Triple Power Supply

Description: Inexpensive miniature transformers typically provide one or two secondary voltages, which are sufficient for generating a set of positive and negative supply voltages, as required for operational amplifier circuits. However, if an additional voltage higher than either of the supply voltages is needed, such as a tuning voltage for a receiver, a solution is necessary. This circuit presents a simple method to address this problem and can be extended for other applications. Utilizing a 2G—15-V transformer, it generates positive 24-V and 12-V supply voltages, along with a negative 12-V supply voltage. The trick for generating the +24-V output involves using IC1 to create a virtual ground, based on a well-known circuit with a voltage divider formed by two equal-value resistors that divide the voltage across the rectifier from approximately 40 V down to 20 V. This Ub/2 potential is buffered by an operational amplifier, allowing the virtual ground to drive a load. The circuit operates on the same principle, but instead of dividing the voltage by a factor of 2, the voltage across the rectifier (approximately 40 V) is divided unequally by resistors R1 and R2. The resulting potential, buffered by the operational amplifier and subsequent transistor output stage, lies approximately 15 V above the lower potential, thus around 25 V below the upper potential. The three voltages are stabilized using standard 100-mA voltage regulators, as shown in the schematic. The supply voltages for the operational amplifier are also asymmetric. Due to low current consumption, this can be managed using two Zener diodes. It is important to note that the secondary voltage generated by an unloaded miniature transformer is significantly higher than its rated secondary voltage. In a test circuit using a 1.6-VA transformer with two 15-V secondary windings, the positive and negative 12-V outputs could be loaded at around 10 mA each, and the 24-V output could be loaded with approximately 20 mA, all without any drop in output voltages. For small circuits such as a 0(4) 20-mA instrumentation loop, this is fully adequate. For more complex circuits or switched loads, additional compensation may be necessary.

The circuit employs a miniature transformer with dual 15-V secondary windings, which serve as the primary source of power. The transformer is capable of delivering higher voltages when unloaded, making it suitable for applications requiring additional voltage levels. The operational amplifier (IC1) is pivotal in creating a virtual ground, which facilitates the generation of the +24-V output. This virtual ground is established through a voltage divider comprising two resistors of equal value, effectively halving the voltage from the rectifier output.

For the unequal voltage division, resistors R1 and R2 are strategically selected to provide the desired output levels. The buffered output from the operational amplifier is then amplified by a transistor output stage, which ensures that the voltage remains stable under load conditions. The standard 100-mA voltage regulators are employed to maintain the stability of the output voltages across varying loads, ensuring that the +24-V, +12-V, and -12-V outputs remain consistent.

The circuit design also incorporates Zener diodes to manage the asymmetric supply voltages for the operational amplifier, allowing for efficient operation with low current consumption. The testing results indicate that the circuit can support specific current loads without voltage drops, making it suitable for instrumentation applications such as 4-20 mA loops. However, for more demanding applications, additional design considerations may be required to ensure voltage stability under dynamic load conditions.Inexpensive miniature transformers normally provide one or two secondary voltages, which is sufficient for generating a set of positive and negative supply voltages, such as are needed for operational amplifier circuits. But what can you do if you need an additional voltage that is higher than either of the supply voltages (such as a tuning voltage for a receiver ).

This circuit shows a simple solution to this problem, and it certainly can be extended to suit other applications. Using a 2G—15-V transformer, it generates positive 24-V and 12-V supply voltages and a negative 12-V supply voltage. The little trick for generating the +24-V output consists of using IC1 to create a virtual ground. This is based on a well-known circuit with a voltage divider formed by two equal-valued resistors, which divide the voltage Ub across the rectifier from approximately 40 V down to 20 V.

This Ub/2 potential is buffered by an opamp, which allows this virtual ground to drive a load. The present circuit uses the same principle, but instead of being divided by a factor of 2, the voltage across the rectifier (approximately 40 V) is divided unequally by R1 and R2. The resulting potential, which is buffered by the opamp and the subsequent transistor output stage, lies approximately 15 V above the lower potential, and thus around 25 V below the upper potential.

The three voltages are stabilised using standard 100-mA voltage regulators, as shown in the schematic. The supply voltages for the opamp are also asymmetric. Thanks to the low current consumption, this can be managed using two Zener diodes. You must bear in mind that the secondary voltage generated by an unloaded miniature transformer is significantly higher than its rated secondary voltage.

The following results were obtained in a test circuit using a 1. 6-VA transformer with two 15-V secondary windings: the positive and negative 12-V outputs could be loaded at around 10 mA each, and the 24-V output could be loaded with approximately 20 mA, all without any drop in any of the output voltages. For small circuits such as a 0(4) 20-mA instrumentation loop, this is fully adequate. For more complex circuits or switched loads, additional compensation may be necessary.