Zener diode and resistor help

[Kimly]

In my learning about Zener Diodes, I took apart a hand control. Looks like it may have been on a chair, or bed of some kind. Here is the schematic.

I don't have voltages etc. for this control.

Could anyone tell me what the resistor and zener diode would be used for. I know this is a vague question, Im just grasping for answers

Thanks

 

 

[Jerry]

A zener diode operates in the reverse bias mode.  Notice that the LED and the zener diode have their anodes connected together.  This means that when the circuit is operating such that the LED is turned on there would be about 1.8 volts across the LED in a forward bias manner.  Now look at the zener diode.  It would also have a forward bias across it causing it to conduct and dropping about 0.7 volts just like a regular diode.  The remaining 1.1 volts would be dropped across the resistor.

Now let's look at what happens when the LED is off.  There is not adequate forward bias across the LED.  This means the voltage has to be about 1.4 volts or less  to have the LED off.  At 1.4 volts the zener diode would still be forward biased, dropping about 0.7 volts and the remaining 0.7 volts would be dropped across the resistor.  Of course there may be less than 1.4 volts and if it is less than 0.7 volts then the zener diode is no longer forward biased.

Now let's look at what happens if we get a surge of power of the wrong voltage.  What I mean by this is that we get voltage across the LED in the reverse bias direction.  The LED is okay with that up to a certain limit.  In this situation, the zener diode would also have reverse bias, but hay, a zener works in the reverse bias mode.  This zener is rated at 1.2 volts, so if our reverse bias voltage exceeds 1.2 volts then the zener will conduct.  Anything more than 1.2 volts will be dropped across the resistor.  This limits the reverse bias that would be applied to the LED and acts as a procetion for the LED.