Automotive Circuit Protection - Part 3

[Bernd Felsche]

Garfield Willis tapped away at the keyboard with:

> These two issues are the first we'll combine together in dealing with
> how to PLAN and SIZE protection for your precious DIY goodies. ...

[snip]

In the example of the load dump, I've read that the energy in the
initial pulse is of the order of 1mJ and that the energy in the
remaining transient is approximately 100 times greater; though with
a voltage decaying from around 40V over approximately 2ms. Is this
correct or are the numbers wrong? If it's wrong, what are
appropriate numbers in terms of energy or voltages and duration?

Some rough calculations indicate that a 100uF capacitor (a size
which is not infrequently-used at the input of a 3-terminal
regulator) starting at 14V would absorb about one third (33.8mJ) of
that energy in rising to 40V. If the amount of energy is correct
(and I haven't messed up the calculation) then that would indicate
that the capacitor alone is sufficient to absorb the bulk of the
damaging transient energy from the load dump. i.e. the initial
high-voltage pulse and the start of the decaying waveform.

What have I missed? Is the semi-conductor device going to be faster
in limiting the maximum voltage? Would a small series inductor not
suffice in presenting a very high impedance to the short voltage
pulse?

How is the TVS better? Their rated energy capacity is greater; of
the order of several joules, but if that capacity isn't required?

-- 
Bernd Felsche - Innovative Reckoning
Perth, Western Australia
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