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Jul 28 2020

LM339 Application Circuit

Introduction

LM339 (Quad differential comparator) consist of four independent voltage comparators. It is a common integrated circuit and is mainly used in high-voltage digital logic gate circuits. Using LM339 can easily form various voltage comparator circuits and oscillator circuits.

Catalog

Introduction

Catalog

I Circuit of Single Limit Comparator

II Overheat Detection and Protection Circuit

III Hysteresis Comparator

IV Over-voltage Detection Circuit

V Double Limit Comparator

VI Using LM339 to form an Oscillator

I Circuit of Single Limit Comparator

Figure (a) shows a basic single limit comparator. Add the input signal UIN (i.e. voltage to be compared) to the in-phase input terminal, and connect a reference voltage Ur at the anti-phase input terminal. When the input voltage Uin > Ur, the output is high level UOH. Figure (b) shows its transmission characteristics.

single limit comparator

Figure 1. Circuit of Single Limit Comparator

II Overheat Detection and Protection Circuit

It is powered by a single power supply. A fixed reference voltage is added to the anti-phase input terminal of 1/4LM339, and its value depends on R1 and R2. UR=R2/(R1+R2)*UCC. The voltage at the in-phase terminal is equal to the voltage drop of the thermistor RT. When the temperature inside the machine is below the set value, the "+" terminal voltage is greater than the "-" terminal voltage, and Uo is a high potential. When the temperature rises above the set value, the "-" terminal voltage is greater than the "+" terminal, and the Uo output is at zero potential, which causes the protection circuit to operate. Adjusting the value of R1 can change the threshold voltage, which sets the temperature value.

Overheat Detection and Protection Circuit

Figure 2. Overheat Detection and Protection Circuit

III Hysteresis Comparator

The hysteresis comparator can also be regarded as a single limit comparator with positive feedback. In the single limit comparator described above, if the input signal Uin has slight interference near the threshold, the output voltage will produce corresponding undulation. This shortcoming can be overcome by introducing positive feedback into the circuit..

Figure (a) shows a hysteresis comparator. The familiar Schmidt circuit is a comparator with hysteresis. Figure (b) shows the transmission characteristics of the hysteresis comparator.Hysteresis Comparator

Figure 3. Hysteresis Comparator

It is not difficult to see that once the output state is changed, the output voltage will be stable as long as the interference near the jump voltage value does not exceed the value of Δ U. Accordingly, it comes to a reduction in resolution. For the hysteresis comparator, it can't distinguish two input voltages whose difference is less than ΔU. The hysteresis comparator with positive feedback can accelerate the response speed of the comparator, which is one of its advantages. In addition, since the positive feedback added by the hysteresis comparator is very strong and much stronger than the parasitic coupling in the circuit, the hysteric comparator can also avoid the self-oscillation caused by the parasitic coupling of the circuit.

IV Over-voltage Detection Circuit

Figure 4 shows the part of over-voltage detection circuit in an induction cooker circuit. When the grid voltage is normal, 1 / 4lm339 u42.8v, the comparator turns over. The output is 0V and BG1 is cut off. The voltage of U5 is completely determined by the partial voltage value of R1 and R2, which is 2.7V. It makes U4 larger than U5, which makes the state after overturning extremely stable and avoids the instability caused by the small fluctuation of grid voltage near the over-voltage point. Due to certain hysteresis, after overvoltage protection, the induction cooker starts to work again when the grid voltage drops to 242-5 = 237v and U4 < U3.

Over-voltage Detection CircuitFigure 4. Over-voltage Detection Circuit

V Double Limit Comparator

The circuit in Figure 5 consists of two LM339 to form a window comparator. When the compared signal voltage Uin is between the threshold voltages (UR1<Uin<UR2), the output is high potential (UO=UOH). When Uin is not between the threshold potential range, (Uin>UR2 or Uin<UR1) the output is low potential (UO=UOL), and the window voltage ΔU=UR2-UR1. It can be used to judge whether the input signal potential is between the specified threshold potential.

Figure 5. Double Limit Comparator

VI Using LM339 to form an Oscillator

Figure 6 shows the circuit of an audio square wave oscillator composed of 1/4LM339. Changing C1 can change the frequency of the output square wave. In this circuit, when C1=0.1uF, f=53Hz; when C1=0.01uF, f=530Hz; when C1=0.001uF, f=5300Hz.

LM339 can also form a high-voltage digital logic gate circuit, and can directly interface with TTL and CMOS circuits.

OscillatorFigure 6. LM339 Oscillator

 

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