jharvey wrote:Perhaps we can plan for high imp, but leave the traces bare. Then those that need low imp, can solder these specific traces giving them the current capability, while not gobbling real estate for the average Joe.
That's another interesting idea. I wonder though, will it act as a trap for those upgrading injectors and not realising that they need to thicken the board? I guess standard assembly instructions could be clear about it and urge users to do it anyway such that if they upgrade later they don't have an issue. Otherwise it might end up working much like your fuse trace idea :-)
jbelanger wrote:Low-Z injectors are usually much more than 1 Ohm. They are usually in the 2 to 3 Ohms range but there are some that are under 2 Ohms.
Sure, but such designs should be based on the toughest conditions imaginable unless they cause the design to become impractical. I was just going for worst case values :-)
Even then, you will not want to let the injector current go to 15A (or whatever the current would be from battery voltage divided by the injector impedance). And the current will not instantly go to this maximum value even if you apply the full battery voltage: it will rise with a certain slope depending on the inductance. Also, you probably don't want to let the current go to the maximum value because it will heat up the injector uselessly since the opening will occur at a lower current.
OK, but where are some of these specs? I'd like to see the specs for the ones that are close to 1 Ohm. We don't want the situation where the driver is configured for 4 Amp peak and the injector needs 6 or 10 etc. Obviously this is on a per injector basis and probably best determined with a dual scope showing current and the output from a transducer on the body of the injector.
For example, if an injector needs 6.5 A to open ASAP and we feed it 4 A then the opening point will move from pale blue to dark blue. This is bad. It's not so much the reduction in current that matters, but rather the shortened time of higher current. If you pull the feed to it prematurely then it could become very dozy to open or intermittently not open.
Do we have any empirical evidence of current vs opening time curves?
As for the heat generated, assuming 15V and 2 Ohm low-Z injectors being held at 1A, by simplifying thing you can see that with 15V you need 15 Ohms to have a 1A current. Since the injector is 2 Ohms that means the BJT has the equivalent of 13 Ohms which means that the power dissipated is 13W (Power = RxIxI = 13x1x1) per injector. For 6 injectors, you have 78W which requires a good heatsink and air flow.
How much heat would be generated in a PWM setup?
However, this is really a worse case scenario because you have 100% duty cycle: in a real application not only will you have a lower duty cycle but you will also have the peak phase which will generate very little heat because the BJT will be fully saturated.
It may be a worst case scenario, but it is definitely a real world scenario for some vehicles that are setup marginally (like mine at the moment). Once I get a decent set of ratios behind my engine it will be shifting from 7500 back to 5000 with 1000/2500 = 40% of the time spent at 100% (and NO opening period of low dissipation) and the rest spent close to 100%. In that case the worst case is realised. We do stuff around with cars so that we can drive them fast afterall, and driving fast = high duty :-)
So from this, I think it is logical to leave this option for an external box. Not only are the board space requirements somewhat higher but the heat management is significantly more demanding. However, this might not be an issue if an enclosure is chosen with this in mind.
I'd tend to agree unless we can use those chips in PWM mode to keep the heat manageable. If we can, then I think it's a goer (as it will buffer the CPU and provide good switch on current to the FET too (24mA they say versus the 4mA or so that a CPU pin can safely push with a full 5v differential though a 1k resistor or so.
Another member PMed me to say he's working on a solution for this, so we'll see what he comes up with. Perhaps it will be suitable, perhaps not. I told him you are the P&H MAN so he should allow your all-knowing eyes to be cast over it and pass judgment :-)
Time to fix those smilies!
Fred.