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Detecting points opening on a magneto question

E

eatmorepies

Jan 1, 1970
0
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
 
J

Jamie

Jan 1, 1970
0
eatmorepies said:
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
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
 
E

eatmorepies

Jan 1, 1970
0
Tim said:
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

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.

2. 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
 
E

eatmorepies

Jan 1, 1970
0
Jamie said:
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

This seems to be the way to go. Thanks.

John
 
R

Rich Grise

Jan 1, 1970
0
eatmorepies said:
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.
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
 
J

Jamie

Jan 1, 1970
0
Rich said:
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
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
 
J

Jasen Betts

Jan 1, 1970
0
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.

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

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
///
There's a capacitor across the points, I did think of detecting a current
spike as the PP3 charged C when the points open.

if the flywheel magnet is in place there will be other effects tat
depend on the speed of rotation etc.
I'd like to try detecting
the coil first. Any help would be appreciated.

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.
 
R

Rich Grise

Jan 1, 1970
0
Tim said:
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.
(slapping forehead) Of course! Duh! We'd set the timing by turning
the distributor itself!

Sorry for the brain fart.
Rich
 
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