Need for zener diode - Page 1
Forum section: electronics / projects

author: ravipal180
author_rank: Newbie
timestamp: October 27, 2015, 01:20:33 pm
content: Hi everybody,Am new to this blog i have some doubt about zener diode in the application of switching.I use p channel mosfet as a switching device and in my reference design they are using the resistor and zener diode connected between the drain and the ground. resistor is used between the drain and ground is common. But why Zener diode is added is in series wit the resisitor.??thanks in advance,Reagards,Ravichandran.

author: DanielS
author_rank: Frequent Contributor
timestamp: October 27, 2015, 01:45:17 pm
content: A link to the reference design in question would be useful.That zener placement does not sound like it makes sense but that could be because you misinterpreted the FET's purpose or omitted to mention something else of importance.

author: Neilm
author_rank: Super Contributor
timestamp: October 27, 2015, 07:37:20 pm
content: Without the schematic (or a link) all we can do is guess. This sounds like a snubber network with the zener to clamp the voltage.

author: DanielS
author_rank: Frequent Contributor
timestamp: October 27, 2015, 08:54:56 pm
content: Quote from: Neilm on October 27, 2015, 07:37:20 pmThat is what I was thinking too, except a snubber would also have a capacitor to soften the turn-off, a diode to spare the FET from shorting the cap on turn-on and a resistor to discharge the capacitor.Just a resistor (and zener) from drain to ground does not make much sense.In a flyback circuit, the snubber would usually be across the transformer to clamp stray inductance.

author: ravipal180
author_rank: Newbie
timestamp: October 28, 2015, 04:35:16 am
content: Its a switching circuit they are using a dual gate p channel mosfet.i attached the image of it. NET L15 AND L16 load is to be connected. the load is led array ROW.The fet datasheet link is belowhttps://www.fairchildsemi.com/datasheets/ND/NDS9948.pdf

author: m12lrpv
author_rank: Regular Contributor
timestamp: October 28, 2015, 08:30:42 am
content: Seems pretty straightforward. Typical Zener voltage regulator that can be switched on and off. If there is anything interesting it would be somewhere in the rest of the schematic.

author: Neilm
author_rank: Super Contributor
timestamp: October 28, 2015, 07:29:53 pm
content: Quote from: DanielS on October 27, 2015, 08:54:56 pmMostly true - but there are some snubber networks that are to ground. These are not as efficient and need to be higher rated than the other sort. They were generally used when there weren't high voltage FETs around. Not in this case however - just a zener regulator that can be turned off.

author: DanielS
author_rank: Frequent Contributor
timestamp: October 29, 2015, 02:13:10 pm
content: Quote from: Neilm on October 28, 2015, 07:29:53 pmBut that was impossible to tell from OP's initial description.

author: ravipal180
author_rank: Newbie
timestamp: October 30, 2015, 09:31:04 am
content: Dear all,Thanks for your replies.in the schematic fet is used as a switch between 0v and 5v and the zener used here is 3v zener.My question is what happens when we dont use zener there, directly short the resistor to ground.Is there any problem in circuit when there is no zener.What is role of zener there??Am not using any inductive load here?load is led matrix.

author: bson
author_rank: Unknown
timestamp: October 30, 2015, 08:42:42 pm
content: The zener just references the circuit to something other than 0 and so moves the shut-off point up a bit. This fixes problems where the drivers don't completely shut off at some logic 0 that isn't quite 0V. In effect, it allows the gates to go below the sources to guarantee a complete shut-off.On a P ch the result is the same, except the gate doesn't have to be pulled all the way up to Vcc to shut off.