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Detecting points opening on a magneto question

Hello

I've been asked to help set the ignition points on a 1950 Triumph motorcycle - it has a magneto. I want to detect when the points open. My first two circuits have attempted to detect the small (around 2.4R) resistance change. Both circuits were based on potential dividers, one with a NAND gate and the second with a bridge circuit and an op-amp. To keep the current drawn from the PP3 low I had to use reistors in the order of low k values - the tiny resistance change is thus hard to detect.

Now for my question. The magneto is basically a low inductance coil that is or is not shorted by the contact breaker - so how do I set about detecting the change from shorted to small inductance?

There's a capacitor across the points, I did think of detecting a current spike as the PP3 charged C when the points open. I'd like to try detecting the coil first. Any help would be appreciated.

John

Reply to
eatmorepies
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The coil is very inductive. Inject a high freq signal into the coil side of the points and use a AC to DC detect circuit from that signal at the point where it is connected.

With the induction there, you'll see the signal until the points close.

you'll need to factor in the cap that is on the points too, so don't get to high in freq..

Most likely something in the range of 1khz from a 555 timer will do it.

Make sure you put a back to back Zener for protection so not to damage your input sensing circuit.

Jamie

Reply to
Jamie

Are you doing static timing? Why not run the beast and use a strobe?

For static timing, a good ohmmeter should resolve the closure. If not, use an inductance meter.

Or run a little current through the points, 50 mA maybe, and measure the voltage drop directly across the points, essentially a 4-wire ohmmeter.

John

Reply to
John Larkin

1: Why in heck can't you get it running and use a timing light? 2: Look for contact closure visually, or if there's a pigtail that can be disconnected do so, and use an ohmmeter or just a light and a battery. 
--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
Reply to
Tim

3: RTFM: find a service manual and do what it says. It's amazing what following directions can do for the reliability of equipment that you're operating. 
--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
Reply to
Tim

This is a 1950 motorcycle using a magneto.

  1. The manual suggests testing for the opening of the points using a small piece of cigarette paper. This method gives +/- 10 degrees of uncertainty in the timing. A visual method would be even worse.

  1. It's a magneto - no pigtail. These things are fully enclosed with just the high tension lead(s) emerging. You get at the points by removing a small cover. You can't even see the capacitor, coils or any wiring without disassembling it - which requires special tools.

Try Googling magneto and have a read.

John

Reply to
eatmorepies

This seems to be the way to go. Thanks.

John

Reply to
eatmorepies

In the olden days, we used to set the points by just setting the gap using a feeler gauge.

Can a dwell meter handle a magneto?

Good Luck! Rich

Reply to
Rich Grise

An audio oscillator of some sort and an earbud would be cool. You could hear it open and close.

It might be possible to measure the points opening and closing by measuring something on the HT wire. The audio thing might do that.

Hey: get anything that plays music or a radio station into an earbud, like one of those FM radios you can buy at Walgreens for $3. Arrange for the points to short the bud. I bet you can hear the open/closed difference.

John

Reply to
John Larkin

Looking back over the years, I find it amazing that the point system worked as well as it did.

Taking into account for worn cam shafts, loose cam bearings, cam pads, spring tension etc.. And let us not forget the crappiest condensers ever made along with the normally stripped out screws holding it all in.

Jamie

Reply to
Jamie

battery life is less of a problem is you use a low voltage op-amp (LM324) or comparitor (LM319) and a 3V supply

You could use a higher frequency than DC, say 10KHZ, the magneto will have a higher impedance at that frequency.

.-->probe 10Khz | |\ /| --------[100R]---+------------| >========| | speaker ------------------------------|/ \| | --- audio amp ///

if the flywheel magnet is in place there will be other effects tat depend on the speed of rotation etc.

Disconnect the magneto from the points and just measure resistance?

put a coil (old degausing coil? 100 turns of telephone wire? a spool of hook-up wire?) near the magnetto supplied with stepped-down AC or an audio signal and connect an amplifier and loudspeaker (pc speakers) to the plug terminal and chassis?

when the points oen the sound should get louder.

--
?? 100% natural

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Reply to
Jasen Betts

That sets dwell, but not the main timing. Changing the dwell does change the opening time of the points, but you're generally less concerned about getting dwell dead nuts on than you are about getting the spark timing right.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com
Reply to
Tim Wescott

(slapping forehead) Of course! Duh! We'd set the timing by turning the distributor itself!

Sorry for the brain fart. Rich

Reply to
Rich Grise

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