Im new to designing a circuits. I've got my basic design done. I'm now trying to get to specific parts but I don't know which ones to buy. mainly because I don't know how to determine what I need. I'm hoping to use an NPN transistor as a switch.


simulate this circuit – Schematic created using CircuitLab

What part should I buy and why. Could you take me through the process of determining the part?

  • 2
    \$\begingroup\$ Well, there's not much right about the diagram. However, it does express the fact that you want to switch 24 V, you want to use an NPN, and that there is a solenoid involved. We do not know what solenoid, or what's going to be driving/operating the switch, or how often it will be operated, or for what duration, or a lot of other things that would be useful to know -- especially when being asked to recommend "what part" to buy or how to figure it out. No one wants to write a curriculum. Could you help out by providing all of the associated details we would need to know to help you? \$\endgroup\$ – jonk Oct 19 '18 at 4:04
  • \$\begingroup\$ Im designing a circuit for a diy paper rocker launcher. The solenoid is from a sprinkler valve. and i would want this on until the system is charged with air. which i will be determining with a pressure switch that will turn off the signal to the base. I don't think it will be on for too long (max a second or two every 20 seconds or so). \$\endgroup\$ – derekjones562 Oct 19 '18 at 4:13
  • 1
    \$\begingroup\$ We need to know how much current the transistor needs to pass, and we need to know what you'll be driving the base of the transistor with. If you measure the resistance of the solenoid we can help you calculate the current (or if the solenoid coil is rated as requiring a certain current at 24V, that'll certainly tell us!) \$\endgroup\$ – TimWescott Oct 19 '18 at 4:39
  • \$\begingroup\$ I corrected the schematic so it would be a rational design to discuss. \$\endgroup\$ – user105652 Oct 19 '18 at 4:53
  • \$\begingroup\$ R1 could be 4.7K and Q1 could be a MJE180. Lots of options here \$\endgroup\$ – user105652 Oct 19 '18 at 4:56

As already mentioned in the comments there is rather important information missing to give an accurate answer to the "what". But here are some steps you can take to come up with a possible solution yourself and then come back with a more specific question like "will transistor xyz fit my application".

  1. Figure out what kind of transistor you want to use in term of NPN or PNP (maybe even a Mosfet?)
  2. Determine the current your transistor has to be capable to draw
  3. Determine to voltage the transistor has to withstand
  4. Define the maximum power loss you can have in your transistor, consider switching losses if you have a fast switching application.
  5. Open the search mask of a big distributor like mouser and try to find a transistor that meets the given criteria.
  6. Check the datasheet for other parameters that might influence your design, if you need further help just come back with specific questions.
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  • \$\begingroup\$ i will do a bit more research and come back with my findings. thanks guys! \$\endgroup\$ – derekjones562 Oct 19 '18 at 6:25

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