Transistors or relays?

Discussion in 'Homework Help' started by The Skeptic, Jan 31, 2006.

  1. The Skeptic

    Thread Starter Well-Known Member

    Dec 27, 2005
    According to,

    "In summary, a transistor is fast, switches a small current, has a long lifetime and works with dc only. Whereas a relay is slow, can switch a large current, has a shorter lifetime and works with ac or dc."

    What do you think? Do you agree? Does this apply to solid-state relays as well? Is there a group of situations in which either of them could be used? Any comments?
  2. chesart1

    Senior Member

    Jan 23, 2006

    The choice between transistors and relays is based on many factors. Some important factors are response time and frequency of state changes as you mentioned in your post.

    A relay also provides isolation. Anotherwords, a low voltage power supply could be used in the circuit that feeds the relay coil, and a high voltage power supply could be connected to another circuit through the relay contacts when they are closed.

    The other choice if isolation is needed is to use a optoisolator.

    If you prefer to use the transistor as a switch, then a good choice would be the metal oxide semiconductor field effect transistor commonly called a MOSFET.

    I've seen relays and optoisolators used in control circuits where the rapid changes of the state of the circuit output is not required. For example, a control circuit for a lamp on a front panel of a test set.

  3. n9352527

    AAC Fanatic!

    Oct 14, 2005
    There are some overlap in relay or solid-state switch applications. Solid-state switches are not only made from transistor (BJT and MOSFET). For relatively low current and low voltage, BJT or MOSFET are common. There are other types for high voltage and high current, such as IGBT and Thyristor (GTO). For AC switching, TRIAC can be used or two Thyristors back-to-end.

    The deciding factor when their applications can overlap is cost.
  4. The Skeptic

    Thread Starter Well-Known Member

    Dec 27, 2005
    What would you recommend for 12V DC circuits and an output current of 1A? Do you have any part # in mind?
  5. Erin G.

    Senior Member

    Mar 3, 2005

    What kind of circuit are you building and how fast does the switching need to be?
  6. Papabravo


    Feb 24, 2006
    We use the MJD350 PNP Transistor as a high side switch in 24V industrial applications. This is a 15W part and resonably priced. As always you should consult the datasheet -- your mileage may vary. If you need the part to survive while driving a short then you'll need some current limiting. If you're driving an inductive load, don't forget the flyback catch diode.

    The datsheet can be found at:

    On Semiconductor Website -- MJD350 Datasheet
  7. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    Again this would all depend on what the application is. Being 12V and 1A doesn't give enough detail. What is the duty cycle? What is the actual O/P doing? But as Papabravo says a heavy duty transistor would be good. A 2N3055 would also be a good and very cheap choice.