Blown Injector FETs

[rob@vems.co.uk]

http://ourworld.compuserve.com/homepages/Bill_Bowden/ohmslaw.htm
Voltage (E) = Current (I) * Resistance (R)
Power (watts) = Current Squared (I^2) * Resistance (R)
Power = I*E = E^2 / R

So each fuse has to be 1amp.
12v / 16ohm injector = 0.75amp

The 12v to the injectors would be
0.75 * 4 = 3amp, I´d go for 5amp fuse

True - but bare in mind that voltage is closer to 14v
14 / 16ohm = 0.875A * 4 = 3.5A
But you will find that your duty cycle is a factor too @ 80% Duty
3.5 * 0.8 = 2.8A
If you duty cycle is less than 80% then you can use a smaller fuse yet.

[GintsK]

Jörgen, how physically FET driver can damage FET?
Yes it can open FET forever, when damaged. But how it can be a cause?

When I had fried FETs, gate resistors was fried too. It works like fuse protecting driver chip. Check resistance!

Gints

[Sprocket]

I do some labor work regarding injector settings.
PWMing can't kill your FETs. It is voltage dependant. But PWM% can only rise from configurated, when voltage falls. When voltage rises PWM% stays as configurated. 100% in this case. 14V seems is breakpoint.

But surprise!!! injrampup_battfac 4080 did not disable correction. It is maximal possible correction in fact!!
works in very strange way. It describe some kind of coefficient for inj_eff_rampup time. No influence @7V
If inj_eff_rampup time is configurated as 1500, it adds 1000us @7V. But injrampup_battfac scale this value @13V by aprox. :
4080 - 0.78
2000 - 0.89
500 -   0.97
0    -   1.0

So if injrampup_battfac is 4080 and inj_eff_rampup time is configurated as 1500 we get 1000*0.78=780us adder to PW @13V.
Very confusing and user unfriendly! Why, why

Both other settings are more straightforward:
Inj_open_time @13.2V is simple adder at any voltage. 1:1. 500us=500us
Battfact is PW added @7V It scales down to 0us @13.2V.

Sorry about offtopic! Rob, can you move it to appropriate place?

Gints

While Im waiting on the band aid turning up in the post, I will explain what I did with the injector settings.

As there is a general lack of explanation, data, and a starting point, I went about working out a theoretical good starting point for the injectors I am using. Now considering the VE table and EGOC will cater for much of the error.

I have some data on the injectors I am using. This was as a result of a fellow Mini enthusiast. He tested the injectors flow rates at various pulse widths using a signal generator. Starting with very low values and working up to a value that gave repeated flow results. 0.9ms was found to give the best repeatable results and as such could be considered the effective ramp up time. Basicaly the injector pintle has fully opened and at the point it fully opened the current was dropped, less than 0.9ms the pintle never managed to reach full open, and so the flow characteristics were inconsistent due to the pintle not opening the same amount each pulse. Effective ramp up time being the total time it takes from when current is applied to where the injector is full open. Open time is the time it takes from when current is applied to when the injector starts to lift the pintle off its seat, and ramp up time is the time it takes for the injector to go from full closed to full open excluding the initial open time. so, to recap, effective ramp up time is the sum of open time and ramp up time.

The flow rate of the injector at this minimum pulse width is a sum of the ramp up and ramp down flow rate, and the flow will obviously vary depending on the size injectors. This is why big injectors can have bad idle characteristics, when you start to get close to the minimum pulse width, as they flow more fuel than the engine needs at idle. I still think that correct sizing of injectors gets overlooked, and why injector staging is a desireble option. The injectors I have chosen, seem to give a pulse width of around 5ms at ilde, and have been calculated to be more than big enough for the expected output of the engine.

Not knowing the injector open time, I made an asumption of 500us and set the effective ramp up to 1500. then using the injrampup battfac scaling factor of (4080) 0.78, calculated the effective ramp up at 13v, this gave 1170us. Too high for the data I have. I then reduced both the open time and the effective ramp up time by 50us in each untill the calculation gave a close enough value to the 0.9ms (900us) the results I have are bellow.



I am a little unsure of the battfac, as the name suggests its a factor, much like the injrampup battfac, but why units of us if its a factor. Is battfac a time value or a factor? For now I have set battfac as if it were a time value, and set it similar to the scaled effective ramp up time (1200x0.78=936us) minus the injector open time (352us) giving circa 600ms for the battfac value.

Now that may well be total rhubarb, but it looks and sounds feasable.

at 7v effective open time is 1800ms (effective ramp up + battfac) (note twice the scaled effective ramp up, coincidence?)

At 13v effective open time is 936 ( effective ramp up x Injrampup battfac)

So with voltage between 7 and 13v the values are scaled acordingly
This by no means an acurate way to set these, but due to the lack of a idiots guide on how to set this up, I applied some reasonable thought to get somewhere close, allowing some small error.

What do you think

[GintsK]

There is need for developers explanation...

My tests show something like this:

For injrampup battfact 4080:

PWefframpup=k*2/3*inj_eff_ramup_time

k=1.0@7V
k=0.90@10V
k=0.78@13V
k=0.737@15V
k=0.70@16V  It is straight line in fact!

If injrampup battfact is reduced, we get k closer to 1.0. As example:
 if 2040 used k=0.85@16V
if 0 - k=1.0 always
====
Battfact is in us bacause it is maximal adder to PW@7V. It linearly scales down to 0@13.2V.
It is straight line too!

Now what? Straight line+straight line=straight line anyway. Why there is two straight line equations used for descscripting one straight line?

