Conex Club Magazine - no.4 - 2006

Conex Club Magazine - no. 4 - 2006 - 120W bridge amplifier with LM3886

Reading time: 5 minute

The main topics presented

1. Alternative technologies for connecting electronic components (IV)

Measurement of the electrical resistance of solders made with alloys without Pb or conductive adhesives

To measure the electrical resistance of a solder based on lead-free alloy or conductive adhesive it is necessary to use a method by which resistances of very low values ​​can be measured reproducibly, as accurately as possible.

As the values ​​of the solder resistances are extremely low, it is necessary to use a measuring system with four terminals to eliminate the resistances of the measuring conductors and contacts.

As seen in Figure 1 (from the article), the finished resistance of the voltmeter may affect the accuracy of the measurement. Some modern measuring devices offer an alternative of equivalent Kelvin samples (figure 2 in the article) which does not eliminate the impedance from the electrode interface. The measuring variant with "classic" instruments, with two contacts, is presented in figure 3 (from the article).

When testing lead-free alloys and conductive adhesives, several typical geometric configurations are used, each with advantages and disadvantages. Some wrong operations can lead to systematic measurement errors.

2. Performance indicator for "burned" fuses

The displayed open fuse indicator shown works with a wide range of DC supply voltages, from 5 at 50Vdc. The LED signals (illuminates) when the fuse has blown.

With the fuse intact, Q2 is kept locked and the base of Q4 is not polarized (by R6). So the LED is not powered. Q5 limits the emitter current of transistor Q4 to 10mA.

The valueless resistors in the diagram (from the article), R6, R2 and R5 are determined by calculating the ratio between the supply voltage of the assembly and the values ​​2, 0.02, the result being in KOhmi (except for R5 which the result is in Ohms).

Example: for a supply voltage of 12V the values ​​used are 560 KOhmi for R2, 5.6 KOhmi for R6 and 560 KOhmi for R5. MPSA65 can be replaced with MPSA92 (0.5A, 300V, 625mW).

3. Active switch for supply voltages

Two supply voltages of different values, coming from the same power supply, can be distributed to a single output by logic control performed based on some transistors (Q6 or Q7).

The scheme allows switching to output 12 or 5V voltages. The application is useful in portable equipment, where various sections of the device are temporarily powered.

Low saturation voltage of the transistor FMMT717 (produced by ZETEX), makes it ideal for replacing MOS transistors in this application and especially in the switching application, the power losses being small.

Those interested can test with MOS transistors instead of FMMT717 (or bipolar BD series). The current capacity is 1A.

4. LM385 (B) - Precision fixed voltage reference source

LM385 (B) is a fixed voltage reference, with two terminals, for fixed voltages of 1.2V - variant LM385-1.2 and 2.5V - variant LM385-2.5. The polarization current, for which stability is guaranteed, is located in the range 10uA… 20mA.

The accuracy is 1% for LM385B-1.2, 2% for LM385-1.2 and LM385B-2.5, 3% for LM385-2.5. The internal compensation of the junction with temperature ensures a variation of the stabilized value of 2Oppm.

The maximum reverse current allowed by the device is 30mA, and the dynamic impedance is very low, below 10 Ohms.

It is generally used in laboratory or portable instrumentation applications. The symbol is similar to that of a Zener diode. It comes in various types of capsule (TO92 - plastic, TO-46-metal, etc.).

5. 120W bridge amplifier, with LM3886

This application is a new reason to meet the most famous member of the series Overture, produced by National Semiconductor, LM 3886.

An extensive magazine series was dedicated to this series. The application adds a new example to the bridge connection of two LM3886 circuits to obtain considerable power with very low distortion. This parameter is the best indicator of the Overture series!

The peculiarity of the assembly consists in the use of a timed circuit with relays that switches the speaker with delay when applying the power supply, to ensure its protection.

The speaker must be 8 Ohms. The application also contains an audio volume filter adjustment system or a power meter for displaying power.

6. Charger for lead-acid batteries (sealed)

This charger, through its electrical qualities, is intended to recover and maintain in perfect working order the lead-acid batteries installed in the security systems.

