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Doubt with this circuit - Back EMF

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Sparky_s

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I have found this circuit

**broken link removed**

Basically it's a circuit to use the colapsing magnetic field to lit the light bulb. A 555 timer generates pulses and the lit bulb is lighted using only the colapsing magnetic field energy, while the coil is not conected to the battery (this is my understanding)

I have some doubts:
1) The light bulb is being lighted exclusively with the colapsing field? or its being lighted with the energy from the colapsing field and the energy of the electromagnet when the electromagnet is energized with the battery?
 
Please correct if wrong:



the MOSFET is sinking current. because of the diode current cannot flow through the bulb, only the inductor. when the mosfet turns off the inductor creates a back EMF opposite the polarity the was flowing through it, which IS in the right direction to pass through the diode and thus the bulb, unlike the current through the mosfet. so yes, only the back emf is powering the bulb i believe, because the diode would prevent this.


note that this might not work if the inductor is to small or the bulb is too big.
 
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Thank you very much,
Now I see more clearly how the circuit works. In the forward case (when the coil is being energized by the battery), no current can flow to the light bulb. When the coil is de-energized (the battery is not conected to it), current can flow because there is back emf and the opposite polarity can permit the flow of the electrical current because Back-Emf is in the good polarity but forward energy is in the wrong polarity for that diode.

Yes, there is a technique called Impedance Matching that people usually use to make the circuit works.
Coil Impedance and Bulb Impedance are the same (correct me if I'm wrong).

How can I calculate the Bulb's impedance?
 
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i think what matters is the Resistance/Impedence(they are not the same thing, Impedence is resistance to AC and often varies by frequency.) of the bulb but the Inductance of the coil.(and pulse frequency)

also the frequency should affect it, as that will be the number of current pulses per second and more pulses = more power transferred.

I'm not sure impedence matching is the right word as there are multiple factors here, and some of them have nothing to do with impedance. and since current is only flowing through the bulb one way i think it would be called resistance although the words are used interchangeably even though i don't think its correct.

however impedence does seem to be the common word for even DC measurement insturments.
i.e a 10 megaohm voltmeter would draw as much current from the circuit under test as a 10 megaohm resistor to ground.

so i'm really not sure about the semantics of the whole thing.

by the way,does the site you got that specify a value for the inductor?
 
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I said Impedance Matching because I have heard it sometimes, while reading about similar circuits.
As far I know, Back-Emf could be similar to a High-Voltage High-Frequency Spike (I don't know if that is correct).

No, I don't have a value for the inductor. I asked the author but he didn't reply. But I know the more turns the coil has, more back emf will be generated. Of course, there is also other values, but the number of turns is one of them.
 
Yes back-emf is a high voltage high frequency(in that it only last for a short pulse) spike.
The voltage and width of the pulse depend upon the impedance of the load, the inductance of the coil and the frequency of the pulses. It could be a continuous sawtooth current through the coil, depending upon the relative value of these parameters.

If the load is 12V, the what's the purpose of the coil? If you just want to vary the current through the load, you can apply the PWM signal directly to the load without the inductor and diode with better efficiency.
 
The posted circuit is similar to a switching mode power supply, except that it is missing a filter capacitor across the load (lamp). Stare at the attachment which is sort-of your circuit, with the added filter, and you will see the relevant currents and voltages....
 

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