Automotive load dump..........design led beacon for it?

[treez]

I worked in a led beacon company who made led beacons for running off 12V car batteries.

The beacons had to run off any voltage between 10 to 100v, because the customers "thought" that load dump could give a battery rail of ~90V for many seconds..........and because the customers thought this, we had to design the beacons to be able to run off 100V permanently.

Were we stupid to do this?

Shouldn't we have just done it for , say 10 to 18v, and left it at that....surely we would have gotten away with it?

We all know that automotive load dump is a myth dont we?

I mean, a 12v car battery isnt going to have 100V across it.

And who in their right mind is going to yank off the battery terminals when the alternator is running at maximum?

OK granted if the battery is dead, but how is a car with a dead battery ever going to start?


So, were we stupid to go to the expense of designing beacons which ran off 10 to 100v?

 

[betwixt]

Answer - probably yes but you will always find some idiot who decides to run them on a 48V aircraft and disconnects the battery in mid-flight!

After far too many years designing things, my advice is you can make things fool-proof but never idiot proof, someone somewhere will always prove me right. :lol:

In a World where common sense prevailed, you would be absolutely right and there is no need for such unreasonable measures.

Brian.

 

[treez]

Thanks, i see what you mean......but in the name of saving a few bucks, can we get away with just designing for 10 to 18V?

....i mean, its much more expensive doing it for 10-100v.

.....And those idiots are going to be few i hope?

 

[betwixt]

I think the usual design limit is 35V maximum but it may vary between vehicle manufacturers. 18V is too low, it isn't the battery voltage that causes the damage, it's spikes of higher voltage caused by relays and charge controllers kicking in and out. The spikes build up in the inductance of the wiring loom and may not be seen at the battery itself. They may only last uS but that's a long as it takes to do damage. If you use sensible interference supression at the power terminals you should be OK to design for 35V. By sensible supression I mean an LC filter and fast acting clamp, TVS or something similar.

Brian.

 

[treez]

thanks, though i dislike TVS's, because if i use , say, a 40V TVS, then if a long lasting >40V transient ever does happen, then the TVS will blow up.... though i suppose a long lasting >40V transeient is unlikely?

(though according to our customers, (who i dont think know what they are talking about but 'the customer is always right etc etc'), these long transients are common.

 

[betwixt]

I doubt the customers are right this time. Do bear in mnd that if you make a 24V version for larger vehicles you have to double all the safety limits as well.

As an alternative to TVS you could look at an automatic cut-off circuit that disconnects your device if the voltage goes over safe limits and reconnects it when the surge is over. Given the nature of beacons, if one didn't flash for a few mS longer than expected it probably wouldn't be noticed. You would have to divulge your designs to get more advice on this method.

Brian.