Electrical circuit blocking a generator using one transistor with a description of the operating principlefor DIY assembly. The transistor can be bipolar or field effect. Blocking was invented at a time when there were no microcircuits yet, but the circuit is still of interest.
Blocking oscillator is a self-oscillator with strong transformer positive feedback, designed to generate short-term pulses with a large ratio of period to pulse duration, i.e. with high duty cycle. The blocking oscillator frequency can range from several Hertz to hundreds of KHz.
The blocking generator circuit and timing diagrams of operation are shown on the tab (clickable). The coupling winding is connected to the emitter-base junction of the transistor VT in series through capacitor C. When the circuit is powered on, a slight increase in the collector current through the coupling winding causes the base current to appear and increase. This process is avalanche-like and leads to the transition of the transistor to a saturation state.
The same current charges the capacitor, thereby reducing the base-emitter voltage. When the capacitor charging voltage becomes equal to the voltage on the coupling winding, the base current and, accordingly, the collector current sharply drop to zero. An almost rectangular voltage pulse is formed in the output winding.
Since, from this moment, the feedback voltage is almost zero, the negative polarity voltage of capacitor C is applied to the base-emitter junction and puts the transistor into a cutoff state. Next, the process of discharging capacitor C begins exponentially through R from the power source. When the opening voltage is reached, an avalanche-like increase in the transistor current begins and a new pulse is formed, the process becomes periodic.
The transistor can be anything with a sufficiently high gain. The transformer is usually wound on a ferrite ring. The collector winding contains 30-50 turns of wire. Communication winding 3-5 turns. The smaller the ring size and the lower the planned generation frequency, the more turns are required. If a field-effect transistor is used, the communication winding contains the same number of turns as the exciting winding, since a voltage of 4 to 20 volts is required to control the key field-effect transistors.
The generator transistor must be protected from EMF emissions. If the transistor is a field-effect transistor, it is enough to place a diode between the gate and the plus of the power source. In this option, the pulse at the drain will be cut off at the voltage level of the IP plus the drop across the diode (0.5 - 1 V). Field-effect transistors are usually protected from drain overvoltage by built-in diodes.
In the simplest case, you can do without a capacitor. In this embodiment, the blocking oscillator switches when the ring is saturated. A simplified circuit can be used for low-voltage power supplies and small ring sizes. The efficiency of the circuit is quite low.
The blocking frequency of the generator is highly dependent on the supply voltage. In this regard, it is better to use pulse generators on microcircuits, especially since you do not need to wind the communication winding. It makes sense to use blocking when the power source voltage does not exceed a few volts, for example, when powered by 1-3 batteries. If you use a germanium transistor, the circuit can operate when the batteries are discharged to 0.5 V.
In this article I will tell you about what is a blocking generator.
A blocking generator is a pulse generator of relatively short duration and long period. It works thanks to transformer feedback. Because of its simplicity, the blocking generator is widely used in compact voltage converters (for example, this circuit can be found in every second electronic lighter circuit).
This is a blocking generator (one of many variations of this circuit):
As you can see, it is really easy to assemble. The most difficult part in it is the transformer. But first things first.
1) Operating principle
First, winding 2 acts as a “resistor”, i.e. a current flows through it and the resistor, which begins to open the transistor. Opening the transistor leads to the appearance of a current in winding 1, and this in turn leads to the appearance of voltage on winding 2, i.e. the voltage at the base of the transistor increases further, it opens even more, and this happens until the core or transistor enters saturation. When this happens, the current through winding 1 begins to decrease, therefore the voltage on winding 2 changes polarity, which leads to the closing of the transistor. That's it, the cycle is closed!
2) Details
Transformer winding 1 is usually 2 times larger than winding 2, and the number of turns and wire diameter are selected depending on the voltage on winding 3 and the current through it.
Resistor usually taken within the range of 1 kOhm - 4.7 kOhm.
Transistor Almost anyone will do.
3) Test
First, let's assemble a basic generator circuit. This is the transformer from the ballast of an energy-saving lamp:
On it I first wound winding 2 (18 turns with 0.4 mm wire)
I insulated it (ordinary electrical tape will do)
And then I wound winding 1 (36 turns with the same wire as the 2nd one)
And finally, I inserted the core and secured it with the same electrical tape
At this point the transformer is ready.
