# Flyback diodes and why you need them

**URL:** <https://forum.arduino.cc/t/flyback-diodes-and-why-you-need-them/675409>\
**Category:** General Electronics\
**Created:** [September 21, 2020, 7:39pm UTC](https://forum.arduino.cc/t/flyback-diodes-and-why-you-need-them/675409 "2020-09-21T19:39:56Z")\
**Posts on this page:** 1\
**Showing post:** 6

<div class="post-metadata">

**Author:** ![PerryBebbington](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/perrybebbington/32/238766_2.png) [@PerryBebbington](https://forum.arduino.cc/u/PerryBebbington)\
**Post date:** [January 28, 2023, 10:37am UTC](https://forum.arduino.cc/t/flyback-diodes-and-why-you-need-them/675409/6 "2023-01-28T10:37:11Z")

</div>

Occasionally I see comments in the forum suggesting that because some MOSFETs have a body diode between source and drain there is no need to put a diode across the inductive load they are driving. This schematic illustrates the situation, with the switch taking the place of the MOSFET and the 1N4004 taking the place of the body diode.

 ![Schematic with diode across switch](https://europe1.discourse-cdn.com/arduino/original/4X/f/d/e/fdefabc3bc0bfc5b1a86233e52559dc77d02d75c.jpeg)

Here is the result of opening the switch, equivalent to turning off the MOSFET quickly.

 ![Diode across switch](https://europe1.discourse-cdn.com/arduino/original/4X/5/b/0/5b0da498985a5cb14e87566510af81ea79304cc7.jpeg)

Note  
The trace is not exactly comparable with previous traces as I was not able to find the exact same inductor I used before, I just don't know for sure what I used, other than it was a winding of a mains transformer. However, the voltage spike clearly goes well below -200V.

---

_[View the full topic](https://forum.arduino.cc/t/flyback-diodes-and-why-you-need-them/675409)._