8/31/2023 0 Comments Transistor emitter![]() Is there any disadvantage to using a 1N4001 instead of a 1N4148? 9600bps means each bit is about 100us long and the 1N4001 has a typical reverse recovery time of 2us. The schematic says to use a 1N4148 high-speed diode, which I have used. The emitter voltage fluctuates with the input. ![]() Would it not make more sense to place the diode between the RS-232 input and R1, or between R1 and the transistor Q1, so as to block any current flow into the pin? Since the transistor stays in forward-active mode, the collector current will be 100 times the base current. Why is the diode placed where it is? If I interpret things correctly, it means that the lowest Vbe would be the same as the diode's forward drop and that there would be a current flow from ground through the resistor R1 into the negative voltage RS-232 pin. FETs ( JFETs or MOSFETs) are devices with three terminals: source, gate, and drain. ![]() My digital multimeter only updates about 5 times per second and I don't have an oscilloscope to see things more clearly, but it previously showed the lowest Vbe voltage at around -5V. The field-effect transistor ( FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. I've constructed the circuit and it seems to be successful: the lowest Vbe voltage is around -0.5V. ![]() The BC548 has a maximum Vbe of -6V, but I'd like to protect the transistor by minimizing any negative voltages across the base-emitter junction.Īfter some searching I came across a post on the Raspberry Pi forums that suggests the following circuit to protect the transistor from negative voltage: However, this configuration results in the transistor seeing a voltage of -5V across the base-emitter junction due to the negative voltage of the RS-232 input. It forms a NOT gate so that when the RS-232 output is high, it pulls the output low and vice-versa.įor reference, the RS-232 device (a GPS receiver) is transmitting at 9600bps and is connected to a Raspberry Pi's UART. We may also share this information with third parties for this purpose.I have a circuit that converts 5V RS-232 polarity signals (logical 0 = +5V, logical 1 = -5V) to 3.3V TTL polarity (logical 1 = 3.3V, logical 0 = 0V) using a BC548 transistor. We will use this information to make the website and the advertising displayed on it more relevant to your interests. Targeting/Profiling Cookies: These cookies record your visit to our website and/or your use of the services, the pages you have visited and the links you have followed. Loss of the information in these cookies may make our services less functional, but would not prevent the website from working. This enables us to personalize our content for you, greet you by name and remember your preferences (for example, your choice of language or region). Functionality Cookies: These cookies are used to recognize you when you return to our website. A bipolar transistor (first invented in 1948) is a three-terminal (base, emitter, and collector), current-amplifying device in which a small input current can control the magnitude of a much larger output current. This helps us to improve the way the website works, for example, by ensuring that users are easily finding what they are looking for. Analytics/Performance Cookies: These cookies allow us to carry out web analytics or other forms of audience measuring such as recognizing and counting the number of visitors and seeing how visitors move around our website. They either serve the sole purpose of carrying out network transmissions or are strictly necessary to provide an online service explicitly requested by you. The cookies we use can be categorized as follows: Strictly Necessary Cookies: These are cookies that are required for the operation of or specific functionality offered.
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