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Generator of short pulses with a large duty cycle
In this article presents a generator of short pulses with a long interval of time between them or in other words — with a large duty cycle, which can reach values of 10 thousand or more. It can generate pulses with a duration of 60 nanoseconds, regardless of the frequency of their occurrence, which in turn can vary in the range of 1 Hz-150 kHz. The pulse duration can be adjusted variable resistor (60 ns to 30 µs), and the frequency of their repetition — with the help of two buttons. This frequency appears on the display, which can be very convenient for many experiments. Generator suitable for experiments and work settings that require very short, powerful pulses, for example, for coils and transformers with ultra-low inductance. All the generator settings to the nonvolatile memory.
Schematic diagram of the generator shown below. It works as follows. Single Board generator DG1 produces at its output TTL square wave given by the operator frequency. Differentiating using the chain C1(C2)R1R2 they arrive at the input of the driver output switch DA1, where it is converted to required to open the transistor VT1 pulses. Adjusting the duration of the output pulse is produced by variable resistor R1, and a range changing switch SW1. Due to the threshold device which is on the driver input, a pulse output is obtained with the highest possible front and decay. Thus there is no need to apply dopolnitelnyyu chip to its formation, though the driver had to use inverting. The yield of the latter is connected with a protective chain R3VD2ZD1 and goes to the gate of the output switch VT1. Protection is absolutely necessary here, because ultrashort pulses can sometimes give a huge emissions in the load circuits, which often incapacitate and the output key and driver.
The scheme involved the relatively inexpensive single-Board digital generator DG1, which is a good stable parameters of frequency and ease of its installation.
Unfortunately, the standard output of the single-Board generator (PWM) provides the necessary values for the front and decay of the pulse because its output is an inverter transistor, to convert the pulses with the amplitude of the TTL voltage pulses with an amplitude of supply. When this conversion is pretty much falling characteristics of the front and the recession, which for us is critical. Therefore, the author used two internal connections, which will be discussed further.
The photo above shows these two connections: TTL and +3.3 V. They also have and concept. +3.3 V is the internal single — Board power generator DG1, and the TTL output of the generator to the output inverter. Care should be taken to solder to these points of thin wires. This is best done by soldering iron with a fine tip.
As the output key are good "light" mosfet: IRF840 or IRF740. If the output required a lot of power, then it is better to apply high-speed IGBT. Driver DA1 is best to use a faster microchip TC4451, then the minimum pulse width is obtained in 50-60 ns. If you take even a faster driver, the output circuit parameters can be even better. The only thing to remember while choosing a driver must investirovat input. And fit more "slow" TC4429, but then the minimum duration of the output pulse is about 100 ns.
For a list of other elements of the scheme:
  • DG1 — digital signal generator 1Hz-150kHz;
  • VD1 — diode 1N4148;
  • VD2 — diode UF4007;
  • ZD1 — suppressor 1.5KE18CA;
  • R1 — any variable resistor with plastic handle; R2 is any trimming resistor;
  • C1, C2 — capacitors with a small HEEL, better tubular;
  • XS1 — standard connector to the power supply at 12V, for example, like this;
  • XS2 — any terminal block on the three pins, for example, like this.
Setting the scheme is to set the minimum duration of the output pulse, which he still observed on the oscilloscope connected to the output DA1 (output 6.7). This is done using trimmer R2 at the minimum value R1 and the open switch SW1. Also, on-Board generator DG1, you need to set the parameter "DUTY" is equal to 50%. This parameter is not changed.
The design of the device can be any, according to your desire. The only requirement: you must position the driver DA1, and all connections as close as possible to the output key. The author chose a bunk option, placed on PCB Board 4x6 cm. On the second tier is a single Board generator DG1, and the first — all the other elements. Appearance and some of the Assembly steps shown in the following photo:
One of the options for using this device can be found here or here, and in the following parts of this note you can get acquainted with a simpler and more advanced version of this generator, which can produce a given number of pulses in a packet and pauses between them.
We also recommend that you familiarize yourself with the pulse generator on a PIC controller, which has two independent channels and advanced characteristics.
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