Wiring schema blogs

Showing posts with label over. Show all posts

Friday, April 5, 2013

Circuit Detector and Disconnecting Over Voltage Schematic

The circuit in this figure is protecting the circuit and the system with power supplies that may exceed safe limits. One example is small consumer products that use external ac adapters; its easy to mistakenly plug in the wrong adapter. Another example is a portable system that uses a rechargeable battery pack. If the battery pack is absent or fails to open during recharging, a high-compliance charger can deliver excessive voltages to the system.

The circuit works using LM4041 adjustable shunt-voltage regulator as a voltage detector. When it operates as a reference, the LM4041 develops a voltage across its positive and negative terminals. This signal forces the voltage across R₁ to equal 1.24V. In this circuit, however, R₃ prevents this servo action. With R₃ in the circuit, V_G is near ground when the voltage across R₁ is less than 1.24V, and V_G is approximately 1V below the positive rail when the voltage across R₁ is greater than 1.24V. You can, therefore, set a threshold voltage by selecting appropriate values of R₁ and R₂. When the supply voltage exceeds the threshold, V_G goes high, thereby turning off Q₁ and removing power from the load. Select R₁ and R₂ according to:

It where V_SHUTOFF is the supply voltage that causes shutoff. With the values shown, the circuit removes power from the load when the supply voltage reaches approximately 6V. R4 provides hysteresis to prevent chattering when the supply voltage is near the shutoff value. IC₁ can accommodate shutoff voltages as high as 10V; clamping IC₁s supply voltage with another inexpensive shunt reference or zener diode (across the positive and negative terminals) allows higher maximum shutoff voltages. Maximum supply voltage with the components is approximately 50V.

[Continue reading]

Thursday, April 4, 2013

Simple Circuit Detects Voltage Over Ranges

Sometimes, a visual indication of whether a sensed voltage is above or below its nominal value can be useful. Most approaches to over voltage or under voltage sensing use two voltage comparators and a resistor divider to form a window comparator. The circuit in Figure 1a is an alternative to the traditional window-comparator approach. It provides different-color indications if the sensed voltage is above or below the preset value; in this case, it is centered around 0V.

The circuit uses a FET-input, low-offset-voltage OPA124 op amp and a dual-color LED. The forward voltages for the red and green LED sections are 2 and 2.1V, respectively. The values of the op-amp feedback resistors R1 and R2 are such that the op amps closed-loop gain, 1+R2/R1, equals VLED/VWIN, where VWIN is the desired positive or negative window threshold. Thus, whenever the input voltage, VIN, exceeds ±VWIN in magnitude, the op-amp stage supplies a voltage that turns on the corresponding LED. When VIN>+VWIN, the red LED turns on; when VIN<–VWIN, the green LED turns on. Whenever –VWIN<+VWIN, both the red and green LEDs are off. R3, typically 5 kV, limits the maximum on-state LED current. You should choose R1 such that the feedback current through R3 is small compared with the on-state LED current. You can ignore the small difference between the red and green LED forward voltages for most applications, or you can balance it by adjusting the op-amp offset voltage.

For asymmetrical window voltages, you can use the configuration in Figure 1b. In this case, you assume |VWIN–|>|VWIN+|, where |VWIN–| is the magnitude of the negative window voltage and |VWIN+| is the magnitude of the positive window voltage. Q1 is an NMOS enhancement-mode MOSFET that has a threshold voltage of approximately 1V. The source terminal of Q1 connects to the negative input of the op amp; thus, it remains at a virtual-ground potential. The gate terminal connects to the op amps output, which turns Q1 on whenever the output voltage exceeds Q1s threshold voltage.

For input voltages greater than 0V, the op amp produces a negative voltage and Q1 turns off. The ratio of R2 and R1 sets the op-amp gain, and the output clamps at the on-state voltage of the green LED, approximately –2.1V. For input voltages lower than 0V, Q1 turns on once the op amps output exceeds the threshold voltage of Q1. In this case, the ratio of R1 and the parallel combination of R2 and R3 sets the op-amp gain, and the maximum output voltage is the on-state voltage of the red LED, 2V. Resistor R4 again serves as a current limiter for the LEDs. The relationship between the resistor values and the positive and negative window voltages is given by the following equations. For simplicity, we use only the positive magnitude of the voltages, and we neglect the difference between the forward voltages of the red and green LEDs.

You should choose the value of R1 such that the feedback current through R4 is small in comparison with the on-state LED current. Choose R3 such that its value is much greater than the on-resistance of Q1. The op-amp configuration in Figure 2 has resistor values that set the VWIN– window at –5V and the VWIN+ window at 0.8V. For the case in which |VWIN–|<|VWIN+|, you can replace Q1 with an equivalent PMOS enhancement-mode MOSFET. When the window voltages VWIN– and VWIN+ have the same polarity, you can also use the circuit in Figure 2.

This circuit inserts a unity-gain difference amplifier (for example, an INA105) in the front end of the circuit in Figure 1a. This added stage subtracts a reference voltage, VR. You can use this type of window-comparator circuit to monitor a power-supply voltage, such as 5V, with preset limits of 4.75 and 5.25V, for example. The following equations yield the window voltages: Source: Mark Stitt, Burr-Brown, Tucson, AZ

[Continue reading]