The charger is designed for lead-acid and gel-coated batteries and is not suitable for NiCd batteries.

In order to have a long service life, a lead-acid battery is recommended to be charged with a current whose value must be 1/10 of the value of its electrical capacity and at a voltage of 2.33V per element.

This means that a battery with a rated voltage of 12V must be charged to a maximum voltage of 13.7… 14V.

Analyzing the electrical diagram of the presented charger, charger at which the values ​​of the maximum current allowed at charging can be predetermined, as well as of the voltage up to which the battery can be charged, its simplicity is observed first of all.

The scheme shows that the realization does not involve a high cost, and the manufacturing process is not complicated. The voltage regulating element is the circuit LM317T.

Technical data:

  • stabilized voltage source;
  • fine adjustment of the output voltage: 12… 14V;
  • maximum current limited at the output, programmable hardware;
  • short circuit protection;
  • optical load and fault signaling.

7. Balance indicator

At the base of the application is the operational amplifier LM324, mounted either in differential amplifier configuration or by comparator, depending on the topological position in the diagram (figure 1 of the article).

If we refer to the input, LM324 is mounted in differential AO configuration, it amplifying the difference between the signals applied to the INL and INR inputs.

If at the output, the amplified signal exceeds the set switching threshold, the LEDs light up, as follows:

  • if it becomes predominantly the R channel, the red LED lights up, mounted with the cathode at GND (U1C is with the output in high);
  • if the L channel is predominant, the other red LED lights up (U1D is with low output);
  • In case of equal signals on the two channels, the AO outputs U1D and U1C are in high state and the green LED in the middle lights up.

In tests, it will be noticed that in the situation of balance of the signals, often two LEDs will light up, sometimes the practical balance, depending on the tolerance of the components used and the adjustments.

8. Automatic / manual video switch

Maximum four video signals (or even audio, low level), from surveillance video cameras, can be switched via the electronic switch presented in this article.

It is possible both manual switching and automatic switching, at the output being distributed, in turn, each video camera. It is also possible to configure the operation to be performed with simple modifications on the printed circuit board.

How at the output the signals are switched by Reed relays, the video signal mass will be totally separated from that of the montage. The signal (input) present at the output is optically signaled by LEDs.

9. Speed ​​regulator for AC motors

The assembly proposed in the article is special due to the particularity that it can also feed inductive tasks. It is dedicated to speed control for drills powered by alternating current, motors that have carbon brushes (on the collector).

Contrary to modern AC voltage regulators, there is no speed adjustment in this assembly by the phase control method on each half-alternator of the mains voltage sinusoid, but only on one, the positive one.

The moment of application of the priming pulse on the triac gate, controlled in phase, determines accordingly the speed of the AC motor with carbon brushes, speed that can be adjusted in the range 5… 95% of the maximum speed that can be obtained at the shaft.

Using this method you get a high torque even at low set speeds. This interesting configuration of the scheme does not mean that resistive loads, such as incandescent bulbs, electric heating elements, etc. cannot be supplied through the assembly.

10. Sensory electronic switch - Touch Switch

Touch Switch is an electronic switch used to control an electrical appliance by simply tapping a finger resistive-capacitive sensor made on a printed circuit.

The SmartKit 1005 project is an easy-to-execute electronic assembly, whose direct application is the digital control of an electrical device (bulb, motor, etc.), by simply touching with your finger some digital sensors made on the printed circuit.

The basic function performed by the electronic circuit in figure 1 (from the article) is a flip-flop with NAND logic gates, physically implemented with the integrated circuit. CD4011.

The internal logic diagram of CD4011 is given in figure 2a (from the article), and in figure 2b (from the article), its logical truth table.

Only two NAND logic gates are used from the CD4011 circuit, properly configured for the flip function (each output of a logic gate is input for the other), respectively pins 10 with 12 and 8 with 11.

The two free inputs, respectively pins 9 and 13, are used as control inputs, SET - ON (S) and RESET - OFF (R).


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