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Location:
Perth
Registered:
June 2002
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I supported Toymods
Banned User
Location:
Brisbane
Registered:
May 2002
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Re: Why are injectors measured in impedence and not resistance??
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Thu, 16 October 2003 20:54
 
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Because they are coils, Just like a transformer.
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Location:
Sydney
Registered:
May 2002
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Re: Why are injectors measured in impedence and not resistance??
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Thu, 16 October 2003 22:12
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Because they are inductors not resistors. Inductors have impedence. Have a look in an electronics book for more details. 
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Location:
Montrose, VIC
Registered:
May 2002
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Re: Why are injectors measured in impedence and not resistance??
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Thu, 16 October 2003 23:10
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In simple terms, impedance is AC resistance - just because something has a DC resistance of, say, 8 ohms at DC (0 Hz) doesn't mean it will have an impedance of 8 ohms at, say, 20 KHz (using a speaker as an example). Just like a speaker, inductor, transformer, etc. an injector is just a coil of wire. If you can't quite get your head around it, don't worry - main thing is that the bigger injectors get, the more high impedance ones SUCK - but low impedance ones put a lot more load on the output transistors of your ECU - so you have to find the balance (or buy a bloody MOTEC and use the lowest impedance injectors you can find  )
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Location:
Perth
Registered:
June 2002
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Re: Why are injectors measured in impedence and not resistance??
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Thu, 16 October 2003 23:29
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MRSHIN---> Impedence is AC resistance yes, but injectors do not operate on AC, rather alternating DC. (i think)
Anyhow......
all this I realise. but there must be a standard voltage/frequency that ALL injectors are measured at. Kind of like a standard accross the board. Because if an injector has its impedence measured at 5V/30Hz, it will be different than at say 15V/600Hz.
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Location:
Brisbane
Registered:
May 2002
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Re: Why are injectors measured in impedence and not resistance??
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Thu, 16 October 2003 23:34
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| gabe wrote on Fri, 17 October 2003 09:29 |
MRSHIN---> Impedence is AC resistance yes, but injectors do not operate on AC, rather alternating DC. (i think)
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I think you'll find you just contradicted yourself. AC = Alternating Current. If you take DC (Direct Current) and "alternate" it, it's now alternating current, is it not? 
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I supported Toymods
Location:
I renounced punctuation
Registered:
May 2002
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Re: Why are injectors measured in impedence and not resistance??
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Thu, 16 October 2003 23:57
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Nope they are different - AC requires a reversal of polarity (IIRC), whilst intermittent DC (what the injectors run on) is a rapid fluctuation between 0V and whatever the driving voltage is. However because the current is rapidly changing, then it is better to measure the 'resistance' as impedence.
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Location:
ACT
Registered:
July 2002
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Re: Why are injectors measured in impedence and not resistance??
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Fri, 17 October 2003 00:02
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Technically no.
Injectors are all DC operated, but its a varing DC. Alternating current means the polarity of the current changes (ie it would go to minus 12 volts). This never happens, the injectors only go from 0 volts to 12V positive, they never receive a negative voltage.
However because the DC voltage does change you treat the circuit like it is an AC circuit even though it doesn't match the exact definition of an AC circuit.
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: Why are injectors measured in impedence and not resistance??
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Fri, 17 October 2003 00:21
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for something that is just 'on' resistance is often a decent enough measurement.
impedance is more like a measure of dynamic resistance...
hmm, to put it in car terms. think of free-revving a motor
resistance would be the internal friction that slows the engine down, and stops it revvign quickly.
impedance is more like the flywheel, as you have to put energy into over coming it's inertial impedance to get the motor revving.
for injectors, even tho two injectors may have the same resistance, the impedance may be much larger in injectors with large coils. this results in more power being needed to get them moving, as you have more 'impedance/inertia' to overcome.
a good example is the massive TBI injectors used on US V8's. these things are huge, and have very large impedance compared to say, MPI denso's. althought he may have same resistance, you need higher ampage to get them to initially get going (for low Z, PWM, peak and hold type injectors), but then similar power to keep them open. when they close, there is also more 'flyback' which is basically the power you put in, kicking back as the solenoid goes back thru the coil....
at least i think that's how it works 
something about inducing a field then moving something thru it, and sort of regenerating fields and current and shit 
Cya, Stewart
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Location:
Montrose, VIC
Registered:
May 2002
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Re: Why are injectors measured in impedence and not resistance??
