Injectors and fuses

[Tommy.Palm at oron.ds.sll.se]

Item Subject: Meddelandetext
> 
>
>> In the 2.5 ohm systems i have checked they don't take more than 1-
>> 1.5amp/injector. Around 2 amp should bee a good choise since the most
>
>
>A little judicious application of Ohm's Law will reveal that at
>12V, a 2.5 ohm load will draw almost 5 Amps!.

With 2,5 ohm systems i meant that the injectors have a resistance of 2,5 ohm.
they are then connected to a resistor, a transistor and diod ( and some more
electronics for overvolt protection). The peak current is checked with a 100
MHz oscilloscope and i have testrunned it with 1 amps transistors for days and
days in bench w/o any problems. The injector gets rather hot in
testbenchdriving. My own system uses a high peak open current and then a lower
hold current than standard systems and don't need more than 0,6 amp fusing
(slow fuses, fast peak currents goes mutch higher on my home made system).
Actually since the serieresistor on stock systems i have looked at had around
6-8 ohms so the max current when shortcurcuiting shold not blow either the
transistor or board on low ohm MPI systems (14.4-06/6=2.3 amps per injector).
Short curcuiting all injectors seemes unlikely. I don't have any comments
concerning high ohm systems.

Considering that
>the standard battery voltage is closer to 14V, the current draw
>becomes even larger. Mind, you, the inductance of the injector
>will self limit the current until the inductor "charges" up, the
>above calculation would be worst case if the injector was on 100%
>of the time.  Most injector drivers are also current limited, but
>this only protects the injector, not the driver circuit.
>
>The 1-1.5 A figure you quote is most likely the average current,
>not the peak. 


 Fuses would have to be rated to handle peak
>currents, with a safety factor of about 25% added on for good
>measure.  I would use an 8-10 A, standard automotive fuse,

I meant 2 amp per injector minimum so a 8-10 ohm for the whole system (if you
have >=4 inj) might give ocillating voltagedrops over the fuse and holders soo
check that its not between batterysupply and controllcircuits. As i wrote
before a low ohm system should not need fusing inside. Any low amp fusing (>15
amp) should always preferably bee done inside the curcuitry. Thats a good
answer wy its not done on the factory made systems.

 this
>will protect the driver from catastrophic failure, but should
>never blow under normal conditions.  I would think that most auto
>makers use some fuse in the circuit, whether it is replaceable
>(read visible) or not is another issue.  May be just a fusible
>link in the wiring somewhere.
>
>regards
>dn
>
>
>
Tommy
>
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> Darrell A. Norquay                   Internet: dn at dlogtech.cuc.ab.ca     
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