Circuit diagram of 9011 transistor switch (1)
So far, we have assumed that there is a complete short circuit between the base and emitter when the transistor switch is on. This is not the case. No triode can be completely short circuited to make VCE = 0. Most small signal silicon triodes have a saturation value of about 0.2 v. even if they are specially designed for switching applications, their VCE (saturation) value can be as low as 0.1 V at most. Moreover, once the load current is high, the VCE (saturation) value will be a little higher Although for most analysis and calculation, VCE (saturation) value can be ignored, but when testing switching circuit, we must understand that VCE (saturation) value is not really 0.
Although the voltage of VCE (saturation) is very small and insignificant, if several triode switches are connected in series, the total voltage drop effect will be considerable. Unfortunately, mechanical switches often work in series. As shown in Fig. 3 (a), the triode switch cannot simulate the equivalent circuit of mechanical switch (as shown in Fig. 3 (b)), which is One of the disadvantages of triode switch.
Figure 3 transistor switch and mechanical switch circuit
Fortunately, although the triode switch is not suitable for serial connection, it can be perfectly applied to the parallel connection mode, as shown in Figure 4.
Compared with traditional mechanical switch, triode switch has the following four advantages:
Fig. 4 parallel connection of triode switch
(1) The triode switch has no movable contact part, so there is no risk of wear and tear. It can be used for unlimited times,
General mechanical switch, due to contact wear, can only be used for millions of times at most, and its contact is easy to be polluted and affect the work, so it can not operate in a dirty environment. The triode switch has no contact and is sealed, so there is no such concern.
(2) The action speed of the triode switch is faster than that of the general switch. The opening and closing time of the general switch is calculated in millisecond (MS), while that of the triode switch is calculated in microseconds (μ s).
(3) The transistor switch has no bounce phenomenon. General mechanical switch in the moment of conduction will have fast continuous open and close action, and then gradually reach a stable state.
(4) When the triode switch is used to drive the inductive load, there will be no spark when the switch is turned on. On the contrary, when the mechanical switch is turned on, the current on the inductive load sample is cut off instantaneously. Therefore, the instantaneous induced voltage of the inductance will cause arc light on the contact point. This arc will not only erode the surface of the contact, but also cause interference or harm.
Circuit diagram of 9011 transistor switch (2)