I chose a powerful transistor: KT805, because the winding has only 36 turns of not the thinnest wire (low resistance).
Resistor 2.2 kOhm.
This is what I ended up with:
As you understand, I will take food from the crown.
So, with a KT805 transistor, a 2.2 kOhm resistor and winding 1 is 2 times larger than winding 2, the voltage oscillogram between the collector and emitter looks like this:
Amplitude 60V, frequency about 170kHz.
Now let's install a 4.7 kOhm resistor. The oscillogram looks like this:
The amplitude is about 10V, the frequency is the same.
Now let's install a 1kOhm resistor:
Amplitude 120V, frequency about 140kHz.
Now let's put back the 2.2 kOhm resistor and swap the windings:
Amplitude 80V, frequency about 250kHz.
4) Conclusion
The greater the feedback coefficient, the faster the signal rises, and the higher the frequency. (The smaller the resistor, and the greater the ratio of the number of turns of winding 2/the number of turns of winding 1, the greater the feedback coefficient). The feedback factor is also affected by the gain of the transistor.
5) Practical benefits
You probably noticed that I didn’t say a word about winding 3. It is needed in order to relieve the output voltage.
Let's see what happens if we wind 3,100 turns of 0.08mm wire into a winding:
First, of course, we need to wind up the transformer. Let's isolate the last layer in the past:
Now we wind 100 turns of 0.08 wire. We assemble the core. WE CONNECT A DIODE TO THE OUTPUT (any one with a reverse voltage of at least 200V can be used. For example, I took the cheap and common 1n4007). Soldering the circuit:
A diode is needed to cut off negative emissions. Let's look at the output oscillogram:
Constant component 50V, pulses amplitude 50V. To remove the pulse component, we place a capacitor at the output. 0.1uF will do:
Oscillogram:
Constant voltage with an amplitude of 100V.
When approaching:
Small fluctuations with an amplitude of 50 mV.
And finally, the complete diagram:
If there is no generation, solder a couple of microfarads capacitor parallel to the resistor.
List of radioelements
| Designation | Type | Denomination | Quantity | Note | Shop | My notepad |
|---|---|---|---|---|---|---|
| Bipolar transistor | KT805A | 1 | To notepad | |||
| Rectifier diode | 1N4007 | 1 | To notepad | |||
| Resistor | 2.2 kOhm | 1 |
For those of you who don't know what we're talking about, a blocking oscillator is a tiny, self-powered circuit that will allow you to light LEDs from old batteries whose voltage has dropped down to 0.5 volts.
Do you think that the battery has already outlived its usefulness? Connect it to the blocking generator and squeeze every last drop of energy out of it with your own hands!
Step 1: Components and Tools
The project will only need a few things that are visible in the photo, but for those of you who like to read, I'll attach a text version of the list:
- Soldering iron
- Solder
- Light-emitting diode
- Transistor 2N3904 or equivalent
- Resistor 1K
- Toroid bead
- Thin wire, two colors
If you find a 2N4401 or BC337 transistor, the LED will burn brighter, since they are designed for higher current.
Step 2: Wrap the toroid with wire
First you need to wrap the wire around the toroid. I found mine in an old power supply. Toroids are similar in shape to a donut and are attracted by a magnet.
Take two wires and twist their ends together (you don't have to do this, but it will make winding the toroid a little easier).
Pass the twisted ends through the toroid, then take the other two (untwisted ends) and wrap them around the toroid. Do not twist the wires, make sure that there is no place throughout the winding where two leads with the same color are located next to each other. Ideally, you need to make 8-11 turns, located at the same distance from each other and tightly adjacent to the toroid. Once you have completed the wrapping, cut off the excess length of wire, leaving about 5cm to connect to other circuit components.
Strip some of the insulation from the ends of the wires, then take one wire from each side, making sure they are different colors. Twist them and your toroid is ready.
Step 3: Solder the Components
It's time to solder everything into one device. You can put everything on a breadboard, but in the instructions I decided to assemble everything on my knee. You can follow the text instructions or solder everything according to the pictures - everything is perfectly displayed there.
First, take the two outer contacts of the transistor and bend them slightly outward, and bend the middle one inward. Also bend the LED contacts outward. This is an optional step, but it will make soldering the components easier.
