Gotcha.  I was thinking of this as two distinct circuits -- one where=20
the current charges the inductor and builds up the stored energy, and=20
then a separate circuit with the inductor having that energy and being=20
dissipated through the various diodes.  I was missing the connection=20
that the current would be the same at the moment the discharge starts. =20
All else falls into place now.

Much thanks,
-Neil.


On 10/20/2015 2:34 PM, Isaac Marino Bavaresco wrote:
> Neil,
>
>
> Remember, the BEMF voltage produced by the inductor has very little
> relationship with the voltage it was subjected before turn-off, and it
> has the opposite polarity.
>
> At turn-off, the 12V becomes irrelevant, just the 4A current is
> important. At turn off this current starts to drop exponentially, and
> the higher the voltage you allow to build up on the inductor, the faster
> the current drops.
>
> If you take Rz in account, I would say that the peak voltage on the
> inductor would be Vz + 4A * Rz =3D 33V + 4A * 10 Ohm, which gives 73V.
> The instant power over the Zener at turn off would be 73V * 4A =3D 292W,
> but just for a brief instant. Knowing the inductance it is possible to
> calculate the current graph.
>
>
> Cheers,
>
> Isaac
>
>
>
> Em 20/10/2015 15:51, Neil escreveu:
>> Hi Isaac,
>>
>> On 10/14/2015 5:34 AM, Isaac Marino Bavaresco wrote:
>>> Hi Neil!
>>>
>>>
>>> In this configuration the current stops early, as others pointed out,
>>> and is recirculated through the power supply
>>> I prefer a different method, that is adding the Zener diode in series
>>> ...
>> I've seen these configurations, and I understand the concept behind
>> those, but...
>>> ... Some calculations required.
>> This is what has me confused right currently... the calculations. And
>> specifically so I can determine the diode power rating.
>>
>> Let's say I charged a coil with 12V and up to 4A.  I can find the energy
>> stored in the coil as (L * I^2)/2.  So far, no problem.
>> Now let's say I shorted the coil (by itself)... when the stored energy
>> dissipates, is the current going to be 4A (because that was what was
>> used to charge the inductor)?  The coil resistance should also apply,
>> but for an example I have here, the coil resistance is <2 ohms, meaning
>> that if I just shorted the charged coil, would the current be up to 6A
>> (12V / 2 ohms)?  I don't think so as the discharge voltage can be much
>> higher, right?
>>
>> Let's take a basic zener-across-the transistor system... assume it's
>> also charged to 12V and up to 4A.  A sample zener datasheet (
>> http://www.mccsemi.com/up_pdf/SMBJ5338B-SMBJ5388B(SMB).pdf ) says that
>> for the 33V zener diode has a max dynamic impedance of 10 ohms, which
>> should apply when the zener is conducting at 33V.  Then I'd think the
>> max current is 33V / 10 ohms =3D 3.3A.  Actually with the coil in place,
>> it should be 33V / (10 + 2) =3D 2.75A.  Is this correct?
>>
>> FYI, all of this is building up to one major piece of confusion I have
>> over the LM1949 circuit -- they specify a zener across the transistor
>> switch or 33V, 5W.  33V at 2.75A =3D 91W!  Perhaps we can get away with
>> only a 5W zener because the time is so short?  Or is there something
>> else going on here?
>>
>> Cheers,
>> -Neil.
>>
>>
>>
>
>

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