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Subject: Re: [geda-user] [off-topic] 24V automotive (truck) design issues
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From: "Richard Rasker (rasker AT linetec DOT nl) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
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Date: Tue, 27 Nov 2018 11:59:10 +0100
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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