hey EE guys Options

[bd1308]

i have a question to ask, but I have to ask it later in the day....

i just wanted to round up the troops....

dont laugh either.....



b

[bondo]

Let's just get this out of the way now...

It'll cost too much.

It won't work.

Why would you want to do THAT?


Pick any two. (IMG:http://www.914world.com/bbs2/html/emoticons/ph34r.gif)

[Aaron Cox]

answers are here:
(IMG:http://www.office-humour.co.uk/includes/images/items/2003/07/801.jpg)

[rhodyguy]

why is this in the garage? why is it not labeled OT? wtf is this post supposed to mean? not meaning to pick on you so much britt. i just seems like after the big shake up people had their minds right. now it appears to be, do what you want, when you want. robots, tasteful camel toe, and 2 eyes seem to be a ok these days.

k

[TonyAKAVW]

Check your ground connections

-Tony

[GWN7]

Clean the fuse block terminals and check your ground strap

[fiid]

I don't think "I'm going to have a topic later" is a valid topic. Sorry to be an ass, but I'm kind of bummed because I was hoping for an interesting topic, and instead I got a "maybe something interesting later".

[SLKWrx]

Is it plugged in?

Did you attempt to put it together yourself?

Thats usually the first round

[jasons]

Its always and I mean always the flux capacitor that fails.

[bd1308]

okay...

i tried to draw you all a pic, but M$ paint isn't really up to the task....

okay...

Silicon Diodes have a property where in the event of a voltage over 100 something volts, the current will be able to flow backwards.

in my experiences, points become pitted and ruined due to excessive arching across the contacts.....possibly due to running lean? (stopped when mix became OKAY again)

would I be able to save my points (and Pertronix) from serious and fatal malfunction by installing a diode in the reverse direction of the current?

so if a spark voltage happens to cross the points, it will flow through the diode instead of through those precious points......



just my therory....thought of it in physics class..... (IMG:http://www.914world.com/bbs2/html/emoticons/smoke.gif)

b

[TonyAKAVW]

Yes. But use a Zener diode, they are made to use that effect at specified voltages. A common one is 5.7 volts, but they exist for higher voltages too.

-Tony

[bondo]

Are you talking about inductive kickback?

I doubt you can find a suitable diode, and if you did I suspect it would totally mess up the function of the coil.

The coil works by building up a large current in the primary winding (points closed) Which builds up a large magnetic field around the secondary winding. When the points open, the current stops, the field collapses, which generates a high voltage in the secondary winding. Without that abrupt inturruption of current, no sparky.

Your points should last plenty long. I've gone 25k miles on one set of points. If they're dying quickly something else is wrong. Have you changed your condenser lately? Do you have the correct coil?

[fiid]

The field collapse causes a voltage spike on both windings. What he is talking about is commonly used when switching a relay with a transistor, and is also used by the megasquirt tach circuit to smooth the input. I would think you could do something to reduce the kickback on the points side of things - it seems plausable to me.

[root]

QUOTE

If they're dying quickly something else is wrong. Have you changed your condenser lately?

(IMG:http://www.914world.com/bbs2/html/emoticons/agree.gif)

Each time the contact points separate, the self induction of the primary winding will cause a spark to arc across the points. This, in time will pit and burn the contacts. Traditionally a capacitor (condensor) is connected in parallel with the contacts to absorb some of the electrical energy between the contacts through charging the capacitor.

You'll (IMG:http://www.914world.com/bbs2/html/emoticons/ar15.gif) any normal size diode you put inline with the point circuit.

Just install a new condensor.... see how that does!

[bondo]

The arcing of points also helps clean them. Slightly used contacts (points or relay contacts, or whatever) of any reasonable quality will actually conduct better than new ones.

I have heard of many cases where points that are only used for a signal (CDI, MSD, etc.) wouldn't work reliably because of contamination.

Also, some points come with a protective coating that needs to be cleaned off.. did you do that? I usually run a folded piece of 400-600 grit sandpaper through mine after installation to make sure they're clean and well aligned.

