Oscilloscope question

That sort of thing was done in the old days, I've never had a car so I don't know how useful it was, or even if it matters now (I gather a lot of cars are now beyond the ability of the average person to do much repair work).

It would have been detailed in countless magazine articles, and likely in all those "101 Things To Do With Your Oscilliscope". That sort of book can be quite useful when starting out with an oscilliscope, you might want to dig up a used copy.

Michael

I've used my 1-channel, 5MHz Heathkit to set the dwell on my '71 Vega, and to diagnose problems with a bad throttle sensor on newer cars. It's a lot less direct than using a dwell meter, but you get to see more of what's going on.

Use a 10:1 probe, set the voltage scale to lots -o- volts/division, hook the thing across the points and -- voila! You can see not only the dwell, but you can see the profile of the low-tension side of the coil as it fires.

ion

Two ways. First, you can look at the primary winding on the spark coil; the opening of the points will make a few hundred volts spike (an attenuator probe is wise here), and the sparkplug firing will determine the trailing edge of that transient. Imbalance of cylinders in breakover voltage is clearly visible, can be caused by obvious things (dirty or misgapped plugs) or unobvious ones (gas mixture in the cylinder changes the breakdown characteristic).

Second, you can use a clamp-on transformer to monitor the current through the HV wires (either the wire to the coil, OR the distributor wires to one plug at a time). Be careful about grounding here, of course. A burden (shunt) resistor on the secondary of your transformer is VERY IMPORTANT; ten to a hundred ohms should be sufficient. The transformer can be a pair of ferrite core halves and a single turn of wire for the secondary.

The LT back emf could spike as high as 600V (maybe even higher) if the scope is all tube you should be OK, but if it uses transistors in the Y amplifiers some caution might be wise. Study the spec for the x10 probe and if it isn't rated for that voltage then make a voltage divider yourself, here again you need to check the voltage rating of the resistors - if necessary make the dropper section of the divider out of a number of series resistors.

It might be prudent to add a neon lamp or a TVS bidirectional zener type suppressor across the scope inputs. I'm not sure how fast a neon lamp triggers, but the type of suppressors used in telephone equipment should be pretty good, and rated about 60 volts.

Here are links:

Paul

rk

=A0I

ion

You could take this oscilliscope, and check the rate of transfer during disconnect of the 12V btty. This will show you when your wires are degrading. And, might even indicate moisture due to acidic influence, a common ailment.

at 60 they'd fire on the ring voltage.

Bye. Jasen

Might not be so bad if the OP builds a scope lead breakout box to add the neon so it can be shunt connected between a x10 probe and the scope input.

If you're interested in the HV current Pulse, You can wrap a couple of turns around a plug wire and load the output to a low value resistor. This will act as a current xformer and provide you a safe a low voltage reference signal for your scope to monitor the current pulse at the electrode on the plug. They make engine scope probes to clamp on plug wires and in line taps accessories. At work, we actually use an automotive HV inline coupling to monitor a signal via a scope on irradiation units.

You could also consider using a HV probe for scopes. The one you would need could be expensive.

And hope the condensor(capacitor) in the ignition system is good as you connect your sensitive scope to this potential arching point.

That's why you use a cheap scope...

Actually, I was trying to figure out if there's a network that one could make that would use a neon bulb or similar to absorb transients. Something that wouldn't mess up the measurement too much, but wouldn't also go 'pip' right before the oscilloscope goes 'pop'.

Invest in some TVS diodes. They'll work great for your application. Just select the ones that hits the limits of your scope input or just under it. Get the bi-directional types and lay one of them across the scope's input.