Inductive load to a transistor switch

[Manchested]

When a bjt switch have an inductive load connected to Emitter, is it necessary to have a diode in reversed biased between the CE junction, or else the back voltage will blow up the bjt?

What is the difference to have the diode between CE and to have it in parallel with the inductive load?
do they have the same function? and do I need a diode for both or either one is actually enough?

 

[alexxx]

Check out the following links. The second one is about relays, but you will get the general idea.

Flyback diode - Wikipedia, the free encyclopedia
https://www.physics.unlv.edu/~bill/PHYS483/relay.pdf


Alex

 

[goldsmith]

Hey buddy!
Hello.
We have a famous law that called ''LENZ'' law. In inductors we have a formula : VL= L*(di/dt). My mean is when your inductor has current , inductor store energy E=1/2*(L*I*I). when you going to open it's current way , Inductor wants to oppose from your work and crate A high voltage in reverse polarity that called Lenz Effect. when you have an inductor In emitter of transistor you should use a diode in paralle with inductor but in reverse bias for protect from your transistor.

 

[Manchested]

Hello,

So what could be the purpose of having those diodes? are they also called the flyback diodes?

https://ww1.microchip.com/downloads/en/AppNotes/00885a.pdf

on page 11

 

[goldsmith]

Hi again.
yes that diode called fly back diode or eliminator diode. we use this diode because of that , when current of an inductor going down the polarity of voltage during inductor going reverse and this diode going forward bias and going to discharge the storage energy of inductor.

Regards
Goldsmith

 

[zuisti]

Hi Manchested;

Simple - not academic - a response to your original questions:

When a bjt switch have an inductive load connected to Emitter, is it necessary to have a diode in reversed biased between the CE junction, or else the back voltage will blow up the bjt?

No, the CE diode doesn't necessary if you have the other ...

What is the difference to have the diode between CE and to have it in parallel with the inductive load?

Only the second (parallel to the relay) diode is needed, the other is redundant.

do they have the same function? and do I need a diode for both or either one is actually enough?

See above : - )