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=20
> those, but...
>> ... Some calculations required.
> This is what has me confused right currently... the calculations. And=20
> 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=
=20
> 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=20
> dissipates, is the current going to be 4A (because that was what was=20
> used to charge the inductor)?  The coil resistance should also apply,=20
> but for an example I have here, the coil resistance is <2 ohms, meaning=20
> that if I just shorted the charged coil, would the current be up to 6A=20
> (12V / 2 ohms)?  I don't think so as the discharge voltage can be much=20
> higher, right?
>
> Let's take a basic zener-across-the transistor system... assume it's=20
> also charged to 12V and up to 4A.  A sample zener datasheet (=20
> http://www.mccsemi.com/up_pdf/SMBJ5338B-SMBJ5388B(SMB).pdf ) says that=20
> for the 33V zener diode has a max dynamic impedance of 10 ohms, which=20
> should apply when the zener is conducting at 33V.  Then I'd think the=20
> max current is 33V / 10 ohms =3D 3.3A.  Actually with the coil in place,=
=20
> 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=20
> over the LM1949 circuit -- they specify a zener across the transistor=20
> switch or 33V, 5W.  33V at 2.75A =3D 91W!  Perhaps we can get away with=20
> only a 5W zener because the time is so short?  Or is there something=20
> else going on here?
>
> Cheers,
> -Neil.
>
>
>



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