Single-Supply Instrumentation Amplifier

The OP284 is a low noise dual op amp with a bandwidth of 4MHz and rail-to-rail input/output operation. These properties make it ideal for low supply voltage applications such as in a two op amp instrumentation amplifier as shown in the diagram. The circuit uses the classic two op amp instrumentation topology with four resistors to set the gain. The transfer equation of the circuit is identical to that of a non-inverting amplifier. Resistors R2 and R3 should be closely matched to each other as well as to resistors (R1+P1) and R4 to ensure good common-mode rejection (CMR) performance. It is advisable to use resistor networks for R2 an and R3, because these exhibit the necessary relative tolerance matching for good performance.


Potentiometer P1 is used for optimum d.c. CMR adjustment, and capacitor C1 is used to optimize a.c. CMR. With circuit values as shown, circuit CMR is better than 80 dB over the frequency range of 20 Hz to 20 kHz. Circuit referred-to-input (RTI) noise in the 0.1 Hz to 10 Hz band is exemplary at 0.45 µVpp. Resistors R5 and R6 protect the inputs of the op amps against over-voltages. Capacitor C2 may be included to limit the bandwidth. Its value should be adjusted depending on the required closed-loop bandwidth of the circuit. The R4-C2 time constant creates a pole at a frequency, f3dB, equal to f3dB=1/2πR4C2. With a value of C2 of 12 pF, the bandwidth is about 500 kHz. The amplifier draws a current of about 2mA.

Read More

+/– Voltage On Bargraph Display

The LM3914 is a truly versatile component. Besides LEDs, only a few other components are needed to make the ‘bidirectional’ bargraph voltmeter shown here. The circuit is similar to a conventional bar display, but it offers a possibility to change the direction in which the LEDs are switched on. This may be useful, for example, when positive and negative voltages are measured. For a positive input voltage, the LEDs are switched on in the usual manner, that is, from D3 to D12, while for negative voltages, the LEDs are switched on in the opposite direction, from D12 to D3. Obviously, the negative voltage must be ‘rectified’, i.e. inverted, before the measurement.

A suitable circuit for this purpose is presented in the article ‘Absolute-value meter with polarity detector’ elsewhere in this website. A set of transistor switches (MOSFETs) controls the direction in which the LEDs light. When the control voltage is high (+6V, according to the schematics, but any voltage that is at least 3V higher than reference voltage will do), T1 and T4 are switched on, while the other two MOSFETs are off. In this way, the LM3194 is configured in the usual manner with the top end of the resistor network connected to the internal voltage reference and the low end connected to ground.


As the input voltage rises, the comparators inside the LM3914 will cause the indicator LEDs to be switched on one by one, starting with D3. When the control voltage is lower than about –3V, T2 and T3 are switched on while T1 and T4 are off. Consequently, the ends of the resistor network are connected the other way around: the top end goes to ground and the low end, to the reference voltage. The first LED to be switched on will then be D12; i.e., the LEDs that forms the bargraph display light in the opposite direction. Although not documented by the manufacturer of the LM3914, this option works well, but only in bar mode (in dot mode, internal logic disables any lower-numbered LEDs when a higher-numbered LED s on, which obviously conflicts with our purposes).

To achieve good symmetry, an adjustable resistor is added to the voltage divider in the LM3914. Using a DVM, adjust the preset until the voltage across P1+R4 equals 1/11th part of Urefout. Sensitivity is determined with the ratio of resistors R5 and P2. If, for example, the reference voltage is set to 2.2 V by means of P2, there will be a voltage drop of 200 mV per resistor in the ladder network (including R4-P1). So, the first LED will switch on when the input voltage exceeds 200 mV, the second, at 400 mV, and so on, and the whole display will be on at 2 V. The circuit draws about 100 mA when all LEDs are switched on.

Read More

PC Fan Speed Controller - For a Low-Noise PC

The fan runs constantly in many PCs, which may not even be necessary. A simple controller circuit can regulate the fan speed according to demand. This not only saves energy, it also reduces irritation from the fan noise. Only three components are needed to allow the fan speed to be controlled according to the actual demand: one adjustable voltage regulator and two resistors that form a voltage divider. One of the resistors is a NTC thermistor (temperature-sensitive resistor), while the other is a normal resistor. If the 12-V power supply is not located close to the regulator, a decoupling capacitor is also required (see Figure 1).


