UPS - Inverter - Power Generator Circuits

General Purpose Schmitt Trigger Schematic Circuit Diagram

Setting Independent Threshold Voltages

The threshold voltages, and thus 21 7 1 the hysteresis, of the Schmitt trigger described can be set independently.

Op-Amp Configuration

The circuit utilizes an op-amp IC1 configured as an inverting Schmitt trigger. The circuit’s output switches levels at two distinct voltages, which can be precisely set using P1 (high level = UH) and P2 (low level = UL). It is important to note that UH must be higher than UL for proper functioning.

U1<Ur ->Uo = U -> Ur = U;

U1> Ur -> Uo =   UL ->Ur = UL.

Thresholds and Output Transition

These two expressions are depicted graphically in Figure 2. The graph illustrates that when the output is in a high state, it will transition to a low state only if the input exceeds UH. It will then return to a high state when the input voltage drops below UL.

Output State and Reference Voltage

The changing state of the output is a result of the shifting reference voltage at the non-inverting input of IC1. Initially, this shift is influenced by weak but swift positive feedback through R6 and R7. The hysteresis introduced by this network is relatively small, around 20 to 30 mV, but it provides the op-amp with effective switching performance.

Transition in Reference Voltage

Once the op-amp changes state, Ur transitions for a second time due to the effect of 1C2a-1C2d modifying the R6-C7 junction, transitioning from UH to UL, or vice versa.

Transition Speed and Signal Processing

The combination of capacitor C7 and the on resistance of the analog switches extends the transition process, ensuring the reference voltage shifts smoothly from one value to another in approximately 250 nanoseconds. However, this assumes that the input signal’s transition speed is not excessively high. To accommodate signals with rapid edge changes, an input low-pass filter is incorporated (R5-C8).

General purpose Schmitt trigger Schematic diagram

Threshold Voltage Adjustment

The threshold voltages are precisely adjusted using a millivoltmeter connected sequentially to UH and UL. Multiturn presets for P1 and P2 are recommended to ensure accurate level adjustments within a millivolt.

Compatibility with Input Signals

The trigger is versatile and can work with both unipolar and bipolar input signals. If only positive inputs require processing, the power supply for the trigger can be configured as asymmetrical. In this setup, the negative supply line is connected to ‘0,’ while the supply (up to 16 V) is linked to both + and 0. When supplied with a voltage of 16 V, the trigger consumes a current of less than or equal to 30 mA.

Switching Performance Assessment

To evaluate its switching capabilities, the trigger is tested using a triangular voltage input ranging from 0 to 5 V. With the thresholds set at 2 V and 4 V, the trigger consistently switches at frequencies up to 5 kHz.

 

General purpose Schmitt trigger Schematic diagram

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