How to detect an inductor's back EMF?

David lopez

how do i detect an inductor's back emf on a breadboard? explain in detail. i am planning to connect an inductor to a switch. i have read when you
open a switch, the inductor creates a back emf. how do i detect this back emf, when the switch is opened? explain in detail.

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BvU

Homework Helper
With something that detects emf , e.g. an oscilloscope, or a voltmeter (if the inductance is big enough).
Or visually: if the inductance and the current are big enough you could draw a fat spark. (probably not the case with a breadbord setup ...)

Post a circuit diagram - perhaps then it becomes clear what kind of detail makes you happy.
And: what equipment do you have ?

Are you aware of the theoretical behaviour ($V_{induced} = L {dI\over dt}$ ) ?

Nik_2213

Given you'd usually fit a hefty inductor with parallel components such as a R-C pair or reversed rectifier diode to quench the 'Back EMF', leaving them off allows you to measuring the voltage 'kick' across the switch.

IIRC, I've flashed neon pilot-lights thus from just a few Volts.
And, it is the basis for a spark-gap transmitter, now something to be avoided...

As the 'kick' may exceed 100 Volts, handle with due care and use appropriate measuring instruments.

Also, consider the possible damage to those switch contacts...

jrmichler

When I was in high school, I had a 10,000 turn air core coil from a failed attempt to wind a transformer for a home made bug zapper. I found that the coil would give me a zap from disconnecting a single D cell battery, so I took it to school and connected an oscilloscope. The 'kick' from a single D cell battery was 1600 volts.

Simple rule: All switched inductors need a bypass diode. That especially includes relay coils, as I pointed out to a client recently when they wondered why their remote control system kept failing.

BvU

David lopez

this is the circuit i am planning to use

David lopez

do you need an oscilloscope to measure the back emf? can that handle high voltage?

BvU

Homework Helper
this is the circuit i am planning to use
That isn't very instructve without
• Inductance and resistance of your inductor
• Internal resistance of your battery (three AA cells in series ?)
Once you have those you can predict I(t) when the button is pressed.
V(t) when the button is released is harder to predict

do you need an oscilloscope to measure the back emf? can that handle high voltage?
I don't see how you can do without
You can adjust the sensitivity or initially use an extra voltage divider just to be sure

David lopez

so if you know the internal resistance of battery and resistance of inductor and inductance of inductor, you can
predict induced voltage?

NascentOxygen

Mentor
so if you know the internal resistance of battery and resistance of inductor and inductance of inductor, you can
predict induced voltage?
No, unfortunately. While the circuit may appear to be a first-order network (comprising R and L), it also involves capacitance of the inductor windings plus stray capacitance and this changes it into an oscillatory second-order system of unknown parameters but of high impedance. When you connect the oscilloscope probe this adds further capacitance and loading, changing the system from what it was before you connected the oscilloscope. So even though the oscilloscope will display a voltage spike, it is generally going to be nothing like what the spike was before you connected the CRO probe. By this I mean that suppose the oscilloscope shows a spike of, let's imagine, 80V, then without the probe's extra loading that spike may manage to overshoot many times higher, for illustration let's say 300V.

anorlunda

Mentor
Gold Member
this is the circuit i am planning to use
That circuit is a crude representation of an old-fashioned non-electronic car ignition circuit.

The car has a battery, a coil, a switch in the distributor, and a spark plug. It generates spikes of about 20000 volts. In your circuit, the switch and the spark plug are the same thing, and you could generate a spark in the switch.

The picture below shows what you might see on an oscilloscope including the capacitance effects that @NascentOxygen mentioned. Note the total duration of the event is about 1 millisecond.

