Soft start of the amplifier power supply with one relay. Soft start SMPS - Power supplies (switching) - Power supplies
Hello to all comrades! The story continues.
Today we have: a power amplifier, a soft start, a power supply for a power amplifier.
Power amplifier LM3886
I once made an amplifier on a microcircuit, now it's time to listen. The circuit is classic, non-inverting. I followed some well-known recommendations. Capacitor C3 is a filter against high-frequency interference. R6 - protects the non-inverting input when the system is turned off (when the internal undervoltage protection system is turned off, there is a possibility of the microcircuit failing). Diodes D1 and D2 protect the output stage from the EMF of the inductive load. It is better to install capacitors C5 - C8 with a larger capacity, but I was critically short of space, and I installed only 200 uF.I took the liberty and changed the gain of the circuit downward (21 → 11). They say that as it decreases, the probability of self-excitation of the amplifier increases, but for me everything is fine even without the R9-R10-C9 chain. I never connected it. And without her everything seems fine, at least by ear. The fact is that at a given gain and at a volume level of 0 dB (volume control value), the maximum undistorted output power is 2x45 Watts (sine on the resistors as a load). See waveforms in the Measurements section.
If it’s louder, then we get into clipping. Eliminating clipping is perhaps the simplest step towards high-quality sound from the system. You can change the gain of the amplifier by placing a divider at the input of the power amplifier. It was possible to limit the signal level in the volume control itself (lower the maximum possible volume programmatically in the parameters). Here everyone decides for themselves what is best.
We use the “MUTE” input signal to eliminate various transient processes when turning the player on and off. To turn on the amplifier, you need to connect the 7th pin of the microcircuit with a negative voltage source through a resistor and provide a current of at least 1 mA. Inconvenient compared to . The optocoupler just begged to be included in the circuit. The 5V voltage to connector X2 will come from the amplifier soft start board - see Figure 3.
UZMCH power supply
Rice. 3. Amplifier power supply and soft start circuit
Usually, for the first launches of their designs (amplifiers, power supplies), radio amateurs turn on a light bulb in series so that nothing shoots out in case of errors. One day I thought - why not leave the light bulb in the device forever. Only, of course, the light bulb should be small; a halogen one will do just fine.
Halogen lamp 50 W at 220 V, type G6.35
In my previous homemade amplifier, I successfully tested a soft-start circuit on a halogen light bulb. I liked it so much that I decided to use it again. I should immediately note that the light bulb does not burn out over time, but in the absence of emergency situations, it is still less reliable than a resistor.
When they crashed (probably from static), I realized that this solution also works as short circuit protection. The speakers were not damaged in the accident.
The essence of the circuit is simple: we shunt the ballast (light bulb) when the voltages on the output capacitors are normal (>27V). And vice versa - if you arrange a short circuit, the light bulb is switched back on to the circuit of the primary winding of the transformer.
A comparator circuit based on TL431 is installed on each arm of the power supply unit. Optocoupler OP1 provides a small hysteresis (less than 15V - failure), OP2 - for the convenience of summing signals from 4 arms.
The circuit starts working immediately after turning on the 5-volt power supply of the audio player. A voltage of 5V is supplied to connector X2, after which relay K1 turns on the transformer through the light bulb. After charging the capacitors, a signal arrives at connector X3, which turns off K1 and turns on K2. That's it, soft start is complete. After some time (set by the R2-C4 chain), we have 5V on connector X7, which opens optocouplers OP1 in power amplifiers. When you turn off the audio player, the 5V on connector X2 disappears and both relays turn off due to the lack of power to them. The transformer is completely disconnected!
To reduce the thermal load on the diodes, a separate rectifier is installed on each amplifier channel.
Implementation. Photos
Rice. 4. Transformer
The transformer wound itself. I once kept, and did not throw away, a burnt-out bourgeois transformer, the iron in it was gorgeous. The frame was made of fiberglass, the window turned out larger than with the original frame. Each layer of all windings is separately impregnated with winding varnish and individually dried in an oven at 100°C.
