It is all too often necessary to augment the power supply of an existing electronic circuit because exactly the voltage that you need is missing. The circuit presented here may provide a solution in a number of cases, since it can be used to convert a single-ended supply voltage into a balanced set of supply voltages. That’s not so remarkable by itself, but the special feature of this circuit is that this is accomplished without using difficult to obtain, exotic ICs. All of the components used in the circuit are ones that every electronics hobbyist is likely to have in a drawer somewhere.
The heart of the circuit is formed by an ‘old reliable’ 555 timer, which is wired here as a free-running oscillator with a frequency of approximately 160 kHz. The oscillator is followed by two voltage-doubling rectifiers, consisting of C1, D1, D2, C3 and C7, D3, D4, C5. They are followed in turn by two voltage regulators to stabilise the positive and negative voltages generated in this manner. The duty cycle of the 555 is set to approximately 50 percent using R1 and R2. The square-wave signal at the output of the timer IC has a DC offset, which is eliminated by C4 and R3.
The amplitude of the output signal from the 555 is approximately equal to the supply voltage less 1.5 V, so with a 12-V input voltage, there will be a square-wave signal on pin 3 with an amplitude of approximately 10.5 Vpp. With respect to ground (across R3), this is this +5 V / –5 V. Although this yields a symmetric voltage, its positive and negative amplitudes are somewhat too small and it is not stabilised. In order to split the square-wave signal into sufficiently large positive and negative amplitudes, C1/D2 are added for the positive voltage, causing the positive half to be doubled in amplitude.
For the negative half, the same effect is achieved using C7/D3. Following this, the two signals are smoothed by D1/C3 and D4/C5, respectively. Both voltages are now high enough to be input to normal 5-V voltage regulators, yielding symmetric +5V and –5V supply voltages at the output. The input voltage does not have to be regulated, although it must lie between +11 V and +18 V. The maximum output current is ±50 mA with an input voltage of 12 V. This circuit is an excellent choice for generating auxiliary voltages, such as supply voltages for low-power opamps. Naturally, the fact that the converter can be powered from the in-vehicle voltage of a car is a rather attractive feature.
The heart of the circuit is formed by an ‘old reliable’ 555 timer, which is wired here as a free-running oscillator with a frequency of approximately 160 kHz. The oscillator is followed by two voltage-doubling rectifiers, consisting of C1, D1, D2, C3 and C7, D3, D4, C5. They are followed in turn by two voltage regulators to stabilise the positive and negative voltages generated in this manner. The duty cycle of the 555 is set to approximately 50 percent using R1 and R2. The square-wave signal at the output of the timer IC has a DC offset, which is eliminated by C4 and R3.
555 DC/DC Converter Circuit Diagram
The amplitude of the output signal from the 555 is approximately equal to the supply voltage less 1.5 V, so with a 12-V input voltage, there will be a square-wave signal on pin 3 with an amplitude of approximately 10.5 Vpp. With respect to ground (across R3), this is this +5 V / –5 V. Although this yields a symmetric voltage, its positive and negative amplitudes are somewhat too small and it is not stabilised. In order to split the square-wave signal into sufficiently large positive and negative amplitudes, C1/D2 are added for the positive voltage, causing the positive half to be doubled in amplitude.
For the negative half, the same effect is achieved using C7/D3. Following this, the two signals are smoothed by D1/C3 and D4/C5, respectively. Both voltages are now high enough to be input to normal 5-V voltage regulators, yielding symmetric +5V and –5V supply voltages at the output. The input voltage does not have to be regulated, although it must lie between +11 V and +18 V. The maximum output current is ±50 mA with an input voltage of 12 V. This circuit is an excellent choice for generating auxiliary voltages, such as supply voltages for low-power opamps. Naturally, the fact that the converter can be powered from the in-vehicle voltage of a car is a rather attractive feature.
Author: L. de Hoo - Copyright: Elektor Electronics
The heart of the circuit is IC2, a dual comparator. The comparator circuit is built without using any feedback resistors, with the indication being stabilised by capacitors C4 and C5 instead of hysteresis. Small 10-µF tantalum capacitors work well here; 220-µF ‘standard’ electrolytic capacitors are only necessary with poorly regulated generators. Voltage regulator IC1 provides the reference voltage for IC2 via voltage divider R2/R3. The onboard voltage is compared with the reference voltage via voltage dividers R4 /R5 and R6/R7, which are connected to the inverting and non-inverting comparator sections, respectively.
Using separate dividers allows the threshold levels to be easily modified by adjusting the values of the lower resistors. IC2a drives the anode of the red diode of LED D4 via pull-up resistor R10. The anode of the green diode is driven by IC2b and R11. T2 pulls R11 to ground, thereby diverting the operating current of the green diode of the LED, if the voltage of the electrical system exceeds a threshold level of 15 V (provided by Zener diode D3). The paralleled gate outputs on pins 10 and 11 of IC3 perform a similar task. However, these gates have internal current limiting, so they can only divert a portion of the current from the red diode of the LED.
The amount of current diverted depends on the battery voltage. The two gates are driven by an oscillator built around IC3a, which is enabled via voltage divider R14/R15 and transistor T1 when the battery voltage is sufficiently high. Depending on the state of IC3a, the red diode of the LED blinks or pulses. The circuit is connected to the electrical system via fuse F1 and a low-pass filter formed by L1 and C1.
The rating of R1 should be 5 W. If the battery to be charged has a different capacity, the theoretical value of R1 in the table must be divided by the battery capacity. Its actual value is the nearest one in the E12 series. For instance, if a 6 V battery with a nominal capacity of 600 mAh is to be charged, the value of R1 must be 20/0.6 = 33R.
