Switching ON/OFF SCR thyristor


(visit  the first or third post in this series)

In previous post, I showed a schematic able to switch ON/OFF SCR. You may wonder how that is possible. Well here is the explanation.

The SCR I used is C106D, which is a sensitive gate thyristor. Those ones have ability to increase their holding current when their gate is negative biased like this:

SCR Negative bias

SCR holding current increases reciprocal to the resistance – smaller resistor increases holding current more than a bigger resistor. In proposed schema

SCR with NPN switch

There is no such resistor as discreet element, but still there is a tiny resistance of the transistor’s collector-emitter junction. Because this resistance is very low, we increase holding current quite significantly. This on other side means, that there will be no enough holding current to keep SCR open when we remove triggering signal from the gate.

We have to be very careful not to burn the transistor, as we will expose it (even this will be for very short period) to current that could be higher than one it can sustain. We can use small resistor attached between the SCR gate and transistor’s collector, to eliminate this danger, but we should not add very high value resistor, because this will lower the holding current and we may lose the switching OFF ability.

Now when we send 1 (true) to transistor switch, it will open and thus cause a reverse current to flow from cathode (at that moment it will be more positive than the gate), through the gate, then sinking to the ground. As the transistor’s resistance is so small, it will make SCR holding current to rise significantly. When the current flowing through the load is below this holding current, the result is that SCR switches OFF.

For now, I will leave you to try and figure out the calculations. I am writing down all that in much more detail including the calculations and will publish it very soon inside next version of my Netduino tutorial (code files) at the forum and wiki.

Last thing to mention is that I successfully tested the schematic with different sensitive gate SCR – MCR22-6.

(visit the first or third post in this series)


6 comments on “Switching ON/OFF SCR thyristor

  1. Pingback: Georgi's blog

  2. Pingback: Netduino tutorial – Replacing Relay with SCR « Georgi's blog

  3. Pingback: Switching ON/OFF SCR thyristor (video demonstration) « Georgi's blog

  4. Pingback: Using NAND logic gate to solve problem with Netduino’s pin output on boot or restart | Georgi's blog

  5. Αγαπητέ Γιώργη Ονομάζομαι Κυριάκος Τριανταφυλλίδης και ειμαι 84 ετων. Ειδα το όνομάσου και βλέπω πως εισαι έλληνας ηλεκτρονικός και θά σε παρακαλέσω νά με
    βοηθήσεις άν βέβαια γίνεται στο παρακάτω πρόβλημά μου.
    Εχω ενα thyritor το 50RIA20 (βαρέως τύπου SCR που σηκώνει περι τά 1000 αμπερ στιγμιαία) και προσπαθώ να το χρησιμοποιήσω παντρεύοντάς το με ενα πυκνωτή 1 FARAD για να φτιάξω ενα Capacitor discharge battery tab Welder
    Υπάρχουν πολλά απο αυτά στο internet.με τίτλο DIY a capacity discharge wel welder for less than $ .100 To προβλημά μου ειναι οτι δ ε ν μπορώ να κάμω το παραπάνω θυριστορ να α γ ε ι πυροδοτώντας το με το + μιάς μπαταριούλας 3 βόλτ μεταξύ της καθόδου του και της gατε και να μείνει αυτο στο latching για να έλθω εγώ κατόπιν να του σταματήσω το laching βραχυκυκλωνοντας τα δυο prongs (electrodes) που θα εκφορτίσουν τον 1 FARAD CAPACITOR πανω σε μεταλλικα
    strips νικελίου και επαναφορτιζόμενες μπαταρίες κάμνοντας κόλληση.
    Ευχαριστώ Κυριάκος Τριαναφυλλίδης
    Τηλ: 2109413380

    • Hi,
      I have tried some translators and this is the best I could get:
      “Dear My name is Georgi Kyriakos Triantafillidis and I’m 84 years old . I saw the name and I see that you’re a Greek electronic and would beg me
      help if the course is below my problem.
      I have a thyritor the 50RIA20 ( heavy duty SCR beaming around 1000 AMBER briefly) and I try to use it Marrying with a capacitor 1 FARAD to make a Capacitor discharge battery tab Welder
      There are many of them on Internet.All title DIY a capacity discharge wel welder for less than $ .100 To My problem is that I n d e up taking the above thyristor to a c e j igniting it with a + batarioulas 3 volts between the cathode and the gate and stay in self latching on to come I then stopped in laching shorting the two prongs (electrodes) that will discharge the 1 FARAD CAPACITOR on metal
      strips nickel and rechargeable batteries kamnontas gluing.”

      Form it I think you mistaken me as Greek – I am Bulgarian 🙂
      Anyway, please, repost your comment in English as currently some of the meaning of your message is a bit lost, and I’ll look how I can help you.


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