[AT] AT Digest, Vol 28, Issue 7
Mark Johnson
markjohnson100 at centurylink.net
Wed May 13 05:11:24 PDT 2020
Steve:
Great explanation of LRC circuits...I know (or used to know) most of
this, but your explanation is one of the best I've ever seen.
I didn't know Snap-On ever sold oscilloscopes - presumably for diagnosis
of automotive ignitions and electrical systems?
With all the GHz-range clocking of modern personal computers, a
knowledge of analog and RF is still very important. At 3 GHz, a 1-inch
trace on a circuit board is an antenna!
Mark Johnson (WB9QLR)
Columbia MO
On 5/13/2020 6:58 AM, Stephen Offiler wrote:
> Glad you mentioned resonance, Carl. This is the next level of the
> discussion that I purposely avoided in my previous comments. With
> points open, it's a LRC circuit. L is the inductance of the coil
> primary; R is the small resistance of the primary; and C of course is
> the capacitance of the condenser. At initial points opening (only
> open a tiny amount thus far) a voltage spike comes out of the primary
> and is absorbed by the condenser. This discharges back thru the coil
> primary in a "ringing" or resonance fashion, and some energy is
> dissipated by the resistance. The "ringing" reverses again, several
> times, with an exponentially decreasing amplitude as energy is
> dissipated. It happens (or should happen, if the circuit is properly
> balanced, as you mentioned) very rapidly. The points re-closing will
> dead-short the condenser but by now there's very little energy left to
> be dissipated.
>
> I have some neat oscilloscope plots of this phenomenon somewhere...
> oh, just realized where they are: in the instruction manual for an
> old Snap-on "Counsellor" oscilloscope from the 1980's. It happens to
> be at work right now. I wonder if I can remember to scan some and
> send them along.
>
> What is really fascinating is that the points-and-condenser ignition
> was developed, as far as I know, before the invention of the
> oscilloscope. Early electrical engineers had a strong intuition for
> analog circuits.
>
> SO
>
>
> On Tue, May 12, 2020 at 7:19 PM <szabelski at wildblue.net
> <mailto:szabelski at wildblue.net>> wrote:
>
> I agree that the condenser is wired across the points, and it
> helps prevent burning of the contacts by helping to suppress the
> arc when the points open. But a condenser can only store so much
> energy and must be discharged at some point, it can’t absorb
> energy forever. The condenser has to be discharged before the next
> opening of the points or it can’t absorb any more energy and help
> suppress the arc. It becomes discharged when the points close
> again. That is also when the plug is firing, so that stored energy
> goes into the spark. The condenser and coil also form a resonant
> circuit which is what is needed to try to have a stable system
> since the energy from the coil rises and falls as the energy is
> created and released.
>
> That is why there are as many different condensers available ,as
> there are cools. You need one that properly works with the coil to
> absorb the level of energy developed by the coil. If it can’t
> absorb enough, points burn. You also don’t have a properly
> balanced resonant circuit.
>
> Carl
> ----- Original Message -----
> From: Stephen Offiler <soffiler at gmail.com <mailto:soffiler at gmail.com>>
> To: Antique Tractor Email Discussion Group
> <at at lists.antique-tractor.com <mailto:at at lists.antique-tractor.com>>
> Sent: Tue, 12 May 2020 10:04:58 -0400 (EDT)
> Subject: Re: [AT] AT Digest, Vol 28, Issue 7
>
> Condenser function: it connects across the points, aka in parallel.
> Points are used to switch current through the coil primary. Coil
> primary
> functions like a big inductor - it stores energy in a magnetic
> field, and
> it uses that field to supply energy to oppose any change in
> current. It
> can't win, because the energy gets depleted, but it does try. As
> the points
> open, that implies a sudden change in current from the steady DC
> value to
> zero. Inductance responds by trying to keep that current flowing. It
> "piles up" electrons at the opening points which elevates the voltage
> there. It would certainly arc, if the condenser wasn't there. The
> condenser charges up and gives that voltage (that pile of electrons) a
> place to go, thus suppressing the arc and extending the life of
> the points
> substantially.