The thermistor has a rated value of 470 Ω. It sets the output voltage of the LM317T to approximately 7 V at 25 ºC. This should ensure reliable starting of the fan. If the temperature rises to roughly 40°C, the output voltage of the regulator reaches its maximum value and the fan runs at its maximum speed. The voltage drop across the regulator is at least 1.75 V for a motor current of (for example) 300mA, and in any case 2V at the maximum current level of 1 A. You thus might want to consider using a low-drop regulator, such as the National Semiconductor LM2941CT.


To be sure, this increases the size of the circuit to a full five components, which are arranged as shown in Figure 2. However, this approach reduces the voltage drop to 0.2 V at 300 mA or 0.5 V at 1 A. By the way, low-drop voltage regulators are not available in a three-lead package. The circuit can be constructed as a well-insulated ‘free-standing’ assembly, or it can be built on a small piece of prototyping board. In either case, it should be fixed to one of the mounting holes of the fan body (via the cooling tab of the TO-220 regulator package for the free-standing construction). The circuit board should be mounted out of the air stream, but the NTC thermistor must extend into the air stream.

Read More

Rugged PSU For Ham Radio Transceivers

This rugged power supply is based on the popular LM338 3-pin voltage regulator. The LM338 is capable of supplying 5 A over an output voltage range of 1.2 V to 32 V with all standard protections like overload, thermal shutdown, over-current, internal limit, etc., built in. In this power supply, some extra protections have been added to make it particularly suitable for use with low to medium-power portable and mobile VHF/UHF (ham) and 27 MHz transceivers. Diodes D4 and D5 provide a discharge path for capacitors C1 and C2. Diode D8 protects the supply against reverse polarity being applied to the output terminals. Capacitor C1 assists in RF decoupling and also increases the ripple rejection from 60 dB to about 86 dB.


If junction R1-R2 is not grounded by switch S1A, transistor T2 starts to conduct, causing the regulator to switch to zener diode D7 for its reference voltage (13 V). The PSU output voltage will then be 12.3 V. Normally, T2 will be off, however, and the PSU output voltage is then about 8.8 V. The high/low switch is useful to control the RF power level of modern VHF/UHF handhelds. Transistor T1, a p-n-p type BC557, acts as a blown-fuse sensor. When fuse F1 melts, T1 starts to conduct, causing LED D6 to light. If, for whatever reason, the PSU output voltage exceeds about 15 V, thyristor THR1 is triggered (typically in less than a microsecond).

Such a high-speed ‘crowbar’ may look like a drastic measure, but remember that this kind of protection is required by digital ICs that will not stand much overvoltage. The crowbar, when actuated, will faithfully destroy fuse F1 rather than allow the PSU to destroy expensive ICs. The two LEDs on the S1B contacts not only act as ‘high/low’ indicators but also as power-on indicators which are turned off when the mains voltage drops below about 160 V. If you envisage ‘heavy-duty’ use of the PSU, then voltage regulator IC1 should be mounted on as large a heatsink as you can get. The minimum we’d say is an SK129 heatsink from Fischer (Dau Components).

Read More

Power Diode For Solar Power Systems

Apart from the sun, solar power systems cannot work without a reflow protection diode between the solar panel and the energy store. When current flows into the store, there is a potential drop across the diode which must be written off as a loss in energy. In the case of a Schottky diode, this is not less than 0.28 V at nominal current levels, but will rise with higher ones. It is clear that it is advantageous to keep the energy loss as small as possible and this may be achieved with external circuitry as shown in the diagram. The circuit is essentially an electronic switch consisting of a high precision operational amplifier, IC1a, a Type OP295 from Analog Devices, and a MOSFET, T1.

This arrangement has the advantages over a Schottky diode that it has a lower threshold voltage and the lost energy is not dissipated as heat so that only a small heat sink is needed. When the potential at the non-inverting input of the op amp, which is configured as a comparator, rises above that at the inverting input, the output switches to the operating voltage. The transistor then comes on, whereupon light-emitting diode LD1 lights. Diode D3 clamps the inputs of IC1a so that the peak input voltage cannot be greater than half the threshold voltage, provided the values of R3 and R4 are equal.


