What is the purpose of 2 diodes on the bottom, i see none?


It is a dimmer circuit, in case you didn't recognize.

Update 2:

light emitting DIODE

Update 3:

"D1 is a rectifier, allowing only forward bias current to flow in the circuit" i thought that but clearly LED is conducting in other direction so we can't speak of rectification.

Update 4:

I knew LED is an indicator, i was really asking about left one.

Update 5:

I know what Zener diode is and how it works, actually i studied it in great depth. Don't assume it's zener when it's not zener symbol.

Update 6:

Also there is R1 and variable resistor to limit LED current

Update 7:

I mistakenly wrote the last update, R1 and variable resistor don't limit the LED current since there is TRIAC gate current, i wonder why is there no resistor for it.

Update 8:

I agree D1 is probably used to provide low voltage for LED.

Update 9:

I find it strange there is no resistor at TRIAC gate (like at amp transistor base/gate). I see nothing to limit the gate current except the load and R2.

Update 10:

Answer is R1+VR1 and C1+diodes act as voltage divider, most of the voltage falling across R1 and VR1 thus limiting the voltage and current at DIAC and TRIAC gate. In fact this is the basic of how dimmer works, variable resistor changes how long it takes for C1 to charge up and fire up the gate.

Update 11:

So there was no need to correct myself above, R1 and VR1 DO limit the LED current.

Attachment image

6 Answers

  • qrk
    Lv 7
    1 month ago
    Favorite Answer

    The LED is an indicator and probably changes brightness with the setting of VR1. D1 is used to prevent damage to the LED during the negative cycle. It keeps the reverse voltage across the LED under 1 volt.

  • Anonymous
    4 weeks ago

    It lite up!    

  • Joe
    Lv 7
    1 month ago

    The LED is an indicator light.  That's easy.

    You would normally have a resistance in series with that LED to keep the current within desired operating limits.  Since there isn't one, and D1 is reverse-biased relative to the LED, I'm going to assume that D1 is a Zener diode, used to limit the voltage on the LED.

    In brief: Zener diodes provide a constant voltage reference when reverse-biased.

    It's mildly interesting that both the LED and Zener diode receive an AC signal; I can't say more about that without more information about this circuit.

    Google "Zener Diode" if you want to know more.

  • 1 month ago

    I like the one answer that says an LED is not a diode. WTF do you think an LED is? A Light Emitting Diode. LOL

    D1 is a rectifier, allowing only forward bias current to flow in the circuit. It blocks reverse bias current.

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  • Damien
    Lv 6
    1 month ago

    what 2 diodes? it clearly shows the other one is a led, not a diode.

  • Jim
    Lv 7
    1 month ago

    As a bleeder or indicator

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