>
> SO
>
>
> On Tue, May 12, 2020 at 9:49 AM <szabelski at wildblue.net
> <mailto:szabelski at wildblue.net>> wrote:
>
> > Steve,
> >
> > The condenser is actually a capacitor, originally called a condenser
> > (probably because a charge condensed internally) and later called a
> > capacitor. There should be no continuity between the condenser
> case and the
> > pigtail wire. A capacitor is made of two thin sheets of metal
> that hold a
> > charge until there is a short between the case and the pigtail.
> The two
> > thin plates are separated by a thin medium that allows the
> charge to build
> > up to a specified amount. The purpose of the condenser is to
> store extra
> > energy and give you that little “extra kick” when discharged.
> >
> > As I said in an earlier post, it is difficult to check a
> condenser, it
> > takes special equipment. Even if it passes, there is no
> guarantee how long
> > it will last since the medium that separates the two plates can
> fail and
> > allow a short between the two plates. The best thing is to try a
> different
> > condenser. It just has to be one with the same capacity to store the
> > required charge.
> >
> > Carl
> >
> >
> > ----- Original Message -----
> > From: STEVE ALLEN <steveallen855 at centurytel.net
> <mailto:steveallen855 at centurytel.net>>
> > To: at at lists.antique-tractor.com
> <mailto:at at lists.antique-tractor.com>
> > Sent: Mon, 11 May 2020 17:39:06 -0400 (EDT)
> > Subject: Re: [AT] AT Digest, Vol 28, Issue 7
> >
> >
> > ----- Original Message -----
> > Message: 7
> > Date: Sun, 10 May 2020 19:53:23 -0500
> > From: Phil Auten <pga2 at basicisp.net <mailto:pga2 at basicisp.net>>
> > To: at at lists.antique-tractor.com
> <mailto:at at lists.antique-tractor.com>
> > Subject: Re: [AT] Project Updates '49 A, '51 A, and '47 B (STEVE
> > ALLEN)
> > Message-ID: <168f1f90-8d7a-1eef-e95e-28fb0fa7e41b at basicisp.net
> <mailto:168f1f90-8d7a-1eef-e95e-28fb0fa7e41b at basicisp.net>>
> > Content-Type: text/plain; charset=utf-8; format=flowed
> >
> > '49A - If your points are clean and reading .5 ohms when closed
> and .7
> > ohms with cardboard separating the points, your points are
> shorted. With
> > the cardboard in, they should read an open circuit, and .5 ohms when
> > closed says your contacts aren't making contact well. If you
> took those
> > measurements with the points installed, disconnect them and
> retest. If
> > the points read 0.0 ohms closed and open circuit with the
> cardboard in,
> > then the problem is elsewhere.
> > '49 A: I forgot to mention, if the points measure correctly
> outside the
> > distributor you may have a bad coil.
> >
> > Phil in TX
> >
> > Phil,
> >
> > Remember that the points set in a Wico X mag is two separate
> pieces. If I
> > take them off, I can screw them back together and check for
> continuity,
> > BUT: while mounted in the mag,
> > the fixed one is in contact with the case of the mag, and the
> other, the
> > one that moves, is in contact with the terminal of the
> condenser. Now, let
> > me speculate here:
> > the terminal in the condenser does not touch the case of the
> condenser,
> > right? So, the two points should not have continuity because
> the one is
> > connected to the terminal
> > and the other is "connected" (through the case) to the mag
> case. UNLESS
> > there is continuity inside the condenser. I do not understand
> condensers
> > well at all,
> > but *SHOULD there be continuity between the case of the
> condenser and the
> > terminal of the condenser*?
> > If not, that may well be my problem (per Cecil). I will investigate
> > tonight.
> >
> > BTW, I know that the shaft is turning and that the points are
> opening and
> > closing: I have verified that much by turning the crank.
> >
> > The "original" Steve Allen
> > Who is avoiding the local Menards
> >
> >
> > _______________________________________________
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