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Are UJT's used anymore?

Discussion in 'General Electronics Discussion' started by NMNeil, Oct 9, 2019.

  1. NMNeil


    Oct 3, 2014
    I have a circuit diagram for a CDI trigger circuit circa the 1970's, that uses a 2N1671 UJT. Newark is the only place I can find them but at $84 each!!
    Looking around for an alternative that's not solid gold and jewel encrusted it seems that UJT are a thing of the past. Mouser only lists 2 and Digikey list 34 but only 3 are kept as stock and the rest are shown as obsolete. Have UJT's been superseded by something else?
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    9.60 € on Ebay + shipping.
    I agree.
    The venerable 555 for one plus everything (well, almost) today is controlled by a microcontroller, e.g. an ATTINY which comes in an SO-8 size package for around 0.60 €.
  3. NMNeil


    Oct 3, 2014
    I was considering using a microcontroller, but have read that the interference from generating a spark in a CDI unit does odd things to the MCU unless you design the voltage regulator properly. Uncharted territory for me as a newbie.
  4. Alec_t


    Jul 7, 2015
  5. NMNeil


    Oct 3, 2014
    I'm not sure how to post photos or circuits, but I'll try and find out. I have already completed and simulated a couple of circuits as an alternative. One using a 2N3906 the other a CD40106.
  6. Alec_t


    Jul 7, 2015
    Use the 'Upload a file' button and browse to the wanted file. Keep graphics files below 1Mbyte and preferably use .png format for circuits.
    If you run a sim you can post the .asc file.
  7. Nanren888


    Nov 8, 2015
    I have not tried to buy ujts since 2015, but they (2n2646) were $16 for a bag full on aliexpress at that time.
    A much underrated device in my opinion. :)
    And, yes, I too bought attinys more recently, but not for the same jobs.

    A quick search suggested 2N1671 listed at about $7 for ten.
  8. Bluejets


    Oct 5, 2014
  9. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    UJTs may not be exactly obsolete, but they have had their "day in the sun" as far as popularity goes. IIRC, their main function was to establish timing pulses for triggering SCRs in so-called phase-controlled dimmer circuits. They also were used to make simple oscillators covering the audio frequency range. I enjoyed "playing" with them on the bench in the 1970s, but as @Harald Kapp noted in his post #2, the ubiquitous 555 "timer" chip pretty much wiped out the market for UJTs... while opening the flood gates to millions of electronics enthusiasts in the process, who embraced the 555 wholeheartedly.

    And, yes, pair one (or several) with microcontrollers allows just about anything possible as far as digital-controlled timing is concerned. Sure, the 555 is an analog timer and not particularly accurate, but let's get creative, people! @NMNeil, post your schematic for the capacitor discharge ignition circuit. These were popular projects "back in the day" before cars got them as "standard equipment" and controlled by microprocessors too. As @Alec_t says, given the right information it can be built. Shielding microprocessors from ignition interference is just part of normal electronic design for the automobile environment. You want to play in that field, you need to gather the tools and experience necessary to succeed.
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