Peak And Hold Low Z Injector drivers

[Sprocket]

This is going to be a bit of a grey area

Will i need to remove the transient supression diode when running the low z injectors on the external peak and hold driver board? I know its bad when PWMing but is that down to the constraints of PWM rather than the low z? Dont forget that the only difference between the Peak and Hold driver and the series resister is the way it limits the current, both could be considered the same type of drive, IE 100%PWM dissabled.

Any help on this apreciated. I may have to go to the wiki on this but the response wont be in the time frame im after.

Thanks

[rob@vems.co.uk]

How does the external board handle flyback?  The VEMS will not be driving the inductive load of the injectors so its flyback will not be needed. You may need to address flyback issues with other inductive loads though.

[Sprocket]

I spoke to Jean about the flyback and which way to go, its the transient diode that we are both unsure of. The external board does have its own flyback diode per driver chanel. However we came to the conclusion that these could be removed and use the onboard flyback. The external board is wired almost as if the injector FETs were 'remote' from the board, its the driving signal from the Genboard processor that is wired to the peak and hold chip, this is taken from the original location of the injector FETs on the Genboard. The grounds are pretty much the same so the VEMS onboard flyback will still be in the circuit. The board requires its own 5v supply which is easy enough.

The only concern is if the FET driver signal is 12v, the peak and hold board requires 5v driver signal. I think the FET driver signal is 5v on the Genboard, can anyone confirm this.

I opted to use the FETs that Jean uses on the external board, and move over four of the FETs i have on the board to four of the unused ignition drivers.

Aparently Jen has had another Vems user asking about these, and I know there is another on this forum.

[rob@vems.co.uk]

The transient diode is used as we've found its the best way of controlling flyback and getting good closing characteristics for the FETs on the VEMS board.  The back EMF is dumped across these FETs when the injector closes, the flyback diodes on the board and transient diode handle this.  If you are going to use a seperate FET driver then you need to use the flyback that he's using, the 5v FET driver on the VEMS board will not see the flyback voltage as its handled by the FET.

You can run a VEMS without the transient diode, in many cases it works quite well but needs the injectors to be tuned more.

Dont remove the flyback diodes on his board, set the thing up as a daughterboard that uses the FET driver feed, and grounds from the board and route the flyback from the P&H board to the EC36-pin23 using a flying wire.

[Sprocket]

Im A bit confused

Ok let me put it another way

If i were to leave the FETs on the Genboard, but unsolder and disconnect from the Genboard, the processor driver signal, what you call it on the fet? Collector? leaving the FET ground and Emitter? as was, with the flyback still intact as intended. Connecting the 'collector of the FET to the PH chip output and then the Genboard processor output that was disconnected from the collector of the FET on the Genboard, connected to the PH chip input. It then is the PH chip and circuitry that is driving the FETwith its signal coming from the processor on the Genboard, the power and flyback side has not been alatered in any way.

This would be the situation if the fets are installed on the external board and hard wired to the Genboard , IE it is remote (longer legs on the FET)

The only other wiring this external board needs is a +ve -ve 5v for the circuitry

Megasquirt, but you get the jist

[rob@vems.co.uk]

Now I get you I'm looking at the board and we have all the flyback diodes in place on the FET 'outputs' which is going to be the route for the back EMF, what are the flyback diodes like on the P&H board?  Are they similar in characteristics to the ones on the VEMS board? 
I'd not be using the transient suppression diode in the flyback.

[Sprocket]

http://catalog.digikey.com/scripts/partsearch.dll?Detail?name=FR303DICT-ND

http://www.diodes.com/products/inactive/_data/ds26003.pdf

These are the diodes Jean has spec'd

I have never really looked at the flyback on the Genboard, i'll have a look at the schematics in a bit, i just know its in that randomly placed looking bank of diodes on the top of the board

I supose running without the transient diode isnt such a bad thing, i read up a bit, it was used to help with badly selected rampup times or something like that. It replaces what would have been the power flyback??

It looks to me that the PH board leads the flyback voltage to ground, where as the Genboard leads it to the +ve 'common' supply of the injectors??

I think we are getting there

[rob@vems.co.uk]

What I would do...

In the first instance I'd bridge the FET drivers directly to the P&H board, and use the board independently of the rest of the ECU.  Maybe use a four pin automotive connector (water proof/resistant connector from the Web Shop) or mount the thing entirely outside of the case so that you can give it some external heat sinking.

Oh, and I'd buy some spare FET driver chips from the webshop

[Sprocket]

Ok then, next question, what voltage is used to drive the FET open. 5v or 12v

[rob@vems.co.uk]

Just scoped it and it looks like 12v.

[jbelanger]

From the schematic, it is 12V.

That means you'll have to bypass the FET driver to drive the p&h board.

Jean

[Sprocket]

Ah ok, ill have a lookie

I found this

http://www.vems.hu/wiki/index.php?page=GenBoard%2FPowerSwitcher%2FFET

[Sprocket]

Ok, Rob, these are the 'spare' FEt drivers?
 http://shop.vems.hu/catalog/product_info.php?cPath=1_7&products_id=59&osCsid=6775c4817555876203e8157ecce3d177

Looking at the schematic (looks like an always hard to read, simplified version) As you have pointed out already Jean, the signal for the Peak And Hold board needs to come from Pre_Inj, this is the 5v signal from the processor.

To the practicality of it. Would simply removing the driver chip, soldering a wire onto the appropriate pads of Pre_Inj for that chip, taking these wires to the 'inputs' on the P&H board, maybe then returning another wire from the P&H driver circuit output to the INJG pads of the removed chip, and solder the P&H FETs inplace of the VEMS Injector FETs. This just leaves the reistors R18, R21,R39 and so on, what to do with these? this would keep the FET heat sinks all in the same place along with the original flyback circuit, with only the add on board needing mounting. Seems daft taking the switched current off the board and then bringing it back again, when you could achieve the same on the 5v signal side of things.

One thing I am concerned with is there is no protection on the processor ? Boom and bust

Also assuming that small gauge wire is only required.

Schematic showing the driver chips Labled with 'U'



And the board Layout again showing the driver chip Chips still labled 'U' (lol, nit exactly the best colours to use together )

[Sprocket]

Fet driver chip data sheet

http://dkc3.digikey.com/Media/PDF/Data%20Sheets/ON%20Semiconductor%20PDFs/MC34151,33151.pdf

[Sprocket]

What about increasing resistor value of R18, R21 and so on, to pull the voltage down, and leave the driver chip well alone, but giving the processor some protection, the driver chip being a rather expensive fuse??

Im not conversant on electronics so dont know if that would work

Ooh, just experienced Dejavou,been here done that sort of thing