Conex Club Magazine - no.1 - 2005

Conex Club Magazine - no. 1 - 2005 - Programmable current source 0-5A

Reading time: 4 minute

The main topics presented

1. 4-channel USB data acquisition mode

Velleman kit, for data acquisition, with PC interface on USB port. Graphical interface with oscilloscope or multimeter display.

The module allows the storage on the computer of the evolution of continuous signals, slowly variable or with long periods. Stored values ​​may be useful for further studies. Due to the connection to the computer's USB port, the module does not require an external power supply.

In the case of experiments or routine measurements, voltage variations are usually tracked using multimeters.

In this way only the instantaneous value can be determined. The K8047 has the quality of recording these values, even for long intervals.

The sampled data is transferred to the computer for storage on the hard disk and graphical viewing. Thus, the evolution of the measured quantity can be quickly evaluated. As an example, you can follow the evolution of the degree of charge of some batteries or the presence of electrical parasites.

With a proper interface, you can also view the variation of non-electrical quantities (pressure, temperature, etc.).

Hardware technical characteristics

  • connection and power via USB port;
  • 4 analog inputs;
  • input impedance: 1 MOhm;
  • sampling rate: 100S / s;
  • input voltage ranges: 3/6/15/30 [V] DC;
  • sensitivity: 10mV;
  • error: 3%;
  • input voltage: max. 30V;
  • LED indicator shows voltage and memory (data storage).

Software technical features

  • analog and digital display;
  • simultaneous memorization of the 4 channels;
  • memorizing the minimum / maximum values ​​of the sampled voltage;
  • sample: 1… 1000s / div;
  • saving the viewed data;
  • option for automatic data storage for long periods of time;
  • markers for amplitude and duration;
  • .DLL libraries included.

2. Programmable current source 0… 5A

There is a programmable power source that can be controlled from the PC via a serial interface on the COM port.

The control on the board is provided by an AVR type microcontroller AT90S2313. Specialized circuits produced by Maxim-Dallas are used.

The current source presented generates between 0 and 5A, if used MAX724 or between 0 and 2A if the variant with is used MAX726. The range of voltages allowed on the load falls within the range of 4V and 36V.

The configuration recommended by the manufacturer and presented in figure 1 of the article, using one of these integrated circuits, offers the following advantages:

  • 12-bit D / A converter made with MAX507 makes the application the program-ball, in 4095 steps (212);
  • Voltage converter, step-down (Buck) used with MAX472, is more reliable than one made with linear transistor;
  • MAX472 provides feedback (reaction) with an amplifier that detects the direction of current in the load without using the mass of the voltage source.

As examples, the application is suitable for charging an accumulator battery or for controlling the speed of the DC motor.

3. AVR Microcontrollers (XIV) - Communication between the UART port of an AVR series CPU and a PC

The presentation of the existing UART port for serial communication to the AVR series microcontrollers, continues in this episode with an application that includes the personal computer.

Full-duplex communication consists in the transmission and reception of some commands through which are "turned on" or "off" three optical elements of LED type, two of them being found in the scheme developed around the microcontroller, and the third being "simulated" in the graphical interface of the program , running on PC.

Description of operation

The diagram in figure 70 (from the article) is made from a module identical to those presented in figure 67 (from the last issue of the magazine). It is developed around the microcontroller AT90S2313-10PI, to which is added the serial communication block with the integrated circuit MAX232.

The connection to the computer is made through the DB9 connector (mother) to the serial communication port COM1.

4. Critical temperature warning

There is a critical temperature warning, very easy to build even on the test wiring. At the base of the scheme is a circuit CMOS CD4001, in monostable configuration (two of the gates, the detection part of the critical temperature threshold) and stable (controlled oscillator, the buzzer audible warning part).

You can use a thermistor with resistance of 4,7 KOhm at 25C (see page 50, past issue). R6 is chosen according to the temperature range of interest, presented in the diagram, and R7 determines (experimentally) the threshold within the respective range.

The buzzer will sound acoustic for 5s, every 4… 5 minutes (adjustable values ​​from capacitors), if the temperature threshold is exceeded.

Applications: gardening, warehouses for agricultural or food products, cars (frost warning), etc.

5. Rectifier controlled with thyristors

The thyristor-controlled rectifier can be used to power a resistive, power heater, or to charge a high-capacity battery. The maximum allowed current is approx. 3.5A.

The application is a thyristor controlled rectifier, of the midpoint type. It can have multiple uses, the prototype being tested both on resistive loads and as a source for charging 12V car batteries.

The control in the grid is done by means of an optocoupler that solves the galvanic separation between the force part and the control part. In both half-alternators, the pulses are sent simultaneously to both thyristors that have common cathodes, but at a given moment only one thyristor is open, namely, the one that receives positive voltage in the anode.

6. Source with 12V / 2A back-up

Linear voltage sources with back-up are the equipments that ensure the good functioning, for a long time, of the surveillance systems. Intelligent charging and battery condition monitoring are required.

The power supply of the installations of special importance, such as the alarm and surveillance ones or of the PC networks is usually made from the double source, the alternating voltage network and the accumulator battery.

The entire operational safety depends on the way these power supplies are built and the response time to events. There is an analog stabilized voltage source, with 220Vac mains supply and from a battery of medium capacity batteries, type SP12-7.2 well known by those working in the field (12V / 7.2A).

Overvoltage protection of electronic equipment

There are methods of protection against overvoltages in electronic circuits and two "crowbar" applications.

At short intervals, sometimes unobservable on the oscilloscope, the current can be exceeded by a device or the voltage applied to it. The phenomenon is characteristic of circuits with reactants, inductive or capacitive, even if they are not obvious, but they exist as distributed elements or as part of power supplies, power supplies, etc.

This is the case of transformers, relay coils, contactor, hydraulic distributor, solenoid valve, motor, etc.

This article discusses some aspects of surge protection. It will be followed in the future by another, which will present data on overcurrents and methods of protection against them.


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