The op amp provides very high small-signal amplification, a small offset voltage, and consequent fast switching. The MOSFET changes from on to off state and vice versa at drain -source voltages in the microvolt range. In the quiescent state, when UDS is 0 V, the transistor is on, so that LD1 lights. The operating voltage (C–A) may be between 5 V (the minimum supply for the op amp and the input control potential, UGS, of the transistor) and 36 V (twice the zener voltage of D1). Zener diode D1 protects the MOSFET against excessive voltages (greater than ±20 V). Diode D3 and resistors R3 and R4 halve the potential across the inputs of the op amp.

This ensures that operation with reversed or open terminals is harmless. The substrate diode of the MOSFET is of no consequence since it does not become forward biased as long as the forward voltage, USD, of the transistor is held very low. The on -resistance, RSD(on), of the transistor is only 8 mΩ and the transistor can handle currents of up to 75 A. When the nominal current is 10 A, the drop across the on-resistance is 80 mV, resulting in an energy loss of 0.8W. This is low enough for a SUB type with a TO263-SMD case to be used without heat sink. When the current is 50 A, however, it is advisable to use a SUP type with a TO220 case and a heat sink since the transistor is then dissipating 12.5 W.

Even then, the voltage drop, USD = 0.32 V is significantly lower than that across a Schottky diode in the same circumstances. Moreover, owing to the high precision of IC1a, a number of transistors may be used in parallel. The circuit proper draws a current of 150 µA when only one of the op amps in the OP295 is used. An even lower current is drawn by the alternative Type MAX478 from Maxim. However, the differences between these two types are only relevant in the low current and voltage ranges. Both have rail-to-rail outputs that set the control voltage accurately even at very low operating voltages.

This is important since the switch-on resistance of MOSFETs is not constant: t drops significantly with increasing gate potentials and decreasing temperature. A experimental circuit may use an LM358 op amp and a Type BUZ10 transistors, but these components do not give the excellent results just described.

Read More

General-Purpose NiCd Battery Charger

There is a wide variety of NiCd (nickel-cadmium) battery chargers on the market, but there are not many that can work from an in car 12 V cigar lighter. Such a charger would, for instance, be of interest to campers and caravanners who do not have a 230 V a.c. mains supply available. To satisfy the needs of these users, a charger could be designed for operation from the cigar lighter, but it is, of course, of far greater interest if it could also work from the domestic mains supply. Furthermore, it would also be very useful if a number of cells, say, 1 to 4, of different format could be charged simultaneously.

Lastly, another benefit would be if the charger would automatically switch off once the battery or cells have been charged fully. The charger described in this article does all that: it accommodates batteries or cells Type R6 and R14. Switching off after a period of 2 h 30 m, 5 h, or 10 h is arranged by 3-way switch S1. The 2 h 30 m period is for charging Type R6 batteries (1/2 charge), the 5 h period for fully charging Type R6 batteries or half charging Type R14 batteries, and the 10 h period for fully charging Type R14 batteries. Light-emitting diode D1 lights when charging is taking place. Charging after the set period has elapsed can be continued, if so desired, only by switching the supply off and then on again.

The time periods are determined by counters IC1 and IC2, Type 4060 and 4020 respectively. The 4060 has an integral oscillator, whose frequency is set to 932 Hz with preset P1 and the aid of a frequency meter. For various reasons, such as the values of the components used and parasitic elements, the oscillator itself operates at a slightly higher frequency – of the order of 1 kHz. The frequency of the signal at the wiper of P1 is divided by 214, so that the frequency of the signal at Q13 of IC1 is 0.056 Hz, equivalent to a pulse every 17.6 s. The signal at Q13 is applied to the input, pin 10, of IC2. When switch S1 is in position 2 h 5 m (output Q10 of IC2), the divisor should be 210 (1024).

However, contrary to what these figures indicate, the time period stops at half that at output Q10. To obtain a charging period of 2 h 30 m, that is, 9,000 seconds, which should correspond to half a period at output Q9 of IC2, the oscillator period must be 9000×2/16.7×106=1.073 ms, which corresponds to a frequency of 932 Hz as mentioned earlier. On power-on, only counter IC2 is reset, since an error of a few seconds that may arise in IC1 is of no significance. This arrangement simplifies the design. When the time set has elapsed, that is, charging is finished, diode D1 goes out.

