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A detailed explanation of role of pull-up resistors on triodes and MOS lamps.


About triode

A brief explanation on triode, if triode operates in saturation region (fully turned on), Rce≈0, Vce≈0.3V, and this is 0.3V, then we think it is directly grounded. Then it is necessary to make Ib greater than or equal to 1mA if Ib=1mA, Ic=100mA, its gain is β=100, and triode is fully on. As shown in the figure below, this is an NPN transistor.

A detailed explanation of role of pull-up resistors on triodes and MOS lamps.

The triode is a current-type drive component, so normally a current-limiting resistor is in series with base, typically less than or equal to 10K, but why is there a pull-up resistor in base? For example. The following figure shows circuit diagram of temperature switch control motor.

A detailed explanation of role of pull-up resistors on triodes and MOS lamps.

As shown in figure, temperature switch controls start and stop of motor. The temperature switch is similar to push button switch. The switch is connected in series to pole B and tube N can be used as a switch tube. The motor in picture is a brushed DC motor. As long as positive pole is connected to 12V and negative pole is grounded, motor will start to rotate.

When temperature switch is turned on, current flowing through circuit I is equal to

A detailed explanation of role of pull-up resistors on triodes and MOS lamps.

The CE triode is fully on, Vce » 0.3V, at this time voltage drop across motor is close to 12V and it can rotate because impedance of triode be is much smaller than 2K resistor R2, so most of current flows through triode, when thermal switch is turned off, there will be no current in ib, and there will be no current in ic.

Because at moment thermal switch is turned off, current on triode ib and ic cannot immediately drop to zero, but slowly drops to zero. This is inevitable existence of production process. During this time, triode operates in amplification zone and is most susceptible to interference. Therefore, it is necessary to connect a pull-up resistor R2, which, firstly, provides a discharge circuit for triode, and secondly, provides an energy dissipation path for point A.

How to understand discharge pattern?

As shown in figure below, triode parasitic capacitance, actual model of triode manufacturing process, has capacitances C1, C2, and C3 between triodes BE, BC, and CE, respectively. The existence of these three capacitors is, on one hand, unnecessary for us, and on other hand, it is inevitably overcome in process, and is an inevitable phenomenon in production process. We usually refer to this capacitance as parasitic capacitance or parasitic capacitance.

Due to presence of capacitance, triode will necessarily have a delay. In absence of current in ib, capacitor C1 begins to discharge, forming a loop I. At this time, voltage at point B drops from 0.7V to 0V. It operates in amplification zone and is most prone to interference. Add a resistor R2 at both ends of C1. Resistor R2 therefore provides a way for capacitor to shed charge, greatly reducing triode's operating time in the amplification zone.

A detailed explanation of role of pull-up resistors on triodes and MOS lamps.

How do you understand providing a decentralized path for energy?

Why is resistor R2 said to provide a power dissipation path for point A. As shown in figure 2, when temperature switch is turned off, point A is suspended at that time, voltage at point A is unknown, and it is in a state of high impedance (infinite impedance), which is prone to false conduction. and is also subject to environmental influences such as static electricity, lightning strikes, etc. may permanently damage device.

When lightning strikes, high-voltage static electricity, etc. in working environment, lower resistor at point A and connect it to ground, and most of current will flow to ground along resistor, providing an energy dissipation path. If this resistor is not connected, when lightning strikes, since impedance on left side of point A is infinite, and right side of point A is connected to triode, impedance is very low compared to left side, so all current will flow in low impedance direction and flow into triode , causing a current. If it is too large, device will be irreversibly damaged.

About MOS tube

Due to space constraints, a basic knowledge of MOS lamps can be found here: MOS Lamp Basics.

The pull-up resistor has two functions:

  • In order to prevent release of charge by static electricity, it is easy to cause electrostatic breakdown

  • When MOS lamp is running in switching state, there is a continuous charge and discharge of Cgs. When power is turned off, part of charge can be stored in Cgs, but there is no reset circuit, and electric field of MOS lamp gate is still there and can last a long time conditions for establishing a conductive channel did not disappear. The next time current-carrying channel is started, MOS lamp immediately generates an uncontrolled huge drain current Id, causing MOS lamp to burn out.

  • A detailed explanation of role of pull-up resistors on triodes and MOS lamps.