Zener diode question. - Page 1
Forum section: electronics / beginners

author: kiwizz
author_rank: Contributor
timestamp: January 23, 2018, 10:59:07 pm
content: Not sure if this question will trigger any facepalms, but here goes:Why does a zener diode, in this circuit (attached), limit the voltage?Looking at the 'path' current would flow, it has two 'options', the diode, and the load. Assuming we have an 'unstable' supply of 12-14v - would the path of least resistance be the DC load until we hit the zener rating (lets assume 12v)? To think of it another way (water seems to be the best way for me to understand these things) - is a zener diode like a pressure release valve?Also, what function does the resistor perform and what would be the issue if we removed it from the circuit.Cheers!

author: danadak
author_rank: Super Contributor
timestamp: January 23, 2018, 11:57:15 pm
content: The zener diode breakdown at a fixed voltage and independentof current in it, it maintains that Vz across it.In the forward direction looks like a "normal" diode, but in reverse direction,when it breaks down, it maintains an approximate constant V independentof current thru it. So if a load is connected in parallel with it, it will maintaina constant V on the load independent of load changes, relatively speaking.The R is to limit Iz so that the zener does not burn out. Limits current at maxVinresistor.Regards, Dana.

author: james_s
author_rank: Super Contributor
timestamp: January 24, 2018, 01:08:57 am
content: The circuit you show is known as a "shunt regulator" because it clamps the voltage to the breakdown of the zener.Without the resistor the zener would be trying to clamp the supply voltage, which means it will try to draw as much current as necessary to do so, that will let the smoke out.

author: BrianHG
author_rank: Super Contributor
timestamp: January 24, 2018, 02:14:30 am
content: https://youtu.be/xXG3VmUBF_o

author: drussell
author_rank: Super Contributor
timestamp: January 24, 2018, 05:59:28 am
content: Quote from: kiwizz on January 23, 2018, 10:59:07 pmHydraulics are a good analogy for electronics in this kind of case. Voltage is the electronpressureand current is theflow.You are correct that in this way, a zener diode acts like a pressure relief valve that allows the pressure (voltage) to build up until it reaches some pre-determined level, then allows the excess flow to bleed off to maintain that constant maximum pressure.QuoteA resistor is a flow restriction. You need one in this case to limit the amount of juice that can try to flow through the zener...If you had a 2" water inlet pipe and a 1/4" pressure relief valve, you're not going to be able to regulate the pressure if the incoming flow is more than the 1/4" relief valve can handle. Something (in the electronics case, the zener diode) is going to blow up if it cannot withstand / dissipate that energy.

author: Zero999
author_rank: Super Contributor
timestamp: January 25, 2018, 09:08:14 am
content: Quote from: kiwizz on January 23, 2018, 10:59:07 pmOne way of looking at this is the zener diode adjusts its resistance, to maintain a constant voltage across it. For example, a 10V zener will have a resistance of 10k at 1mA, 5k at 2mA and 1k at 10mA.In reality, no zener diode is perfect, so the voltage will increase slightly, as the current increases.

author: paulca
author_rank: Super Contributor
timestamp: January 26, 2018, 09:47:33 am
content: In the water analogy a zener is like a weir.