Tach Jumpy at Idle

[JGeezer]

I have the RVI tach (one transistor), stock ignition. At idle, the tach needle jumps a lot, but at a few thousand RPM is settles down to a reading. What do you think, tach problem, or something in the ignition?

[billwill]

Could be either, but first inspect and get your distie points clean & flat.

You could be getting extra spikes in the current flow due to dirty/uneven contacts.

[JGeezer]

Could be either, but first inspect and get your distie points clean & flat.

You could be getting extra spikes in the current flow due to dirty/uneven contacts.

Hmm, good thought. Will try come morn! Thanks.

[Chancer]

The condensor is there to remove any spikes on the LT circuit which would reduce the spark energy or affect the tacho operation, the tacho could however be affected by transients from other sources like the dynamo, the current/voltage regulator cut out, air horn compressor, relay coils etc.

[Quart Meg Miles]

The condensor is there to remove any spikes on the LT circuit which would reduce the spark energy or affect the tacho operation, the tacho could however be affected by transients from other sources like the dynamo, the current/voltage regulator cut out, air horn compressor, relay coils etc.

I would disagree!

The condenser reduces the rate of change of current in the coil when the CB opens to provide a controlled voltage spike. If the CB is intermittent then it would release and restore the current/spike which could trigger the tach.

Another possibility is that the tacho's retaining nuts are loose and making a poor ground connection.

[Chancer]

When I said "spikes" I meant sparks across the points but the capacitor will also absorb any secondary transients, its interesting to look at the LT circuit on an oscilloscope with condensor, without and also with a failing or wrong capacitance condensor. The following is a cut n paste.

If the LT and HT windings, the CB and everything else were perfect, then the voltage induced across the LT winding would rise to infinity in an infinitesimally short time, and the voltage across the HT winding would try to become many times infinity. But things aren't perfect. In particular, even though the CB opens quickly, it takes a finite time, and even when it is fully open, the CB contacts are only a very small distance apart. Immediately the CB begins to open, the large voltage that appears across its contacts can be enough to strike an arc between them so that an electrical current continues to flow through the CB and the LT winding. As a result, the voltages across the LT and HT windings become significantly less than infinite. Indeed, the HT voltage may well be insufficient to cause a spark at the sparking plug, and much of the electrical energy that was generated by the LT winding spinning in the magnetic field gets dissipated at the CB as heat, light and damage to the CB contacts.

To prevent, or at least significantly reduce, the arcing at the CB, a condenser or capacitor is connected between the contacts of the CB and therefore between the ends of the LT winding. By contrast to a coil that tries its best to prevent changes in electrical current through it, a capacitor tries its best to prevent changes in voltage across it. Put the two together and they dance or resonate with each other at a particular rate dictated in part by the capacitance of the capacitor. What is important in a magneto is the very first step of that dance.

So, just before the CB opens, energy has been stored up in the LT winding in the form of an electrical current flowing through it and the closed CB, while the capacitor is shorted out by the CB. Immediately the CB begins to open, the LT winding no longer sees a short circuit between its ends, but instead sees an uncharged capacitor. The current continues to flow around this new circuit of LT winding and capacitor, but decays at a rate dictated in part by the capacitance. Meanwhile, the voltage across the LT winding and capacitor (and therefore also across the CB) starts ramping up at a rate dictated in part by the capacitance. So, we can now exercise some control over how quickly the voltage changes.

The aim is to choose a capacitor such that, while the CB is opening and the distance between its contacts is increasing, the voltage across the capacitor (and therefore across the CB) never exceeds the breakdown voltage of the air between the CB contacts. If that is done, then an arc and consequent energy loss between the CB contacts is prevented.

[Quart Meg Miles]

Chancer,
I misinterpreted your meaning of "LT circuit" and our descriptions agree. If the contact breaker is not perfect though, any chatter on opening will allow the current to fall a little but be restored intermittently, possibly causing the tachometer to misfire as described. The circuit is low impedance so should be immune to other interference unless the power source is dodgy.

Geezer's car is an early +2 so, if standard, will have a dynamo. At low speeds it should be on smooth battery power only but possibly the regulator is twitching the supply until the charging current reduces. I guess this could be tested by interrupting the dynamo by pulling off its field wire (or the Brown/Green F wire from the regulator).

[JGeezer]

Chancer,
I misinterpreted your meaning of "LT circuit" and our descriptions agree. If the contact breaker is not perfect though, any chatter on opening will allow the current to fall a little but be restored intermittently, possibly causing the tachometer to misfire as described. The circuit is low impedance so should be immune to other interference unless the power source is dodgy.

Geezer's car is an early +2 so, if standard, will have a dynamo. At low speeds it should be on smooth battery power only but possibly the regulator is twitching the supply until the charging current reduces. I guess this could be tested by interrupting the dynamo by pulling off its field wire (or the Brown/Green F wire from the regulator).

Sorry to have rudely never responded, other projects intervened. I really appreciate all the time you and Chancer took trying to help.

The +12 (white wire) seems to be a reasonably clean level, maybe ,1V jiggle to it. No jumps up and down corresponding to the tach movement, and the same with the field wire off the regulator.

Onto into looking for spikes on the signal...