakly. I wanted to add a speed control, but I was too lazy, and decided that a separate converter would be enough for manual installation any speed you like.

I decided to modify the “primary” power supply - slightly increase the voltage in order to get at least 24V at the output of the entire device. Taking into account the limitation of the maximum input voltage of the used converters to 28V, I decided to “overclock” the power supply to 26V. To do this, solder a 22 kOhm resistor parallel to resistor R19.

Well, the result:

Now let's move on to testing.

First of all, how does it work? top small line - set values current and voltage. large numbers are the measured values ​​at the output, and below - input voltage(minimum difference between input and output is about a volt). The icons on the right show the current status: blocking, status (ok/not ok), output mode (cc/cv) and output status - on/off. When turned on, the output is turned off. Turn the output on and off using the button under the encoder. The icon is off - red, on - green. Locking by long pressing the encoder.

When we press the set button, we have the opportunity to change the current current and voltage values. the variable digit is highlighted in red in top line, and switches by pressing the encoder. Rotating the encoder changes the value. when moving from 9 to 0, the most significant digit increases.

When you press set again, we get to the “advanced” settings menu. And in the top line, accordingly, they begin to display current parameters output - current and voltage.

Here we have the output voltage, output current, voltage/current/power of the protection, backlight brightness, and the current memory cell. There are 10 of these cells. M0 is the “manual” mode, that is, what we are dabbling in now. these values ​​are saved and restored the next time you turn them on.

Select a parameter using the up/down buttons, then press the encoder and change the parameter, exit with the set button. in order to save values ​​​​to some memory cell, you must first select it in lower point menu, then change everything you need, and then go to the cell number in the bottom menu item and hold the set button for two seconds. The number of the cell in which it was saved will appear on the left between the icons.

On|off in the lower menu item on the right is the output state when selecting this memory cell. off - turned off, on - “as it was.”

The controls are, of course, a little strange. To be honest, I still don’t understand how these “protections” work; I simply use them in current limiting and voltage stabilization mode.

Next. the next press of the set button takes us to “ home screen" The selection of a memory cell is carried out either by holding the up button to select M1, or the down button to select M2, or the set button - and then use the encoder to select the cell number. It’s a shame that when switching memory cells, the current and voltage stored there are not displayed. It would be logical and convenient - but no.

Now - measurements. I put it in a table, and, to be honest, I won’t even really count and comment, because the pot is no longer cooking something;) Set is what we set, meas is what it measures at its output, tester is what it shows accordingly tester. At low currents it lies quite significantly, but IMHO this is forgivable. From 100mA and above - it consistently lies at 3mA (underestimates), at lower currents - not so much, but it also lies. In my opinion, the error at adequate currents varies (0.5% +2 digits). Let the metrologists correct it if anything;) At low currents, of course, it’s a miss.

Oh, I almost forgot. interference and ripple measurements.

At low currents:

At high (2.5A it seems) currents:

AC 0.2V 500µS.

When turned on, the voltage gradually increases, turns on in CC mode, then switches to CV mode:

If you connect the LED and then turn on the output, it lights up for approx. If you first turn on the output and then connect the LED, it doesn’t even have time to make a sound, it burns out instantly, which is predictable.

To sum it up: I really like it. IMHO for this money (up to 50 bucks) there are simply no alternatives. In terms of work, IMHO he will be no worse than any other Chinese laboratory worker. It’s not the most thoughtful control, but it’s not all that scary either - I think you can get used to it quickly enough, and what’s there to really control... you set it up once and be happy, but then turning the voltage is a matter of the button and the encoder. According to the design of the power supply, I’m no longer sure that the sockets should have been made on the left, perhaps it was worth moving them to the right - which, however, can be done simply by turning the front panel over. Undoubtedly, in the comments they will throw out links to cheaper options, but even for this amount, everything is quite good.

The product was provided for writing a review by the store. The review was published in accordance with clause 18 of the Site Rules.