From: John De Armond
Date: Fri Apr 5, 2002  7:18 am
Subject:  Re: [megasquirt] Looking for cheap secondary ignition probe (kV)
X-Source: The Megasquirt mailing list

On Thu, 04 Apr 2002 20:35:42 -0000, "pbanders" <pbanders@y...>
wrote:

>Anybody got a line on a US source for a cheap secondary ignition 
>probe for an oscilloscope? Something like this:
>
>http://www.picotech.com/auto/secondary_ignition_pickup.html
>
>I'm also interested in any home-built probe designs.

Well, since I directly connect to ignition secondaries all the time, I
have to disagree with the premise for that probe. I do NOT like
capacitive voltage dividers (what that thing actually is) in
uncontrolled environments.


The easiest and most direct method is what I use - a Tek P6015 high
voltage scope probe. This probe is good for 40kv at 75 mhz bandwidth.
I have three of these probes. The least I paid was $20 and the most
was $150. This is pretty much the range of street prices for used
probes. Note that the P6015a is the more modern solid dielectric
probe that usually sells used in the hundreds of dollars. 

Getting this kind of bandwidth at this voltage is not easy. Tek did
several things that would be difficult to do at home. The main
dropping resistor was insulated with pressurized freon. Most probes
you find used have lost their freon charge but I've found that cig
lighter butane works as an acceptable dielectric fluid. The line from
the probe to the compensation box is air insulated transmission line
to minimize capacitance. I've never seen this sort of line anywhere
else so I suspect Tek (had it) made it for this application. The
compensation box contains a complex network with half a dozen
adjustments.

If a gross outline of the waveform is OK, then any of the HV probes
designed for DC measurements with DVMs will do. I have the Tek "Red
Finger of Death" probe which costs about $125 new. I have used it
with my scope for ignition work. While the P6015 will show all the
nuances of the ignition waveform, the DC probe shows a rough outline.
Frankly, it's probably as good as that capacitive prope your URL
references.

You can roll your own. My tek probe has a 990 megohm high voltage
resistor in the long red finger that connects to the positive lead and
a 100k resistor + cal pot in parallel across the pos and negative
leads to form a 1000:1 voltage divider. Adding some pf of capacitance
across the 100k resistor would negate some of the rolloff caused by
the long time constant formed by the high value resistor and
distributed capacitance.

I'd just watch Ebay and labx.com and get myself at least one P6015 if
I was you. Putting "P6015" in google usually turns up several other
sources. If you find one without freon, contact me and I'll tell you
how to do it with butane.

John