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From:	"Luis de Arquer (ldearquer AT gmail DOT com) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
Date:	Tue, 27 Nov 2018 16:54:38 +0100
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Subject:	Re: [geda-user] [off-topic] 24V automotive (truck) design issues
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Hi Richard,

On car, we normally use a TVS with a high enough value so that
supressed load dump doesn't affect it, yet the other pulses can be
drained through -so 36V to 40V. 33V TVS showed to burn on load dumps.

Anyway, let us know what you find out! I am curious to see why it failed :)

Luis

2018-11-27 11:59 GMT+01:00, Richard Rasker (rasker AT linetec DOT nl) [via
geda-user AT delorie DOT com] <geda-user AT delorie DOT com>:
> Hi Luis,
>
> Op 26-11-18 om 23:06 schreef Luis de Arquer (ldearquer AT gmail DOT com) [via
> geda-user AT delorie DOT com]:
>> Hi Richard,
>>
>> There are quite a few spikes that can damage electronics on the
>> truck. You can find most of them on ISO 7637 (second part has the
>> pulses definition).
>
> Thanks, I, er 'found' a copy of this, and the worst I see, are
> negative-going 1ms-pulses to -600 volts @ 50 ohms for 24V systems.
>
> Then again, the TVS diode I used (SMAJ33CA) should handle this just fine.
>
>> The worst I can remember for car is -150V for 2ms
>> approx, surely more on trucks. Positive pulses of over 100V happen
>> often when parallel inductive loads are disconnected (such as windows,
>> etc), but this are normally under 100us.
>>
>> Load dump is fairly unusual though, since that requires the battery to
>> be removed from the truck while alternator is running. Its strength
>> depends on whether the truck has load dump supressor or not (cars now
>> normally have it, but I don't know trucks...). With load suppresor,
>> you may see about 58V for up to 300-400ms. It is currently defined in
>> ISO 16750-2 (2012 or later).
>
> For 24V systems, I see load dump peaks up to 200 volts for 350 ms at
> only a few ohms -- this would definitely fry the TVS diode I used and
> everything behind it. I guess I'll have to employ either a far more
> rugged TVS diode or an overvoltage disconnect circuit to handle this.
>
> But as you say, this occurrence (battery disconnect with running
> alternator) should happen only on rare occasions, so it doesn't explain
> two failures within a week.
>
> AFAICS, all the other pulses should be no real problem for the TVS diode
> (I used this simple setup several times with normal 12V cars, and never
> had any trouble).
>
>> This may help:
>> - Try to get the boards that failed for analysis. Did the TVS fail?
> I'll see tomorrow, when the boards arrive.
>> - Revise all input capacitors voltage ratings
>> - It can help having a high voltage series diode at the input such as
>> BAV21W, followed by a tanky 220uF or higher capacitor: This should get
>> rid of most pulses, except load dump.
>
> Unfortunately, there is no room on the PCB itself for a hefty capacitor
> on the input (as it would need to have a working voltage of 200 volts
> for load dump handling), but I don't think that's necessary in case of a
> disconnect solution. It's a non-essential gadget, and it's no problem if
> it cuts out for a fraction of a second on rare occasions.
>
>> - For load dump, you really need to know if there is a supressor or
>> not. The difference is huge.
>
> I think that I simply will have to take the worst case scenario. These
> people have no (haha) truck with conditional reliability; things should
> 'just work, period'.
>
> Anyway, thanks for your elaborate reply!
>
> Best regards,
>
> Richard Rasker
>

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