Rice. 5. Soft start board (top view)
Rice. 6. Soft start board (bottom view)
Now I cover the boards with acrylic varnish PLASTIK 71. The varnished boards look amazing, I recommend it.
Rice. 7. Diode bridge (top view)
Rice. 8. Diode bridges (bottom view)
Rice. 9. Amplifier
The amplifier board turned out to be extremely distorted, all this due to the lack of space in the case. I had to bend the pins of the microcircuit and make the board double-sided. The left and right channel boards are slightly different; some components had to be moved because they rested on the soft start board.
Rice. 10. Output connectors
The output connectors are made from old powerful Soviet (military) connectors, or rather from their pins (male/female).
Rice. eleven. Output connector installed in the housing
Rice. 12. 220V and Ethernet connectors
UMZCH measurements
Rice. 13. Photo at the time of testing the maximum possible output power
All measurements were made with an oscilloscope with the channels loaded to a resistive load of 7.8 Ohms. The goal is to determine the maximum power for a given power supply.
Rice. 14. Supply voltage (idle)
I wonder how much the supply voltage will drop under maximum load. Let me remind you that during measurements, my transformer will be loaded with two channels, and power measurements are obtained on the diode bridge of one channel, since I have my own diode bridge for each amplifier.
Rice. 15. Power supply voltage drop for one channel under 45 W load
The voltage dropped by 3.6 V. Between the maximum output value of the sine and the supply voltage is about 3 V. Of course, it could have been made a little louder, but then clipping begins.
Rice. 16. Supply voltage ripple under load 45 W
The ripple is no more than 1 V, a slight modulation of 1 KHz is observed (test signal 1 KHz).
Figure 17. Output L R channels 1KHz
In Figure 17, the long-awaited sines are 1 KHz, 2x45 W. (45 = 18.8×18.8 / 7.8)
Rice. 18 Output L, R channels 20 KHz
It wouldn't hurt to look at the spectrum; I'm too lazy to connect it to a PC; I'll have to make a divider. Let's take a look with an oscilloscope and that's it. See Figure 19.
Rice. 19. Signal spectrum 1 KHz (top), 20 KHz (bottom)
As a spectrum analyzer, an 8-bit oscilloscope is inferior to a sound card. But at least in the 60 dB range there was no disaster, thank God.
These two are power device circuits with a toroidal transformer. Typically the starting (inrush) current is very high for a short period while the smoothing capacitors are charging. This is a kind of stress for capacitors, rectifier diodes and the transformer itself. Also at such a moment the fuse may blow.
The soft start circuit is designed to limit the starting current to an acceptable level. This is achieved by connecting the transformer to the mains supply through a resistor, which is connected for a short time using a relay.
The circuits combine soft start and push-button control, thus creating a ready-made module that can be used in power amplifiers or in conjunction with other electrical appliances.
Description of soft start circuits
The first circuit is built on CMOS logic chips (4027), and the second on the NE556 integrated circuit, which consists of 2 combined in one package.
As for the first circuit, it uses a JK flip-flop connected as a T flip-flop.
T-flip-flop is a counting flip-flop. The T-trigger has one counting (clocking) input and one synchronizing one.
When J2 is pressed, the trigger state changes. When transitioning from the off state to the on state, the signal is transmitted through a resistor and capacitor to the second part of the circuit. There, the second JK flip-flop is connected in an unusual way: the reset pin is driven high, and the SET pin is used as an input.
In the truth table, you will find that when the reset pin is high, all other inputs are ignored except the SET pin. When the SET pin is high, the output is also high in reverse.
Resistor R6 and capacitor C6 are used to delay the signal at the moment of switching on. With the values indicated in the diagram, the delay is 1 second. If necessary, parameters R6 and C6 can change the delay time. Diode VD2 bypasses resistor R6, as a result of which when turned off, the relay turns off without delay.
The second circuit uses a dual NE556 timer. The first timer is used as a push-button switch, and the second as a switch associated with the delay created by elements R5, VD2 and C6.
