# How Can You Narrow Possibilities When Measuring Inductor Back EMF?

• David lopez
In summary, the discussion revolves around measuring the back EMF produced by an inductor when a switch is opened. It is difficult to predict the voltage in this scenario, but using a semiconductor switch and a small capacitance can provide a more predictable peak voltage. The concept of the inductor equation V = L*dI/dt is also discussed, with examples of how the voltage can vary depending on the loading impedance. Ultimately, it is agreed that simulation is a more efficient way to predict circuit behavior in this situation.
David lopez
i am planning to measure the back emf produced by inductor when you open a switch. i know it is very hard to predict the voltage. but is there any
way to narrow the possibilities?

Try to simulate the circuit first with something like (free) LTSpice.

i am thinking i will leave the switch open for a short period of time. then i will close the switch. is the back emf easy to predict then?

David lopez said:
i am thinking i will leave the switch open for a short period of time. then i will close the switch. is the back emf easy to predict then?
Yeah, it's zero when you do that, so much easier.

For the case of opening a switch in series with an inductor, many things come into play. There is switch bounce (what kind of switch?), and if the voltage can rise high enough, you have to account for arcing.

A better approach is to use a semiconductor switch to eliminate bouncing, and use a small capacitance to give you a predictable peak voltage from the back EMF. That's how flyback transformer circuits for CRTs work, for example.

David lopez said:
i am planning to measure the back emf produced by inductor when you open a switch. i know it is very hard to predict the voltage. but is there any
way to narrow the possibilities?

In principle, current of inductor cannot be changed instantaneously.

The inductor equation V = L*dI/dt means that if the current changes suddenly, a very large voltage will be generated.

This means that after the switch is turned off, the inductor current will try hard to flow continuously at the same amplitude.

In short, let Ia be the current just before turning off the switch, then the instantaneous voltage of the inductor just after turning off the switch is equal to Ia multiplied by Za (loading Impedance).

Namely Va = Ia * Za, where Va is instantaneous voltage of the inductor just after the switch is turned off.

Prediction Examples : -
Va will be very large if Za is a very high resistance
Va will be extremely large if Za is another high inductance inductor with zero initial current
Va will be nearly zero if Za is a high capacitance capacitor with zero initial voltage
Va will be very difficult to be predicted if Za is a complex network which containing L, C , R and other power sources.
...etc

Of course, the long-term varying of the inductor voltage/current after the switch is turned off is another story.

All in all, in today's times, using simulation to predict circuit behavior is much easier than analysis.

Last edited:

## 1. What is back EMF and why is it important to measure?

Back EMF stands for back electromotive force, which is a voltage that is induced in an inductor when the current through it changes. It is important to measure back EMF because it can cause damage to electronic components and can also affect the performance of a circuit.

## 2. How can I measure back EMF caused by opening a switch in series with an inductor?

To measure back EMF caused by opening a switch in series with an inductor, you will need a voltmeter. Connect the voltmeter in parallel with the inductor and switch. When the switch is opened, the voltmeter will measure the back EMF voltage.

## 3. What is the purpose of adding a switch in series with an inductor?

The purpose of adding a switch in series with an inductor is to control the flow of current through the inductor. When the switch is closed, the inductor will allow current to flow through it. When the switch is opened, the inductor will produce a back EMF voltage.

## 4. How does the value of the inductance affect the magnitude of the back EMF voltage?

The value of the inductance directly affects the magnitude of the back EMF voltage. A higher inductance value will result in a higher back EMF voltage, while a lower inductance value will result in a lower back EMF voltage.

## 5. Are there any safety precautions I should take when measuring back EMF?

Yes, it is important to take safety precautions when measuring back EMF. Make sure to use a voltmeter with a high enough voltage rating and to wear protective gear such as gloves and safety glasses. Also, be sure to disconnect the power supply before opening the switch to avoid any potential hazards.

Replies
25
Views
4K
Replies
10
Views
1K
Replies
19
Views
1K
Replies
38
Views
4K
Replies
23
Views
3K
Replies
4
Views
1K
Replies
12
Views
2K
Replies
10
Views
1K
Replies
64
Views
5K
Replies
13
Views
2K