- One-IC 50W Audio Amplifier Using TDA1562
- Maximum Temperature Detector For Fan Controller
- Garden Lighting Using Solar Cells
Posted: 25 Apr 2011 12:03 AM PDT
The integrated output amplifier described in this article consists of little more than one integrated circuit. It is intended especially for use in motor vehicles and other battery-operated applications. Although it appears simple and hardly worth looking at, the amplifier can produce an appreciable audio power output. The circuit diagram in Figure 2 emphasizes how few external components are needed to construct a complete output amplifier.
For instance, the new device does not need compensation networks to enhance the stability. Also, because of the absence of switch-on phenomena, there is no need for a switch-on delay network. There is, of course, still a need for supply line decoupling capacitors. Capacitors C5 and C6 are required for Class-H operation, about which more in the box. The value of input capacitors C1 and C2 is relatively low, thanks to the high input impedance of the IC. Switched RC network R4-C4 at the ‘mode select’ input (pin 4) serves to switch the IC to ‘mute’ or ‘standby’.
When the supply voltage is switched on, the IC is first switched automatically to the ‘mute’ mode and to ‘on’ only after a short delay. The time constant R4-C4 is a few tenths of a second and this delay between the two states is sufficient to obviate disturbing (and annoying) switch-on phenomena. Switch S1 enables the amplifier to be switched to ‘standby' when the use of the amplifier is not needed for a period of time. When that time has elapsed, the amplifier is quickly reverted to normal operation. The current drain in the standby mode is virtually negligible at only 200µA. Resistor R3 prevents a short-circuit current ensuing when S1 is being closed at the instant C4 is being discharged.
Measurement results (at Ub=14.4 V)
Posted: 24 Apr 2011 11:45 PM PDT
The fan controller circuit for the Titan 2000 and other AF heavy-duty power amplifiers, has an output that sets a voltage if the fan controller reaches the end of its range. Since the controller responds to temperature, this signal is seen by the amplifier protection circuitry as an over temperature indication. The disadvantage of this output is that the maximum voltage for the fans is not constant, but depends on the load (number of fans, defective fans) and the mains voltage. This variation is caused by the fact that the supply voltage for the output stage is taken directly from the ﬁltered transformer voltage.
If the fans should fail, for example, the maximum temperature limit would lie at a considerably higher level than the desired value. The accompanying circuit, which compares the magnitude of the fan voltage to a fixed reference value, has been developed to allow the maximum temperature to be reliably detected. This circuit is tailored for 12-V fans. The reference voltage is generated by the ‘micro power voltage reference’ D1 and the FET T1, which is wired as a current source. These components are powered directly from the applied fan voltage. The current source is set up to deliver approximately 50µA.
D1 can work with as little as 10µA. The supply voltage for the IC is decoupled by R10, C3 and C4, with D4 providing over voltage protection. A maximum supply voltage of 16 V is specified for the TLC271. This opamp works with a supply voltage as low as 3 V and can handle a common-mode voltage up to approximately 1.5 V less than the positive supply voltage. Accordingly, 1.2 V has been chosen for the reference voltage. The fan voltage is reduced to the level of the reference voltage by the voltage divider R2–R3–P1. The limits now lie at 11.2 V and 16.7V.
If you ﬁnd these values too high, you can reduce R2 to 100 kΩ, which will shift the limits to 9.5 V and 14.2 V. The output of the voltage divider is well decoupled by C2. A relatively large time constant was selected here to prevent the circuit from reacting too quickly, and to hold the output active for a bit longer after the comparator switches states. A small amount of hysteresis (around 1 mV) is added by R4 and R5, to prevent instability when the comparator switches. D2 ensures that the magnitude of the hysteresis is independent of the supply voltage. Two outputs have been provided to make the circuit more versatile.
Output ‘R’ is intended to directly drive the LED of an optocoupler. In addition, transistor T2 is switched on by the output of the opamp via R7 and R8, so that a relay can be actuated or a protection circuit triggered using the ‘T’ output. The high-efficiency LED D3 indicates that IC1 has switched. It can be used as a new ‘maximum’ temperature’ indicator when this circuit is added to the fan controller. The circuit draws only 0.25 mA when the LED is out, and the measured no-load current consumption (with a 12.5V supply voltage) is 2.7 mA when the LED is on.
Posted: 24 Apr 2011 11:25 PM PDT
Completely ‘self-supporting’ garden lamps using solar cells as their energy source are gradually becoming more and more common. How do they actually work? We took one apart to ﬁnd out. From environmental and technical considerations, buying such a solar-cell garden lamp has a lot to recommend it. It’s a great thing that the energy necessary for the lamp to burn in the evening can be drawn from the sunlight that is available for free during the day. In addition, such a lamp is enormously practical, since you can place it in any desired location in the garden without having to dig a trench through the lawn or ﬂowerbeds.