Take one of the toroid wires that are left unconnected (that's right, one of the wires not twisted together). Solder it to one side of the resistor. Solder the other end of the resistor to the middle pin of the transistor.
Take the second single wire of the toroid and solder it to the collector of the transistor. Solder the positive contact of the LED also to the collector, and the negative contact to the emitter.
All that's left to do is solder the extension wire to the negative terminal of the LED. Take the piece of wire you had before and solder it to the emitter of the transistor.
Step 4: Trying the device in action
All is ready! You have completed your single transistor blocking oscillator. Attach the twisted toroid wires to the positive terminal of the battery and the extension wire to the negative terminal. If everything is assembled correctly, the LED will light up. If the LED does not light up, try wrapping the toroid with a thinner wire.
Sometimes you need to use a cold cathode fluorescent lamp from the backlight of an old LCD monitor, but do not have an inverter available. A homemade blocking generator will help us! The scheme is quite simple:
I took a ready-made choke from the electronic ballast of a compact fluorescent lamp. This winding, containing the largest number of turns, will produce the highest voltage for the lamp.
You need to carefully remove the core from the inductor, insulate the winding with tape and wrap the collector winding on top with wire of approximately the same thickness. I got about 24 turns. It is necessary to wind turn to turn. Just one layer is obtained.
We glue a layer of tape on top of our winding and wind the base winding onto it - about 6 turns with a wire of the same thickness. We put the core back on. We got a coil with 6 terminals.
Transistor KT835A. You can use others, but not just any one. From my stock, many transistors gave poor results or did not generate high voltage at all.
The transistor must be placed on a radiator - it gets very hot! The resistor also gets very hot, so I used 5 pieces of 10 Ohm each. And 2 capacitors. How everything looks and works in the photos below.
This device was run from a computer power supply. Current consumption 1A. If the lamp does not fully glow from 5 volts, then you can gradually increase the voltage. After ignition along the entire length, the voltage can be reduced so that the lamp heats up less.
Also, the blocking generator allows you to turn on fluorescent lamps even with a burnt-out coil.
Here is an example of how a compact fluorescent lamp works. By the way, the choke was taken from exactly such a lamp.
And this is not the end of the application of this invention! Instead of lamps, you can connect a voltage multiplier to high-voltage wires. Then at its outputs a high voltage is obtained that can penetrate air, i.e. we'll see some lightning!
Only at the multiplier should not be located next to the blocking generator!!! High voltage damages the transistor!!! I burned out a few times before I figured out what was going on.
To view in a larger size, you need to click on the link with the name of the video, or on the YouTube button during playback!
And a voltage multiplier circuit. Capacitors are only suitable for the type shown in the photo, any diodes.
More can be done economical blocking - generator using a horizontal scan transformer (TDKS) from an old TV or monitor. Due to its ability to operate on low voltage, it is also called the joule thief or joule thief. I used one 1.2 V battery. But the device can be powered with higher voltage - I connected a maximum of 19 Volts. Approximate diagram:
Only I used an MJE13003 transistor and a 680 Ohm variable resistor. To properly connect the transformer, you need to find two terminals with the lowest resistance (mine are 0.5 ohms) and two with the highest resistance (mine are 1 ohm). In different lines, the location and resistance of the terminals will be different. Testing the circuit on video:
To view in a larger size, you need to click on the link with the name of the video, or on the YouTube button during playback!
:: Help
Operating principle of blocking generator
When the power is turned on, the transistor opens slightly due to the bias current through resistor R1. Since voltage has not been applied to the transformer before, no current flows through the windings (the current through the inductor cannot change instantly, and current cannot immediately arise through the load, since there is always some coupling or leakage inductance). So the entire supply voltage is immediately formed on winding 2. Consequently, a voltage appears on winding 1, determined by the ratio of the number of turns of windings 2 and 1. An additional current appears in the base circuit, sufficient to saturate the transistor.
The circuit remains in this state until the voltage on the capacitor reaches such a value that the current through resistor R2, depending on the difference in the voltage on winding 1 and the voltage on the capacitor, becomes less than necessary to saturate the transistor. The transistor begins to close. The voltage on winding 2, and therefore on winding 1, changes polarity. A turn-off voltage equal to the voltage drop across the open diode VD1 is now applied to the base junction of the transistor. The transistor turns off completely.
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