# Hef on a Cen-tech multimeter

#### dl324

Joined Mar 30, 2015
12,245
When the transistor opens, the inductance will attempt to keep the current moving in the same direction (from top to bottom of the inductor).
This causes the voltage to drop at the transistor inductor interface until the voltage goes about 0.7V negative, which forward biases the diode.
If everyone would have just kept repeating
$$\small V_{emf}=-L\frac{di}{dt}$$
It probably would have sunk in faster (after I did some research to convince myself it was believable). Thanks for the help.

#### dl324

Joined Mar 30, 2015
12,245
Examine the, "several ways to say the same thing" and work with the one that seems to make the best sense to you.
It makes sense to me now... Thanks.

#### WBahn

Joined Mar 31, 2012
26,398
I found a physics lecture from Rutgers that seems to have cleared it up for me.
View attachment 93727
The formula to calculate back EMF has a negative sign because the current is decreasing:
$$\small V_{emf}=-L\frac{di}{dt}$$

I must have been distracted when the negative sign was introduced for the collapsing field case.

Now having or not having ground or earth ground is immaterial because the inductor generated a negative voltage so KVL is obeyed.
View attachment 93728

Did anyone else learn something today? Be truthful
But if you introduce a negative side, and if the current is decreasing, then you have two negatives that multiply out to a positive. Remember, the derivative itself is negative.

#### dl324

Joined Mar 30, 2015
12,245
But if you introduce a negative side, and if the current is decreasing, then you have two negatives that multiply out to a positive. Remember, the derivative itself is negative.
You're right. It'll make it clearer to anyone trying to follow this thread.
$$\small V_{emf}=L\frac{di}{dt}$$

#### #12

Joined Nov 30, 2010
18,216
clearer to anyone
It depends on the audience. I don't speak calculus and I can see that my flow model approach doesn't work for you, but a major strength of this website is that when several people explain the same thing, one of them will almost always find the right approach for whomever is questioning at the moment.

Congratulations on removing some of the magic. When your mental model is correct and you can quantify the results, you have the tools you need!