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Fri, 17 October 2003 02:19
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That's why I said 'in simple terms' - I realise there is a LOT more to it that AC resistance, but didn't see any need to start going into too much detail
And, as for converting DC to AC, it's something I'm working on at this very minute trying to build myself a simple circuit to step voltage up for my ignition (I'm far too tight to just go and buy an ICE voltage stepper...) - and it's a pain in the arse, to be honest! Think I've got it pretty much sorted, though..
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: Why are injectors measured in impedence and not resistance??
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Fri, 17 October 2003 02:28
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LOL, sorry  it's the scientist in me trying to break out.....
stepping up voltage for ignition??? what for??? can a coil handle more than 14V? (maybe if you reduce current?) how much voltage are you after... and what ar eyou using to do it?? i was told ages ago about either a diode or transistor cascade that steps up DC voltage,, but can't remember which....
i need something similar to run a laptop... only 16V tho, not sure on current..
Cya, Stewart
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Location:
ACT
Registered:
July 2002
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Re: Why are injectors measured in impedence and not resistance??
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Fri, 17 October 2003 03:09
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Any PWM IC will do the job. The TL494 is commonly used for simple supplies. Then all you need is a switching transistor (or mosfet) and a transformer with the right turns ratio. Then you rectify and filter the output voltage like a regular linear DC supply.
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Location:
Montrose, VIC
Registered:
May 2002
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Re: Why are injectors measured in impedence and not resistance??
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Fri, 17 October 2003 04:09
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That's basically exactly what I'm doing  Using a 494, then a set of MOSFETS to feed a transformer.
For your laptop, there was a project in August or September Silicon Chip magazine that does exactly what you want - go check it out.
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Location:
Perth
Registered:
June 2002
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Re: Why are injectors measured in impedence and not resistance??
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Fri, 17 October 2003 04:35
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This is how I am thinking.
Impedence is the opposition to current flow.
Impedence is the combination of 3 things. Straight resistance(of the copper itself), Inductive reactance(Back EMF and the like) and Capacitive Reactance. On straight DC, Inductive and Capacitive reactance are non existant, leaving only resistance to oppose the current.
Now introduce a variation to the DC waveform, and you introduce Inductive and Capacitive reactance. One thing that has an effect on the ammount of Inductive and Capacitive reactance is the speed or frequency of the variation in voltage.
So I was curious is there a set frequency and voltage that is used across the board when stating an impedence.
Take a speaker for example, I think 600Hz is used when giving a speaker impedence. This value does not vary from manufacturer to manufacturer.
Feel free to correct any of what I have said here
Cheers
Gabe
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Toymods Social Secretary
Location:
Sydney
Registered:
July 2002
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Re: Why are injectors measured in impedence and not resistance??
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Fri, 17 October 2003 09:14
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| mrshin wrote on Fri, 17 October 2003 14:09 |
For your laptop, there was a project in August or September Silicon Chip magazine that does exactly what you want - go check it out.
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You'll find this kit only operates up to 2.5A, and is not nearly enough to run most laptops which usually require a hearty 3-4A at 15-25Vdc. Jaycar does sell a little device that suits your purpose Stewart, just give me a PM about it
As for stepping up a DC source, i think you have all covered the basics of it - it basically requires some switching circuits and a suitable transformer..... as you have mentioned... but there is a little more to it if you want to make the output decent.
But now if we wish to get into AC "inverting" from a DC source, this is where it gets tricky. You need crystal frequency locking, and some damn good switching circuits as well.... and thats just to get "sqaure wave AC". I am at a loss as to how pure sinewave inverters work!
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