You are also free to change your mind about the best location for the lamp - something that would have unpleasant consequences with ordinary garden lamps. What makes up a typical solar-cell garden lamp? A certain number of elements are in any case necessary for it to function. It’s clear that there must be a light bulb and some solar cells. However, the bulb is naturally not powered directly from the solar cells, so there must be a storage battery and a suitable charging circuit to allow the battery to be charged by the solar cells. In addition, the idea is that the lamp should only burn during the evening and the night, and that needs a twilight switch with a light-sensitive cell.
It’s not necessary to do anything to switch off the lamp, since that happens automatically as soon as the battery is fully discharged. Some of the more luxurious models have a small fluorescent tube in place of a normal light bulb, and in this case a small converter is also necessary. However, the model that we examined contained a small 2.5V/75-mA halogen bulb, and thus did not need a converter. As far as the electronics are concerned, the whole thing can thus remain very simple.
Simplicity wins out:
Our garden lamp consists of a simple plastic structure. Eight solar cells are mounted at the top, and inside there are a small halogen bulb, two penlight NiCd cells and a small printed circuit board for the electronics. As can be seen from Figure 1, there isn’t all that much inside. This lamp costs around 15 pounds, and it can be found in several different shops. The electronics also turn out to be extremely simple. Figure 2 shows the complete schematic of the internal circuitry. The twilight switch is on the left, and its output controls the lamp via transistor T4. To the right are the on/off switch, a diode and the eight solar cells.
During the day, as long as there is sufficient light, the voltage generated by the solar cells is 8 × 0.45 V under ideal conditions, with a current that depends on the size of the cells — in this case, approximately 140mA. With less light, less current is supplied. The charging circuit consists simply of a single Schottky diode (D1). The current generated by the solar cells passes through this diode, with its typical low voltage drop of 0.3 to 0.4 V, and charges the NiCd cells. There is no overcharge protection. It is not actually necessary, since all NiCd cells can handle a continuous charging current equal to 1/10 of their capacity (60mA in this case), while modern cells are so robust that twice this amount of current does not cause any problems.
The advantages of using a somewhat higher charging current are naturally that the battery is already fully charged after several hours of sunlight, and that a certain amount of charging takes place even on rainy days and during the winter. Solar cells act as light-dependent current sources, so the more light there is, the more current they produce. The voltage is determined by the load, but it can never be higher than the previously mentioned 0.45 V per cell. Approximately 2.8 V is necessary to charge two NiCd cells. If we add the voltage drop across D1, we arrive at a required voltage of 3.2 V. This is 0.4 V per solar cell.
Charging takes place continuously, even when switch S1 is off. It is important to make sure that both NiCd cells are fully charged the ﬁrst time. Otherwise, one cell may become fully discharged before the other one when they are discharged. As a result, this cell may have a reverse-polarity voltage applied to it, which will shorten its useful life. Therefore, when ﬁrst putting the lamp into service, you should place it outside with S1 switched off for at least one day in full sunlight, or two days if the weather is cloudy.
When S1 is closed, voltage is applied to the part of the circuit containing the light bulb. An LDR is used to determine whether it is light or dark outside. During the day, the resistance of the LDR is low, and the voltage on the base of T1 is also low, so that it is cut off. T2, T3 and T4 are then also cut off, so that the bulb is not illuminated. As soon as it becomes dark, the resistance of the LDR increases, and the voltage on the base of T1 rises. T1 starts to conduct when the voltage is around 0.65 V. This causes T2, T3 and T4 to conduct as well, and the lamp starts to burn. T1 then receives a bit of extra current via R4, so that positive switching takes place when the circuit is sitting ‘on the edge’. This is called hysteresis. It means that a threshold is set such that the light level has to drop a bit more before the lamp will switch on again once it is off, and vice versa.
This means that the circuit does not react to every passing cloud or insect that is flying around. As long as it remains dark, the lamp continues to burn until the battery is fully discharged. A fully charged battery has a capacity of 600 mAh, which is enough to supply the 75-mA bulb for approximately eight hours. This is sufficient for the evening and a large part of the night. In the winter, this is not possible, since the battery will probably not be fully charged due to a lack of sunlight. When the battery becomes fully discharged, its voltage drops. If the voltage drops below 1.25 V, T2 and T3 are cut off, since their base-emitter junctions are in series and thus need at least this amount of voltage. The lamp is then switched off, and the battery is not further discharged.
In the long term:
NiCd batteries usually have a lifetime of around 500 to 1000 charge/discharge cycles. After two to three years of continuous use, therefore, the two penlight cells of the garden lamp will probably be ready for replacement. However, these cells are presently so inexpensive that this is not a serious disadvantage. Naturally, there is also a limit on the life-time of the light bulb, but here again, making a replacement is quick and inexpensive.