Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Can an SCR handle high voltage?

  1. Aug 30, 2012 #1
    I'm looking on digikey for an SCR and there is a parameter that concerns me:

    Voltage - On State (Vtm) (Max)

    It seems the maximum voltage rating for this parameter is 3.7V. I must be misunderstanding what this parameter means. Is there somewhere I could look up what it means?

    I plan to run a 12V motor pulling 3A and use the SCR as a switch instead of a big bulky relay. But if the max voltage is only 3.7V, then I must look for an alternative solution.
     
  2. jcsd
  3. Aug 30, 2012 #2
    Well that scr is not going to be much use to you.

    The on state voltage is the voltage across the device when it is on ie conducting.

    So if you SCR drops 3.7 volts you will only have 8.3 volts left for your motor.

    further the scr will be loosing 3.7 *3 = 11 watts.

    By contrast the contact resistance of a good relay is measured in milliohms so pretty well all of your 12 volts will appear across your motor.

    Page 8 here

    http://www.onsemi.com/pub_link/Collateral/HBD855-D.PDF
     
    Last edited: Aug 30, 2012
  4. Aug 30, 2012 #3

    dlgoff

    User Avatar
    Science Advisor
    Gold Member

    You sure it's not the maximum gate trigger voltage, Vgtm, they're referring to?
     
  5. Aug 30, 2012 #4
    That's VGM in my reference.
     
  6. Aug 30, 2012 #5

    dlgoff

    User Avatar
    Science Advisor
    Gold Member

    Actually, your reference for gate trigger voltage is VGT

    Edit: You're right for the max. Sorry.
     
  7. Aug 30, 2012 #6

    uart

    User Avatar
    Science Advisor

    It sounds like that is probably an absolute maximum rating at absolute maximum current. Assuming that the current rating for your SCR has been correctly chosen, then the actual on-state voltage under normal operation will probably be in the 1.0 to 1.5 volt range.

    BTW. Is this a 12V DC motor? How do you plan to turn the SCR off?
     
  8. Sep 8, 2012 #7
    it is a 12V motor and i planned on turning it off by forced commutation.
     
  9. Sep 11, 2012 #8

    NascentOxygen

    User Avatar

    Staff: Mentor

    That's getting ambitious, but would make an interesting project. :smile:

    You could search for an SCR with a lower voltage drop, or maybe you could change to a power supply of say 13.5V on the assumption that you'll lose 1.5V or so across the SCR?

    Is your goal here to get electronics experience, or to construct a reliable working circuit for a larger project? If the latter, then I side with those suggesting a relay as an off-the-shelf solution.
     
  10. Sep 15, 2012 #9
    This is for experience. The goal is to control high current loads with a micro controller. Motors, large DC light bulbs, etc. Basically all with semiconductor devices (aka no relay). A relay is the obvious choice but I've already completed operating these loads using a micro controller and a relay. My goal is to do this without using a relay. It seems to be very tricky to do but I like challenges. I'm not getting much support on this project and I don't know why but I will continue on! :D

    Here is what I have as a basic idea:
    3501lk1.png

    I don't know how to size the cap or r2 but r1 will be the load. the SCR I am experimenting with now is a S6020L and the transistor I am experimenting with is a MJH6287. It's a PNP. I meant to order an NPN version but I got em mixed up. I want high true but pnp gives me low true. Not really a problem just more of a slight inconvenience to the way I normally think of things. Anyways. Is my though process on the right track here or am I trying to do something impossible?

    Basically a positive pulse from the microcontroller will turn the circuit on and a different pulse from a different pin of the microcontroller will cause the transistor to close and force current the opposite direction of the circuit for a second causing the scr holding current to go low enough therefore turning it off.
     
    Last edited: Sep 15, 2012
  11. Sep 16, 2012 #10

    NascentOxygen

    User Avatar

    Staff: Mentor

    That's the basic idea, and interrupting 11 mA in a resistive load should be no problem. :smile:
    But what happened to the 12V 3A motor?
     
  12. Sep 16, 2012 #11
    Oh. I'm sorry about that. What I was trying to say is that R1 will be REPLACED with whatever load I want. All 12V loads ranging from 3A-10A or more. Idk yet. The goal is the 12V 3A motor. Sorry about the confusion. I guess I should have just put a motor schematic symbol instead of the load, flyback included. I just threw down a basic idea but yes that r1 will be REPLACED by a real load.
     
  13. Sep 21, 2012 #12
    Bump :(
     
  14. Sep 21, 2012 #13

    uart

    User Avatar
    Science Advisor

    Just use the correct tool for the job (switching 10A @12V DC), a MOSFET.

    If you're doing this to learn about SCR's and forced commutation then that's fine (though you'll need to beef up your commutation circuit to switch 10 amps), but let us know what the purpose is here.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Can an SCR handle high voltage?
Loading...