# Oscilloscope question

Discussion in 'Electronic Basics' started by Dan Beck, Sep 13, 2008.

1. ### Dan BeckGuest

Hello all,

years ago I was given a 10 MHz Heathkit oscilloscope. Both channels work
fine, and I use it to probe circuit boards powered up on the workbench. I
now own an old hobby car with the old fashioned plugs/points/capacitor
ignition. Is there any way I could use the 'scope to observe the ignition
signals, and would there be any value to it, besides academic information?

Thank you in advance for any and all thoughts!

Regards,
Dan

2. ### Michael BlackGuest

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

3. ### neon

1,325
0
Oct 21, 2006
YES you can . the max car frequency is not 10mhz more like 400hz. at the points there is 12 volts being switched on-off. looks like you could put a scope there right NO! when the secondary releseases the energy there is the same ratio 1:xxxx reflection so there is probably 3-400v spikes can you see that yes but i sugest you putting a resistor attenuator of 1000:1 so the cheap scope doesn't break down the input. same applies to the secondary at spark gap output. there is usualy about 100mw of power there so don't kill the circuit with low resistors. have fun.

4. ### whit3rdGuest

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.

5. ### ian fieldGuest

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.

6. ### Paul E. SchoenGuest

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.

http://www.onsemi.com/PowerSolutions/supportDoc.do?type=datasheets&category=824
http://www.rectron.com/data_sheets/1.5ke.pdf

Paul

7. ### Guest

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.

8. ### Jasen BettsGuest

at 60 they'd fire on the ring voltage.

Bye.
Jasen

9. ### ian fieldGuest

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.

10. ### JamieGuest

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.

http://webpages.charter.net/jamie_5"

11. ### JamieGuest

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

http://webpages.charter.net/jamie_5"

12. ### JamieGuest

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.

http://webpages.charter.net/jamie_5"