I have a post up about the situation here ( ) which is where the PWM-parallel-resistor approach was suggested. I started out searching for a digital pot, but I am having a lot of trouble finding one that has the exact parameters that I need. You may be better off with a digital potentiometer instead. Having a resistor switched in & out could play havoc with current & voltage levels. ![]() Mechanical relays are not good for driving with PWM. What are the voltages and currents in play for these 2 examples? Analog switches have integrated control logic, like level shifters and digital enables so the design task is easier. FETs are used when the control circuits are expertly designed. I use the CD4016 and CD4051 analog switches for tiny currents. An FET switch usually drops a Vt voltage from the load so full rail to rail switching is not achieved with simple drive circuits. The CMOS analog switch is more sophisticated than single FET switches which often drop a Vt threshold voltage in common circuit configurations. PMOS and NMOS form complementary switches (CMOS). Magnets and mechanical gripping circuits can retain the state of the relay even during a power loss to the control circuit, while the load power is still continuing to draw current and maintain a voltage.Īn analog switch uses enhancement mode MOSFETs that are n-channel (NMOS) and p-channel (PMOS) so that a full voltage swing can be conducted. Or they can have a latching construction so they remain closed or open even if the control circuit is not powered up. For example, a relay control circuit can lose power, but the load current can still flow through the normally closed relay terminals (nc). Enhancement mode FETs are (no), normally open and the gate usually needs its gate driven by a control circuit for it to conduct, depending on circuit details.Ī relay can also be conductive without being enabled by electrical control. To stop it from conducting, its gate must be driven beyond the voltage on its source. The depletion mode FET is normally a conductive switch, even if the gate is not driven to a voltage away from its other terminals' voltages (nc). The FET is a simple switch that have depletion mode or enhancement mode. In the two examples I list above, what would you use, and why? Other than these characteristics, I'm not sure what else might differentiate these devices. So I could see that in a more complex switching application (3P2T), or an application where you needed a lot of switches, an analog switch IC would make sense because of its compactness and simplicity. They also have both a NO and an NC output, whereas an FET, by itself, is either NO or NC, but not both. That is basically all I know about them.Īnalog switch ICs come in a wide variety of designs, with multiple switches and multi-pole/multi-throw configurations. ![]() The relays I have seen also seem to have slower switching speed than the other two: the solid state relays seem to have switching speeds in the ms, where the others have switching speeds in the tens of ns.įETs are, in a simple sense, like a SPST switch. They are usually larger and more expensive than ICs. They can be designed such that there is a very large difference in voltage between the control circuit and the switched circuit. ![]() Relays, especially mechanical relays like the solenoid in your car, excel at switching large currents. But I would like to have a broader understanding of when the use of each of these types of devices is appropriate. The two specific applications I'm considering are 1) replacing a manual NO switch in a device with a switch that is controlled by the Arduino, and 2) feeding a PWM input to a switch to cause a parallel resistor to cycle in and out of circuit, thereby approximating a digital pot via the PWM duty cycle. Frankly, I'm a little overwhelmed, and I wonder if anybody could help me understand what qualities differentiate these devices, that would cause someone to choose one of them over another. I have been thinking about switching, and I have come up with at least three ways to accomplish switching: with an FET, with an analog switch IC, and with a relay.
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