[root]

I want some BOSCH Platnum Points!
Wander if anybody ever made such a thing? (IMG:http://www.914world.com/bbs2/html/emoticons/idea.gif)

[Dr. Roger]

the condensor deletes something called CEMF. counter electro motive force.

when the points open the electromagnetic lines of force collapse causing a high voltage on the secondary windings. when this voltage is induced and sent to the the plugs that induced high voltage creates it's own eletromagnetic lines of force on the primary windings, which induce, once again, a voltage and it continues again and again.

on an oscilliscope is is the long wiggly line just after the heart beat looking line.

the condensor grounds out the CEMF.
=-)

[lapuwali]

Yup. The MS tach input circuit has a 200-300V Zener that will shunt any signal that high to ground coming from the - post of the coil. It's true that any diode will eventually hit a breakdown voltage and will conduct "backwards", but a Zener is designed specifically to do this, and the breakdown voltage is consistent. A non-Zener may end up conducting backwards permanently, which will stop you just as fast as if you had your points fail completely.

I'd hardly consider points to be "precious". They're under $3 a set.

[jsteele22]

Hmmm... just thinking out loud here, so don't take this as definitive.

The role of the points is to make and break contact in the primary circuit (12-ish volts). Contact is made, current builds up and up until there's a good bit of energy stored in the magnetic field, then click - the contact is broken. Imagine there is no secondary coil. The primary coil is an iductor; it's windings don't like to see an abrupt change in magnetic flux : a reverse EMF (voltage) V = -L * dI/dt results. From the point of view of energy, the energy that was stored in the magnetic field is transformed to energy stored in an electric field between the points. If the field gets high enough, there is a breakdown of the air and a spark results.

One way to avoid the spark is to have a secondary circuit. When the primary current is cut off by the opening of the points, not only is a reverse EMF produced in the primary, as before, but also in the secondary (due to mutual inductance). Because of the huge turns ratio, the secondary EMF is much, much greater (it's a step-up transformer), so if the secondary is connected (via spark wires and an appropriately timed distributor rotor) to a spark plug, then the spark should appear in the spark plug gap first. Once the spark occurs, the spark plug gap has low resistance, and current can flow through the secondary circuit across the gap to ground. This current flowing through the secondary (actually, the time derivative of it) creates a time-varying magnetic flux in the coil that opposes the change caused by the points interrupting the primary current. So the reverse EMF across the breaker points is eliminated before it gets too large. From the point of view of energy, the energy from the original magnetic field is converted to heat in the spark gap.

So far, it sounds like the way to fry your points is to disconnect the spark plug gap from the circuit. The obvious way is to yank the plug wire. A less obvious way is to change the timing so the rotor is not making contact when the breaker points cause the EMF to spike. So when you say "running lean" do mean running with the timing too far advanced/retarded ? I'm sure the actual presence (or absence) of fuel in the air in the spark gap has some effect, but my hunch is that it's not huge.

Anyway, your question was about a different way to avoid frying the points. The idea is to hook up a diode so that when the EMF appears across the points gap the diode is reverse biased. As you mentioned, a diode won't conduct in the reverse-biased direction until a certain voltage (the "reverse voltage, or "peak inverse voltage", PIV) is exceeded. From the above discussion, it should be clear that one thing you DO NOT want to do is choose a diode with too small a PIV. Then you would short out the primary before the spark plug had a chance to fire. No harm there, except your engine won't work. The other thing to think about is the energy involved. Pretty much all of the energy that would have gone into the spark plug gap, or into the breaker points gap, will be dissipated inside the diode. I Better make sure the diaode can take that much power dissipation. I don't know how much (in Watts), but it's enough to toast stainless steel, or whatever points are made of. I bet there is a diode big enough, but I bet the cost is more than a new set of points.

An interesting variation on your idea is not to use a diode, but a "sacrificial" gap. Make a gap that is a little bit smaller than the points gap. Hook it up in parallel to the points. Under normal operation, it shouldn't have any effect (provided you don't make it too small), and under the no-secondary-discharge situation, it will produce a spark first, saving your points.

Good luck.


Jeff

[jsteele22]

Wow, got a lot of good replies when I was composing my tome. I'm sure I agree with all of 'em (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

Probably the most useful detail is to check the condenser (the old-timey name for a capacitor.) They can go bad and not give much in the way of obvious symptoms. I bet one of 'em is prematurely burnt out points.