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Zener in series to LED...

Discussion in 'General Electronics Chat' started by Externet, Nov 10, 2016.

  1. Externet

    Externet Member

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    Would a LED turn on only at voltages above Vz + Vf + Vdrop if hooked in series ? (Vdrop for the series current-limiting resistor)
    -Like to indicate valid supply, not undervoltage, above the operative minimum, or battery not in need or recharging, or signal peaked-

    Have not seen that in circuits...
     
  2. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    It will work to some degree, but the brightness will vary depending on the level of the voltage.
    The LED starts to turn on at a lower voltage than at full brightness. First light may only be 20ua for example.
    You could however use an LM431.
     
  3. spec

    spec Well-Known Member Most Helpful Member

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    Further to what you say, this approach has the advantage of precision and flexibility. It would be very accurate and the no-go voltage could be set to any voltage within certain limits. By the way, for low voltage monitoring there is the TLVH431 which has a 1.25V reference, but its maximum voltage is limited to 18V compared to 36V maximum for the LM431.

    spec
     
    Last edited: Nov 10, 2016
  4. dave

    Dave New Member

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  5. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    I used it in a circuit I designed for an audio amp. I kinda wanted all of the rails (4 of them) to be within 2/3 of 50V, but the 2/3 didn't have to be anywhere near exact,
    The design did exactly what it was supposed to do, but the design could have been better. If any rail fails to be >2/3 Vcc my line current limiting resistor will fry. During start-up audio is removed by an optoisolator and the speakers are disconnected. That resistor and 40,000 uF of capacitance determines the turn-on delay.
    If a rail fuse pops, that fusible resistor pops too.

    No method was employed to remove power if (so many seconds passed by on start-up) AND (immediately after a Power Good signal) AND any supply was <2/3 of the final value was implemented along with "some light".

    The home made AMP didn't actually have a power switch either.

    I accidentally reversed the PNP and NPN output transistors. The only damage was that resistor that's supposed to pop.

    Now, there was ONE ISSUE: The opto's would break until a ZNR was placed across each capacitor. There was a 12 V logic supply that came up immediately. I had a relay that shorted the RESISTOR, but no means to disconnect the AC line power totally. The amps "switch" was external and was a triac built into a "fancy timer".
     
  6. dr pepper

    dr pepper Well-Known Member Most Helpful Member

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    As you reach the zener voltage + the led voltage the led wil turn on gradually, the zener will have some affect on this too.
     

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