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I am relatively new to transistor switching and am having a hard time understanding what is going on with my transistor switch.

I would like to 'switch on' my transistor only when the vin is >= 4.7V.

I thought using a 4.7V Zener attached to the base would work.

What I am seeing is that as the voltage across my circuit is increased above 4.7V, the current increase is gradual rather than instantaneous.

my circuit

I want the current to flow when the base threshold is reached or not at all.

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2 Answers 2

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There are other devices which behave like a transistor with a sharp turn-on characteristic when the 'base' voltage reaches a specific value.

Take a look at the TL431 or LM431; you just need to add a potential divider to the Ref pin, such that its voltage is 2.5V when you want the device to switch on.

Although these devices are advertised as shunt regulators, there is no necessity for the signal being monitored to be the same as the signal being controlled; you can just use them in place of an NPN transistor, in applications such as this.

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  • \$\begingroup\$ I found the major issue with the comparator (after trying out andy's suggestion) was that it needs a constant supply of 4.7V for reference. That becomes rather cumbersome in very simple designs. I really just wanted this as a way of seeing without a multimeter if supply is being provided at 4.7v. But then i realized it could have other uses. \$\endgroup\$
    – Triangle4
    Feb 7, 2022 at 14:22
  • \$\begingroup\$ That is why I suggested the TL431, which is a simple 3-terminal device; no need for a reference supply. \$\endgroup\$
    – jayben
    Feb 7, 2022 at 15:39
  • \$\begingroup\$ Okay thank you! I will try that out today! \$\endgroup\$
    – Triangle4
    Feb 8, 2022 at 19:54
  • \$\begingroup\$ Works like a charm. Thank you for the help! \$\endgroup\$
    – Triangle4
    Feb 11, 2022 at 7:25
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You have your zener diode incorrectly wired to block the 4.7 volts. At the moment it just behaves like a forward biased diode and will drop about 0.7 volts across it.

It should be like this: -

enter image description here

If the diode in series with the transistor is an LED you could get rid of the transistor and do this: -

enter image description here

In the first example, the zener probably needs to reduce to 3.9 volts and also, when implemented correctly it still will switch gradually but much better than when you originally had it wired incorrectly. In the 2nd example a 2.7 volt zener plus the natural 2 volts dropped across the LED will prevent it from fully illuminating until 4.7 volts is reached.

Transistors, Zeners and resistors are not digital circuit elements and don't behave with absolute digital precision so, there will always be a range of input voltages where there will be a gradual change. If you wish it to be more "digital" in nature then you should consider using a comparator and voltage reference. You might also consider adding hysteresis also.

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  • \$\begingroup\$ Ok. I was thinking comparator (relatively new to electronics) but I don't know enough about them to come to that conclusion. I will do research and see if that path is more fruitful. Thanks for clarifying the difference between digital and analog. I never realized that. \$\endgroup\$
    – Triangle4
    Feb 7, 2022 at 12:20
  • \$\begingroup\$ @Triangle4 you should probably take the two minute tour to understand why folk give their time for free to help. \$\endgroup\$
    – Andy aka
    Feb 7, 2022 at 12:26
  • \$\begingroup\$ @Triangle4 let me explain something here. I pointed out that your zener was the wrong way round in your diagram and I explained that using Zeners will not switch rapidly AND I even mentioned using a comparator and voltage reference (of which the TL431 can be used). So, to say my answer did not solve your problem is not to recognize the usefulness of my answer and not to recognize that I did suggest using a comparator..... \$\endgroup\$
    – Andy aka
    Feb 14, 2022 at 8:54
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    \$\begingroup\$ @Triangle4 I'm a moderator on this site: Part of my task is to watch people disagree with and misunderstand each other and to decide when intervention is more desirable than inaction. In this case your frustrations are understood - but you are also ignoring useful technical input due to your inexperience. Not understanding is fine - it's part of the learning process. But if you start making technical assertions which are incorrect when advised otherwise then it impedes your longer term learning prospects. You do not have to like Andy's style, but his technical advice is usually excellent. ... \$\endgroup\$
    – Russell McMahon
    Feb 15, 2022 at 11:47
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    \$\begingroup\$ The TL431 IS a combined comparator, voltage reference and power switch. It does not need a separate power supply due to the way it is designed and works. || Your original circuit with the zener reversed and perhaps a somewhat different zener value will perform your task within the limitations of the circuit. Adding hysteresis would make it "a little snappier". The TL431 has the advantage of high gain and low sense current so works far better. || I have personally upvoted your question, Jayben's answer and Andy's answer because ALL of them "are useful" which is the main criterion for upvoting. \$\endgroup\$
    – Russell McMahon
    Feb 15, 2022 at 11:53

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