# Back EMF diagram

#### bloguetronica

Joined Apr 27, 2007
1,541
Well, it should make no difference since the original circuit would not work in practice either (only works in theory, and that's the problem). Considering that the emf is generated according to the load "shorting" the inductor, it would be natural, for a load with relatively low resistance, that the inductor would generate less back emf (an inductor will generate a costant current pulse, with the same intensity of current that crossed if before). This factor and the fact that the bulb filament needs time to heat, you would never see the back emf, because the pulse generated is too short. Although, if you replace the bulb by a digital voltmeter and a zener diode in paralel (so the voltage across the voltmeter won't exceed a value), the story might be different. You could also try a white LED (they are more sensitive to pulses than the ordinary ones), if you are willing to waste one.

To see the circuit working, you might need a much bigger inductor, or a bulb with much less power consuption (or higher resistance), or both. Certainly you'll have to generate a back emf pulse much higher than 6V.

#### thingmaker3

Joined May 16, 2005
5,083
Harvard is one of the most over-rated schools in America, unless one wants to be a physician or lawyer.

Let's do the math. (Physicians and lawyers are not required to do math as often as engineers.)

We are not given a power rating for the 6 volt bulb. Let's assume it is rated at 10 watts. (E^2)/P=R, making lamp resistance (6^2)/10 = 3.6 ohms. TC for the inductor and lamp is 3.6 x 0.012 = .043 seconds. You might notice (unless you are a physician or lawyer) that the formula for time constant is independent of EMF - picovolts work the same as gigavolts here. As Cumesoftware noted, 0.043 seconds is not enough time for the filament to begin glowing. Larger wattage bulbs will have even shorter TCs, also longer heat-up times.

Try a flash-bub instead. They are specifically designed for quick light-up at higher voltages.

#### thingmaker3

Joined May 16, 2005
5,083
Oh, and keep a close watch on your battery temperature. 12 milihenries or 12 megahenries - its all zero ohms at f = 0.

#### boco

Joined Jul 5, 2007
7
Thank you Cumesoftware & Thingmaker!

I have used splice blocks for my connections versus soldering. Do you think such connections will reduce the back emf as well?

#### thingmaker3

Joined May 16, 2005
5,083
Splice blocks work just fine if used correctly. Get the conductors all the way under the screws and tighten the screws snugly. Resistance should be every bit as negligible as with soldering.

#### boco

Joined Jul 5, 2007
7
I was also hoping to build a computer model which could forecast the back emf produced by this modified circuit. As a total rookie (its been 25 years since I tinkered with this) I don't know how to start. I reviewed the formulas in Vol I DC book but still cannot quite make a formulaic connection.

Can someone recommend a source for developing such a model?

#### recca02

Joined Apr 2, 2007
1,212
i think there are quite many applications out there that might help u in your objective.
one way i think wud be using matlab .