Al

 

Don't think it is a worry.   All can be on for a short time (possibly long enough to blow a fast acting fuse) but not long enough to heat a 16 ga wire significantly. 

Tracy

 

Tracy;

 

Well; the worry is this.  Safe wiring practice requires being able to protect the wire, especially if it is in a bundle or close proximity with other wires, against any failure which would overload the wire.  Running the numbers says that the maximum safe current for a 16 awg wire is 15 amps.  If I interpret you correctly; it is possible for my coil power lead to have to carry 27 amps (for 3 coils) for a short time, which would trip a breaker, or blow a fuse.  Under these conditions one can not properly protect the wire against a potential short – unless a much larger wire is used.  My wires run in a wiring duct, which isn’t exactly a bundle, but the analysis says for 27 amps in a bundle you need about a #10 wire.

 

Further; I went to great lengths to maintain the redundancy that you also strived for in the ECU with the dual controllers and the isolation switches.  I have the engine critical bus connected to the dual battery system, and isolated by 40 amp diodes.  I think you’re suggesting that it’s possible for the coils to momentarily draw 54 amps (6 x 9), plus pump and injectors; maybe up to nearly 70 amps.  Ain’t gonna work.

 

So what am I misunderstanding here, or how do we get around this?  Under what temporary condition can this high current exist?  Is there an operational sequence to avoid this possible condition?

 

Thanks,

 

Al

 

Oh darn, I knew this would happen. 

 

 The momentary 9A load on each coil only lasts in the range of 50 - 100 milliseconds under worst case conditions during start sequence.  There is no way to avoid it but FWIW, it just isn't a problem.   The 16ga wire would not have time to get even warm.   The specs you are discussing relate only to long term events and do not address short term transients.  

 

Tracy

 

Tracy;

 

I am in full agreement that the short spike is of no concern to the wire, even though I checked and find that I used #18 for the 3 coils; not #16.  It is a factor, not a problem, only in regard to circuit protection.  As I’ve said in another post, the wire needs to be protected against overload for reasons other than the spike.  And even in a broader sense, the spike is not an issue – IF you know it’s there and design the circuit and select components accordingly.  I designed based on the info that I had that the coils (all of them) drew 6-8 amps in normal operation.  So a #18 wire and 10-15 A circuit protection is fine.

 

If the spike really is only 50-100 ms then it still isn’t much of an issue, particularly for only 2 coils on the power lead.  But John’s 15A fuse blew with 2 coils; which raised this issue.  The average automotive 15A fuse will blow at 20A steady, 30A for 1 second, 70-80 A for 100 milliseconds.   (The typical Time-Current Curve can be seen at http://www.bussman.com/library/bifs/2009.)  There is, of course, a tolerance on the current at which they melt; but John’s fuse was either faulty, there was some other reason that it blew, or the spike was higher and longer duration than we think.

 

I checked the Schottky bus isolation diodes that I installed, and they are rated for 60A and should withstand up to about 400A for 100 ms, so that’s no problem.  I need to consider what change, if any, to make on the fuse selection; so I’d appreciate whatever info you have, if any more than you’ve already posted, on the startup spike; especially it’s possible duration.  I assume the 9A max is pretty firm based on the coil circuit design.  And I’m curious when it happens – when the EC2 power is on and coils are on, or after you start cranking?

 

BTW; in the argument (sorry, discussion) of fuses vs CBs, this is a case where a CB has the advantage.  They are slow acting, and will allow 10 times there rating for up to a second. (See http://tycoelectronics.com/product/circuitbreaker/w2331_DS.pdf ).  This is more like a slo-blo fuse.  I find that you can’t get a slo-blo in a blade type except in the ”maxi” version, and then the lowest rating is 20A.  You can get them in the glass tube type.

 

Al