Resistors R8 - R10 have a resistance of 150 Ohms and a power of 10W. They are connected in parallel resulting in a 50 Ohm resistor with a power of 30 W. On the PCB, two of them are located side by side, and the third is in the middle on top of them. The power of transformer Tr1 is about 5 W with a voltage in the secondary winding of 12-15 V. Connector J1 is used if 12 volt power is needed for other external devices.
Relays K1 and K2 are 12V, the contact groups of which must be designed for switching 220V / 16A. The value of fuse F1 must be selected in accordance with the device that will be connected to the soft starter module.
Both circuits have been tested on a breadboard and both work, but the second circuit is susceptible to interference if the wire going to the button is long enough, which in turn causes false switching.
Most resistors, capacitors and diodes are SMD. Lately I've been using more and more SMD elements in designs because there's no need to drill holes. If you decide to use either of these two PCBs, check them carefully because they have not been tested.
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The article uses materials from an article by Alexey Efremov. I had the idea of developing a soft start device for a power supply a long time ago, and at first glance it should have been implemented quite simply. An approximate solution was proposed by Alexey Efremov in the above-mentioned article. He also based the device on a key based on a powerful high-voltage transistor.
The chain to the key can be represented graphically like this:
It is clear that when SA1 is closed, the primary winding of the power transformer is actually connected to the network. Why is there a diode bridge at all? - to provide direct current power to the switch on the transistor.
Circuit with transistor switch:
The given ratings of the divider are somewhat confusing... although the hope that the device will not smoke or bang remains, doubts arise. And yet I tried a similar option. Only I chose a more harmless power supply - 26V, of course, I chose other resistor values, and used not a transformer as a load, but a 28V/10W incandescent lamp. And the key transistor used BU508A.
My experiments have shown that a resistor divider successfully lowers the voltage, but the current output of such a source is very small (the BE junction has low internal resistance), and the voltage across the capacitor drops significantly. I didn’t risk infinitely reducing the value of the resistor in the upper arm, in any case - even if we find the correct current distribution in the arms and the transition is saturated, it will still be only a softened, but not a smooth start.
In my opinion, a truly soft start should occur in at least 2 stages; First, the key transistor opens slightly - a couple of seconds will be enough for the filter electrolytes in the power supply to be recharged with a weak current. And at the second stage it is already necessary to ensure the complete opening of the transistor. The circuit had to be somewhat complicated; in addition to dividing the process into 2 stages (stages), I decided to make the switch composite (Darlington circuit) and as a source of control voltage, I decided to use a separate low-power step-down transformer.
*Ratings of resistor R 3 and trimmer R 5. To obtain a circuit supply voltage of 5.1V, the total resistance R 3 + R 5 must be 740 Ohms (with R 4 = 240 Ohms selected). For example, to ensure adjustment with a small margin, R 3 can be taken 500-640 Ohm, R 5 - 300-200 Ohm, respectively.
I believe there is no particular need to describe in detail how the scheme works. In short, the first stage is launched by VT4, the second is launched by VT2, and VT1 provides a delay in switching on the second stage. In the case of a “rested” device (all electrolytes are completely discharged), the first stage starts after 4 seconds. after turning on, and after another 5 seconds. the second stage starts. If the device is disconnected from the network and turned on again; the first stage starts after 2 seconds, and the second - after 3...4 seconds.
A little tweaking:
The whole setup comes down to setting the open circuit voltage at the stabilizer output, set it by rotating R5 to 5.1 V. Then connect the stabilizer output to the circuit.
You can also choose the value of resistor R2 to your taste - the lower the value, the more the key will be open at the first stage. At the nominal value indicated in the diagram, the voltage at the load = 1/5 of the maximum.
And you can change the capacitances of capacitors C2, C3, C4 and C5 if you want to change the turn-on time of the stages or the turn-on delay of the 2nd stage. The BU508A transistor must be installed on a heat sink with an area of 70...100mm2. It is advisable to equip the remaining transistors with small heat sinks. The power of all resistors in the circuit can be 0.125W (or more).
