Programmable Unijunction Transistor == Programmable Zener Diode?

Discussion in 'General Electronics Chat' started by dsharp02, Aug 24, 2015.

  1. dsharp02

    Thread Starter New Member

    Aug 8, 2015
    I recently started trying to learn electronics, and came across the programmable unijunction transistor. From the description it sound very similar to a zener diode, but with a programmable breakdown voltage.

    Am I overlooking something here? Or can a PUT be used in this manner?

    I tried to google it, but couldn't find anything that seemed to answer this specific question.

  2. MikeML

    AAC Fanatic!

    Oct 2, 2009

    A Zener has a knee in its IV curve. A PUT has a negative resistance region (fold back or snap back) in its IV curve.; quite different.

    A programmable Zener is a TL431.
  3. OBW0549

    Well-Known Member

    Mar 2, 2015
    A Zener diode and a unijunction transistor, programmable or not, are as different as night and day.

    If you're after an analogy to the PUT, think of a reversed-polarity SCR with an extremely sensitive gate. Not quite the same, but close...
  4. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    There is a partial similarity between the two devices. As the voltage across the devices steadily increases, there is no current conduction for a while. Then, when the voltage reaches a value that is much higher than a normal diode, like 5.1V or 12 V, the devices start to conduct. But this is where the two devices are different. When a zener diode starts to conduct, the voltage across it remains almost constant. This is the main property about a zener diode that is useful. But when a PUT starts to conduct, the voltage across it decreases. This is useful too, but in a very different way.

    Last edited: Aug 24, 2015
    MHDS likes this.
  5. ian field

    AAC Fanatic!

    Oct 27, 2012
    The PUT is a 4-layer device that can be duplicated with a cross-coupled complementary pair of small signal transistors. The PUT is basically an SCR with the gate at the anode end instead of at the cathode end. The transistor pair is more nearly equivalent to the Silicon Controlled Switch (SCS) which has both gates brought out on leads.

    In the usual textbook PUT oscillator example; the gate is tied to a voltage divider across the supply, the anode is connected to a capacitor that charges through a resistor. Since the gate is opposite to that of an SCR, it needs to be negative with respect to the anode - making the anode positive with respect to the gate means the same thing, as the capacitor charges; the anode voltage rises and at some point gets higher than the gate. Usually a resistor is inserted in the cathode lead to extract the output pulse.

    Because of the complementary asymmetry - you can turn the circuit upside down and do something similar with an SCR.

    Typical example part numbers are; 2N6027/6028 for a PUT and 2N5061 for a suitable TO92 style SCR.

    A typical example of a programmable Zener is; the TL431 - its a TO92 (and various SMD equivalents) device containing a comparator with a built in 2.5V reference. Its invariably used in a high negative feedback loop, so the comparator operates in linear mode.
  6. GopherT

    AAC Fanatic!

    Nov 23, 2012
    Dave, when you are exposed to something new, your mind works in overdrive to find correlations to help you learn. As people become more experienced, they start looking for differences instead of similarities. So, you coming in new, notice a similarity and ask experts about it. They start criticizing you and say, night and day difference. So, it is more a matter of perspective (experience) than correctness or wrongness.

    You are right, Zener and UJT have some similar responses except for the negative notch at the critical point on the UJT. However, a Zener cannot be used to replace a UJT in a UJT circuit. And a UJT doesn't make a good Zener (it will likely oscillate severely instead of acting like a rock solid voltage reference.

    Unfortunately from an academic view, the UJT is a dying technology. Looking at, there is only one model/part number stocked and that one is at a highly inflated price vs the good old days.
    dsharp02 likes this.
  7. dsharp02

    Thread Starter New Member

    Aug 8, 2015
    Thanks for your response. I'm pretty thick skinned, so none of the replies offended me. In fact, I'm probably as guilty as anyone here of being, errr... blunt. Professionally, I'm a software guy, but I like to learn new things, so here I am.

    I'm not sure I fully grok the replies, but that's likely due to a lack of background in the basics on my part. The whole negative resistance thing is messing with my head. I *think* that what I'm being told is that:

    1. When a zener breaks down is conducts just enough current to pull the voltage down into some sort of equilibrium at the breakdown voltage.
    2. When a UJT breaks down it starts to conduct increasing the current until the source can't provide any more and the voltage drops to near zero, which then resets the UJT so it stops conducting. So the UJT doesn't turn off until the voltage drops to zero?
  8. ian field

    AAC Fanatic!

    Oct 27, 2012
    If you take either a PUT or an SCR and don't connect anything to the gate - its basically a 1 direction only diac.

    The diac of course is bidirectional and has a specific breakdown voltage defined in the datasheet, but a PUT or SCR will break down in a similar way if its voltage is exceeded.

    The distinction from a zener being that the device latches with a volt drop of about 1V until the holding current is released, whereas a zener just conducts more leakage current when the applied voltage reaches the "knee curve".