Here can be some answer:

If active PW comes close to 2/3*inj_eff_ramup_time (at very low injection times), Inj_rampup becomes scaled down. Smaller PW cause smaller  Inj_rampup adder.

It is my free translation from Russian from Andreys description in http://rus.vems.hu/punbb/viewtopic.php?id=42

Here is original text:

Если pulse_width < 2/3*Inj_rampup, то есть при очень малом времени впрыска идет дополнительное масштабирование Inj_rampup в меньшую сторону, чем меньше pulse_width тем сильнее масштабирование.

How it exatly works is not clearly understandabble. Need additional bench tests. And very preferable some description of original idea.

[Sprocket]

If active PW comes close to 2/3*inj_eff_ramup_time (at very low injection times), Inj_rampup becomes scaled down. Smaller PW cause smaller  Inj_rampup adder.

It is my free translation from Russian from Andreys description in http://rus.vems.hu/punbb/viewtopic.php?id=42

Here is original text:

Если pulse_width < 2/3*Inj_rampup, то есть при очень малом времени впрыска идет дополнительное масштабирование Inj_rampup в меньшую сторону, чем меньше pulse_width тем сильнее масштабирование.

How it exatly works is not clearly understandabble. Need additional bench tests. And very preferable some description of original idea.

I think I understandthis part.

Consider the effective flow rate of the injector at minimum effective pulse width (effective ramp up time) (in my example 0.9ms). The PW at idle is a calculation of the required fuel PW divided by the VE, plus, the scaled effective ramp up time. You need this effective ramp up time for consistant injector opening irrespective. If the injectors are so big that the PW at idle is such that PW minus effective ramp up time is too small, the effective ramp up time itself is scaled, so to increase the required fuel PW (VE value)(the bit in the middle of ramp up and ramp down). Otherwise VE at idle would be near zero?

So much going on in my head, but with all this rambling im starting to understand this more.

Next lesson, idle control PID

[Sprocket]

Right need a little help

I have replaced the FET driver and P259 chips. I came to replace the driver chp capacitors. The shcematic shows 0.22uf capacitors, and the rescue kit says there are 8*220nf capacitors, but the most of one capacitor in the reskue kit is 6 I have assumed for now that these are the 220nf capacitors PLEASE CONFIRM

Also, I measured the resistance of the FET gate resistors all except one read 18 ohm, which acording to the code 180, this is correct. The other one, read 39 ohms and the code was unreadable, just black. I removed it and measured again, still 39 ohm. It also had a black line through it so it has obviously been smoked. Now there are no 18 ohm resistors in the rescue kit, but there are 33 ohm, which, are the same ones available in the PWM kit using the same FET Driver chips. Do I try and source an 18 ohm resistor? (dificult) or do I replace all the resistors with the 33 ohm ones? PLEASE CONFIRM

All the old FETs whether they were good or not have been removed ready for all new one.

I dont think I did too bad a job. Obviously not as pretty as the flow solder, but everything looks to be a good contact and no tracking.

Also, can anybody see any issues with the grounding wires I remade?

Nearly there

[rob@vems.co.uk]

Nothing wrong with trying the 33ohm resistor, if the original was a 33 and you were suggesting using an 18 then I'd be wary, but you're not so that's all good.

[Sprocket]

Thats what I ws thinking Rob. But what about those capacitors?

[rob@vems.co.uk]

Good question and unfortunately one I cannot answer

I'll check my rescue kit this evening and see if I can find an answer for you.

Rob

[Sprocket]

Bump

[rob@vems.co.uk]

Bugger

[Sprocket]

Bump

[rob@vems.co.uk]

The ones I have are all in strips of 6.
I've put a question on the wiki - I'll be asking the developers...

[Jorgen]

Jörgen, how physically FET driver can damage FET?
Yes it can open FET forever, when damaged. But how it can be a cause?

When I had fried FETs, gate resistors was fried too. It works like fuse protecting driver chip. Check resistance!

Gints

I have yet to find a broken gate resistor in any of the boxes I service and I have serviced a fair number of boxes. Is this something that is exclusive to the recent batch of boxes using the 18ohm resistors?

I can only guess why a damaged FET driver kills the FET but Sweden is a large market for VEMS and I have seen more then a few broken boxes over the years. But as I'm one of the original developers of the v3 I tend to get involved sooner or later whenever a box has anything but trivial damage. For example when the FET's fail repeatedly. The initial reason for the failure has usually been because of a fucked up install but in many cases the boxes has continued to fail after being repaired. A few years ago we found that the FET driver is the reason for this and that replacing it fixes the problem. Most other cases can be explained with broken external hardware like injectors, flyback or that the install is still fucked up in some way or another.

A FET will often fail when it's turned on or off slowly, it wouldn't be far fetched to guess that a FET driver could fail in a way that makes it turn on and off slowly. Stranger things happen, like for example FET drivers exploding with enough force to make a crater in the board if the capacitors for the FET drivers is left out.

Sprocket, I have verified that the strip of 6 in rescue kit 2 are 220nF capacitors. I did this by measuring them on a recent kit.
The 33 ohm resistors is a good replacement for the 18 ohm resistors.

Jörgen

[Sprocket]

Jorgen.

Thank you, that is exactly what I was after, I didnt want to power up the board before I had this info. Time will tell how I get on now.

I still dont know what cause the failure, but i will assume it was the grounding, that has been re done, as you can see above. However, i have one last thought. The injector and ignition power supply comes from the  main relay which is switched when the RPM rises above 100rpm. would this cause any problems?

Thansk for everyones help

Sprox