Source: https://www.denso.com/global/en/products-and-services/automotive-service-parts-and-accessories/plug/basic/spark/

David lopez

so there is no way to predict the back emf?

hutchphd

As has been said before: If you know the internal R of the battery and the inductance of the coil, the "back" emf can then be calculated subsequent to when you close the switch. Get a book on inductance.
When you open the switch, internal arcs in the switch make the response not reliably repeatable and therefore not easy to calculate.

sophiecentaur

Gold Member
so there is no way to predict the back emf?
OK as long as you don't allow a spark to form (for a low enough battery emf). When the non linearity kicks in that makes life harder. If you can characterise the circuit parasitics (including the switch) well enough then you're just calculating the linear step response of a circuit.

hutchphd

But the inductive response to the step current going to zero gives you an infinite voltage!! I think the result will be driven by only exact shape of resistance of the switch contact at that time. You could assign a finite time to the triangular "step" but that is a pretty large kluge. Any simple method is likely to be wrong by factors of 2 or 3 or maybe 10.
My point was that the "on" switch was very amenable to "exact" analysis while the "off" is much less so! Putting a capacitor across the switch to assure no arc will bring it back to the linear realm for both circumstances.

sophiecentaur

Gold Member
But the inductive response to the step current going to zero gives you an infinite voltage!!
It's a step voltage change from a constant voltage source. There will be a parasitic Capacitance in parallel with the battery. High induced voltage but not infinite.

hutchphd

It's a step voltage change from a constant voltage source. There will be a parasitic Capacitance in parallel with the battery. High induced voltage but not infinite.
Sorry but I don't see it. The idealized step function is actually an instantaneous step in resistance of the ideal switch from 0 to very very large. The current in the loop goes to zero regardless of the capacitance of the battery. Only if the capacitor is across the switch will it help. Am I missing this?

sophiecentaur

Gold Member
Only if the capacitor is across the switch will it help
And there will be a parasitic capacitance just there. I think you are right to propose a model of a step change in current and the presence of a Capacitance across the switch will take care of your problem with an infinite change. The Capacitance will be, as a minimum, the capacitance between the cables to the switch and each end ov the inductor. Say a few pF. But there will be the self capacitance between the windings of the Inductor. An inductor like that could have a self resonance of several or several tens of MHz.

David lopez

so there is no way to calculate back emf in advance?

sophiecentaur

Gold Member
so there is no way to calculate back emf in advance?
It’s the same for all circuits. You need a good estimate of all relevant circuit component values if you want a good prediction. In this case, it’s an RF problem so it’s harder

David lopez

so it is hard to get good estimates of the relevant circuit component values? if you had good estimates of the
relevant circuit component values you could predict the back emf in advance?

anorlunda

Mentor
Gold Member
so it is hard to get good estimates of the relevant circuit component values? if you had good estimates of the
relevant circuit component values you could predict the back emf in advance?
Yes, as in any circuit.

However, the answer is not a single value, it is a function of V versus time. If you plotted the curve, it would look something like that in #10. You could spot the peak voltage from that.

The simple circuit applies until the moment a spark is ignited (if ever). During the spark, models are very difficult.

David lopez

then i will settle for narrowing the possibilities. what are the relevant parameters for estimating back emf?

anorlunda

Mentor
Gold Member
Have you studied basic electricity? Ohm's Law. Kirchoff's voltage and current laws?

Tom.G

If it is really the back EMF you are after then you really do need to find the relevant circuit parameters, the inductance of the coil and the capacitance of both the coil and the stray capacitance of the wiring, etc.

If you just want to suppress the back emf enough to not damage the switch, then here is a starting point Rule-of-Thumb:
Wire a 0.1μF capacitor in series with a 100Ω resistor, then wire the assembly across the coil. The capacitor should have a noticeably higher voltage rating than the battery!

That configuration gives the most effective voltage suppression. The drawback is when/if the capacitor fails as a short, the supply voltage appears across the 100Ω resistor when the switch closes.

Another configuration wires the resistor-capacitor across the switch contacts. When/if the capacitor fails shorted, the load (coil) remains partially powered even with the switch "Off."

Cheers,
Tom

sophiecentaur

Gold Member
The old mechanical contact breaker style of ignition used a 2.2μF 'condenser' and the coil took some time to develop (more experiment than calculation, I bet), apparently. Iirc, the core design, before ferrite was used for such things, consisted of a bundle of iron wires. This worked better than other choices (probably cheap too). In a car ignition system the important thing is Spark Energy as long as there were sufficient volts. Most of what I read in my search for the value of the Condenser simply stated that the C was there to 'damp' the arc across the contacts. There was more to it than that. The 2.2μF capacitor resonated with the coil inductance and was best suited to the spark plug gap and the pressure in the combustion chamber. You can still get them, of course on eBay, for a very few quid.

"How to detect an inductor's back EMF?"

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