The charging current is fixed by darlington transistor T3, which is a classical design of a current source with negative feedback. The transistor tends to hold its emitter potential at 1.3 V, but this requires the aid of a zener diode, D2. In this type of design, the thermal stability is, in fact, totally acceptable, because the temperature of the zener diode, in view of the small current this draws and its consequent low temperature rise, hardly affects the charging current Transistor T1 is either on or off and serves to power the on/off indicator LED. It is needed to prevent an overload on the output of counter IC1 if this would be required to absorb the total current (about 7mA) drawn by the diode.

Transistor T2 discontinues the charging when the time set by S1 has elapsed by earthing the base of darlington T3. Diodes D3–D14 are connected in threesomes across the terminals of the batteries to be charged: D3–D5 across those of battery Bt1, D6–D8 across those of Bt2, and so on. Diode D15 prevents the batteries to be charged from being discharged when the supply fails. When the charger is used in a vehicle, additional precautions should be taken to ensure that any spurious surges on the vehicle power lines do not adversely affect the charger’ s operation. The battery holder should be one that can accommodate four size R6 (AM3; MN1500; SP/HP7; mignon) or R14 AM2; MN1400; SP/HP11; baby) batteries.

The length of these batteries, but not their diameter, is the same (about 45 mm). When the charger is used at home, it may be powered via a suitable 15V mains adaptor. It draws a current of about 150mA. A final word of warning: it is possible for batteries to be connected to the charger with incorrect polarity. This may result in a very large discharge current and even destruction of the battery. It is, therefore, imperative to verify the correct polarity of the battery before inserting it into the holder.

Read More

Absolute-Value Meter With Polarity Detector

This circuit breaks an input voltage signal down into its components: (1) the absolute value and (2) the polarity or ‘sign’ (+ or –). It will handle direct input voltages as well as alternating voltages up to several kHz. With a supply voltage of ±9 V, the input level should remain below ±6V. The circuit consists of two sections, each having its own function. Operational amplifiers IC1a and IC1b form a full-wave rectifier, its output terminal supplying the absolute value of the input signal, while operational amplifiers IC1c and IC1d examine the polarity of the input voltage. For negative input voltages, the output of IC1a goes high.

Consequently D2 is reverse-biased so that IC1a has no effect on the rest of the circuit. IC1b then acts as an inverter because its amplification is (–R5/R3) or –1. Thus, the output voltage is positive. For positive input voltages, D2 conducts and the amplification of IC1a is -1. The output voltage is then determined by the sum of currents that flow through R3 and R4. Taking into account the polarities and the value of all resistors, the overall amplification becomes

–R5/R3 + (–R5/R4) ↔ (–R2/R1) = –1 + 2 = 1

This means that the value of the output voltage at the output terminal is the same as the input voltage, but the polarity is always positive. The accuracy of the rectification process is determined by the accuracy of resistors R1-R4; close-tolerance (1%) types are recommended. At low input voltages (smaller than 20 mV), the input offset voltage of the operational amplifiers may introduce significant errors. If this is the case, use individual operational amplifiers instead of an array of four (TL061, TLC271, AD548, ...), because they have pins for offset voltage compensation. Alternatively, use an operational amplifier with a low offset voltage like the OP07. In the polarity detector IC1c acts as a comparator, with a certain amount of positive feedback due to R7 and R8.

This feedback causes a hysteresis of 20mV that prevents oscillation when the input voltage changes slowly. IC1d is an ordinary inverter. For input voltages above 10mV, the SIGN output terminal will swing to almost the positive supply. When the input voltage is below –10mV, the SIGN terminal drops low, almost to the negative supply voltage. For input voltages between these two thresholds, the output voltage is well defined, too, because it stays at its previous level. This circuit is the perfect complement to the ‘+/– voltage on bargraph display’ circuit discussed elsewhere in this blog.

The |Uin| and SIGN outputs of the present circuit may be directly connected to Uin and CONTROL IN inputs of the bi-directional bar display. The ±6 V sign indicator signal may be used as the control voltage for the +/– voltage display as long as the reference voltage remains smaller than 3 V. Although presented as a pair, both circuits may of course be used individually for other purposes.

Read More