thepest
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| posted on 22/9/10 at 04:26 AM |
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Oscilloscope - how to use
Hello All, its been a few months since I've been on this forum & I'm back into tuning my machine again.
I bought myself a digital oscilloscope because I would like to set the dwell with my Megasquirt. I'm using the standard ford coil pack. Has
anyone done this before?
To where should I be connecting the oscilloscope poles & what settings should I use (Time base etc)?
Thanks 
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wylliezx9r
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| posted on 22/9/10 at 07:12 AM |
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It all depends what your trying to measure. The amplitude and timebase sholud be set so that you can display at least 1 cycle on the screen.
If you can tell me roughly what sort of signal you are trying to measure I could give you the rough settings.
I don't know what type of probes you are using but the normal type is sort is a probe with a crocodile clip coming off it. The clip would go to
earth (reference) and the probe would go to the point where you want the signal.
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Bluemoon
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| posted on 22/9/10 at 07:57 AM |
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As above, try playing with it so you understand what it's doing, there should be a square-wave test point probably on the scope. You can use
this to play with, try to measure the period of the wave to make sure you understand how it works..
Is it a dual beam scope (i.e. two channel)? If not you will need to use the trigger to measure the dwell..
Dan
[Edited on 22/9/10 by Bluemoon]
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matt_gsxr
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| posted on 22/9/10 at 08:45 AM |
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http://www.picoauto.com/tutorials/ignition-primary.html
there are some helpful pages here on using their digital oscilloscopes.
Have a play, they are great tools.
Matt
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MikeRJ
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| posted on 22/9/10 at 09:06 AM |
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When setting up dwell, the actual event you are interested in is the saturation of the coil primary. To see this you ideally need to monitor current
rather than voltage.
You have three options - the best by far is to use a hall effect current probe. This requires no circuits to be broken or any components added, and
does not affect the circuit you are measuring in any way. They aren't especially cheap however.
The other way is to insert a low value resistor into the ignition supply wire to the coil and measure the voltage drop across it. You need as low a
value as possible to minimise the effect on the primary current, consistent with being able to get enough voltage to measure with a reasonable degree
of accuracy. For a coil with a saturation current of, e.g. 7 amps something like a 0.05 Ohm resistor would give you 0.35 volts which is plenty
(assuming even a half decent scope). You really need two channels to do this since the resistor is not ground referenced in this position.
The third option is to measure voltage on the switched side of the coil. Since the transistor switching the coil has a finite resistance, the voltage
drop across it will increase with primary current. However, you don't know what this resistance is, so you can only get a relative measurement.
You also need to be aware that there will be a high voltage at this point when the transistor switches off to generate the spark, this could be
around 300 volts, so you need to make sure your scope can deal with this without damage.
[Edited on 22/9/10 by MikeRJ]
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thepest
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| posted on 23/9/10 at 06:53 AM |
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Thanks guys, My oscilloscope has 2 poles the probe and the crocodile clip which would be the earth.
Is there any other way I can use this tool? I mean if the current is increasing going into the coil then drops when it discharges wouldn't the
voltage do the same?
As Mike says I am trying to measure coil saturation but my probe can only measure voltage and not current.
I can set my readings as follows:
Frequency, cycle, duty, Vpp,Vram,Vavg, DC voltage.
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MikeRJ
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| posted on 23/9/10 at 04:06 PM |
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quote:
Originally posted by thepest
Is there any other way I can use this tool? I mean if the current is increasing going into the coil then drops when it discharges wouldn't the
voltage do the same?
Not by much, only due to the resiatnce of any wiring and the coil driver transistor. It's inductance that limits the current rise in the coil
during the dwell time, until you reach saturation where it's the resistance of the coil.
I would try simply measuring the voltage on the switched side of the coil with respect to ground - the voltage during the dwell period will rise a
little and you should be able to detect saturation. Check the maximum voltage rating on your scopes input though, if it's less than ~300 volts
then use a 10:1 probe or you may find the voltage generated during a spark event could damage your scope.
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thepest
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| posted on 24/9/10 at 06:19 AM |
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Any other "Locost" method that I may use to detect coil saturation?
Probes of this type cost a penny or two more than I want to spare 
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needforspeed
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| posted on 26/9/10 at 09:14 PM |
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just ask phil on extraefi.co.uk unless you really want to do it yourself :-)
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