Diode bridge VD1 - any ordinary one for 10A, VD2 - any ordinary one for 1A.
The voltage in the secondary winding TR2 is from 8 to 20V.
Interesting? Need a signet or practical recommendations?
To be continued...
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This simple device can improve the reliability of your radio equipment and reduce network interference when turned on.
Any power supply for radio equipment contains rectifying diodes and high-capacity capacitors. At the initial moment of turning on the mains power, a pulse current jump occurs - while the filter capacitors are being charged. The amplitude of the current pulse depends on the capacitance value and voltage at the rectifier output. So, at a voltage of 45 V and a capacitance of 10,000 μF, the charging current of such a capacitor can be 12 A. In this case, the transformer and rectifier diodes operate briefly in short-circuit mode.
To eliminate the danger of failure of these elements by reducing the inrush current at the time of initial switching on, the one shown in Fig. is used. 1.7 diagram. It also allows you to lighten the modes of other elements in the amplifier during transient processes.
Rice. 1.7
At the initial moment, when power is applied, capacitors C2 and SZ will be charged through resistors R2 and R3 - they limit the current to a value that is safe for the rectifier parts.
After 1...2 seconds, after the capacitor C1 is charged and the voltage on the relay K1 increases to a value at which it will operate and bypass the limiting resistors R2, R3 with its contacts K1.1 and K1.2.
The device can use any relay with an operating voltage lower than that at the output of the rectifier, and resistor R1 is selected so that the “extra” voltage drops across it. The relay contacts must be designed for the maximum current operating in the amplifier's power supply circuits. The circuit uses a relay RES47 RF4.500.407-00 (RF4.500.407-07 or others) with a rated operating voltage of 27 V (winding resistance 650 Ohms; current switched by contacts can be up to 3 A). In fact, the relay operates already at 16...17 V, and resistor R1 is selected as 1 kOhm, and the voltage across the relay will be 19...20 V.
Capacitor C1 type K50-29-25V or K50-35-25V. Resistors R1 type MLT-2, R2 and R3 type S5-35V-10 (PEV-10) or similar. The values of resistors R2, R3 depend on the load current, and their resistance can be significantly reduced.
Rice. 1.8
The second diagram shown in Fig. 1.8, performs the same task, but allows you to reduce the size of the device by using a timing capacitor C1 of smaller capacity. Transistor VT1 turns on relay K1 with a delay after capacitor C1 (type K53-1A) is charged. The circuit also allows, instead of switching secondary circuits, to provide a stepwise voltage supply to the primary winding. In this case, you can use a relay with only one group of contacts.
The value of resistance R1 (PEV-25) depends on the load power and is selected such that the voltage in the secondary winding of the transformer is 70 percent of the rated value when the resistor is turned on (47...300 Ohms).
Setting up the circuit consists of setting the delay time for turning on the relay by selecting the value of resistor R2, as well as selecting R1.
The given circuits can be used in the manufacture of a new amplifier or in the modernization of existing ones, including industrial ones.
Compared to similar devices for two-stage supply voltage given in various magazines, those described here are the simplest.
This circuit limits the current through the power wires to 5A for about 1.5 seconds. After this, the time relay will close and the current consumption will no longer be limited. This is a very useful device, because if you have a large transformer or electrolytic capacitors of significant capacity, then at the moment of switching on they will act as a short circuit for a short period of time.
The power delay circuit is implemented by temporarily connecting several power resistors into the circuit, thus minimizing the large inrush current.
The relay is used at 24 volts, with contacts that can withstand 0 amperes and above. The delay time depends on the total capacitance of C2 and C3, as well as their charging rate, determined by capacitor C1, which acts as a ballast resistor. The soft start device will also work perfectly in tandem with electric motors.
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Essentially, the soldering iron tip is hardened due to a short circuit. The secondary winding contains half a turn, the voltage is about 1 volt, but the current reaches 15 Amperes! It is precisely because of the reduced voltage that the load is not so great, and during operation the parts are almost cold.
