The use of barrier-resistive elements - baristors in power supplies. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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Baristors, or barrier-resistive elements (Fig. 1), are designed to separate signals whose amplitude is above or below a certain user-defined threshold value - the barrier. Ideally, such devices at the low-level output should transmit the input signal without distortion if its amplitude does not reach the threshold (barrier) value. When the threshold (barrier) value is exceeded, the input signal automatically switches and passes without distortion to the high-level output.

Baristors can be used in miniature economical power supplies, for amplitude selection of signals, as well as in frequency multipliers.

The operating principle of the baristor is clear from Fig.1.

As a threshold (barrier) Z-element, a semiconductor device is used, which has a current-voltage characteristic with a section of negative dynamic resistance (Fig. 2).

If the input voltage does not exceed the switching voltage of the barrier Z-element, its resistance is infinitely high. A low-level voltage is supplied to the control input of one of the key elements, an inverted, high-level voltage is supplied to the input of the second. Accordingly, the input signal will pass without loss through the enabled (switched on) key element. When the input voltage level exceeds the threshold, the resistance of the Z-element will drop abruptly to some finite value. Key elements will switch automatically.

Zener diodes, bipolar avalanche transistors, dinistor and thyristor structures and their controlled and uncontrolled analogs can be used as a threshold (barrier) Z-element.

Schematically, the baristor and its current-voltage characteristics for a certain load resistance RN are shown in Fig.3.

A practical diagram of the use of a baristor in a transformerless power supply is shown in Fig. 4, 5.

Diagrams of the signals present at the input and outputs of the baristor (Fig.3-5) are shown in Fig.6.

The device contains a bridge circuit for rectifying the voltage supplied to the device, threshold and key elements. At the output of the bridge circuit, a signal characteristic of this type of rectifiers is formed (Fig. 6). When the amplitude of the voltage taken from the bridge circuit does not exceed the breakdown voltage of the controlled analogue of the dinistor (transistors VT1, VT2), the key element on the composite transistor VT3, VT4 is open. The input signal of a small (pre-barrier) level passes unhindered to the output of the low-level baristor (load resistance RN and the SF filter capacitor connected in parallel to it).

In accordance with the level of the signal taken from the bridge circuit, the key element is switched, see also the diagrams shown in Fig. 6. As soon as the input voltage exceeds the barrier voltage, the analog of the dinistor is unlocked, the voltage across it drops, the transistors VT3 and VT4 are locked, the current does not pass through them.

The level of the barrier at which the switch of the device key occurs can be controlled manually by the potentiometer R3 or automatically, by using the tracking feedback and turning on the controlled element (optocoupler, field effect transistor) instead of the potentiometer R3.

When the device is operated from a high voltage source, high-voltage transistors must be used as the output composite transistor and the values ​​​​of the resistive elements (R5 and R4) must be corrected.

Author: M.A. Shustov

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