Paging the Voltage Regulatoristi

As I just had to replace a dead battery on the Sprint I decided to check the
regulator. I'm getting 14V at just above tickover but this drops to around
13.65V as the revs go up. Is this correct? (I half expected the opposite
ie;13.5V at tickover rising to 14V).

 

Sounds fine. You will be putting a bigger load on the system at higher revs
as the spark plugs etc will need to produce more bangs per second. If the
alternator is the typically weedy affair fitted to most bikes then the
tickover voltage with a very low load i.e. no headlight glowing, could be
where you see the highest voltage.

As long as the voltage across the battery remains over 13V then you will be
sticking plenty of electrons back into the battery.

 

Cheers Wessie

 

Actually the coils - well er being coils, are inductive, that means as the
frequency increases the impedance increases.

With the old Kettering ignition (points) it was commonplace to either wind
the LT coil with high positive tempco wire or use a series ballast resistor.

This is because if the engine stalls with the points closed, the LT coil can
draw enough current to burn out.

 

Speaking as UKRM's resident electronics expert, I think#~{{&%"¬`#NO
CARRIER

 

Speaking as UKRM's resident electronics expert, I think#~{{&%"¬`#NO
CARRIER


??????

 

wtf? are you on crack?

 

Oh dear. You must be too young.

 

Haha

 

Given what happens when you charge a battery and the way that the
surrounding environmental issues can affect what you read on the meter the
question is reasonable. Person anally I would be happier to see a higher
voltage at around a quarter to a third revs but a good old fashioned zero
tech eyeball check of how much the lights brighten when going from tick
over to this speed is just as good as any reading you can take from a
meter.

 

You're on about dwell angle I was talking about frequency.

 

Traditional ignition systems (points or transistorized) store a finite
amount of energy for each spark: I doubt that would change much with
engine RPM. You're looking at it from a pure inductor/frequency POV,
which doesn't really apply here.

If it's a basic CDI system then again, the capacitor will store a
finite amount of energy for each spark.

Having said that, I'm not certain that Wessie's correct here. While
the ignition & FI (if it has this; CBA to Google) current draw will go
up with revs, so will the alternator output current. I guess it comes
down to which curve has the steeper slope.

 

well, it could be that his alternator brushes are fucked and make less
contact at higher revs. For now, the 13.65V will do and rick won't have to
spend any more on the bike, today.

Of course, we could send him out to the shed to test the alternator under
the load of his headlamp which would give a better diagnosis of the
charging system's health. As rick has publicly stated that he is skint I
don't think he wants to know...

 

I'm so skint I can't even afford a shed

 

You were wibbliung on about the frequency with which the spark happens and
are wrong.

The spark itself is unchanging, a mere transient pulse a few
thousandths[1] of a second long. Engine speed will not affect this at
all. What will happen with increased engine speed is that this extremely
short lived pulse of energy will occur more often. Thus leading to the
confusion of the ill educated in the manner you so ably demonstrate.

[1] Actually I don't have a clue how long this is but I do know it is
almost as quick as a libdem dumping manifesto pledges. Which, as we all
know children, is pretty dammed quick.

 

I can give you the facts and you can look up the formulaes and figure it out
yourself.

With points the dwell angle is fixed and doesn't change (notwithstanding
points bounce at high revs).

When you apply a voltage across a coil (inductor) the current starts at next
to nothing and rises linearly as the lines of flux build up around the
core - when the core saturates, the inductance value disapears and the
current is determoned purely by the DC resistance of the winding wire
(shouldn't happen in normal operation).

When you interrupt the current, the lines of flux collapse around the core -
which happens much more rapidly than due to the linear rise in current that
put the energy there in the first place - that's the bit that produces the
much higher voltage.

Because the dwell angle is fixed, as the revs increase the time period for
current flow into the coil decreases in direct proportion of 1/T, the
current is interrupted earlier on the linear rise in current therefore the
average current draw is less.

I rather doubt that any TAC pulsers use a monostable (fixed charging period)
as making the charging period short enough to fit the available time period
at maximum revs would make it too short for efficient operation at tickover,
and cold starting a real pig!

CDI depends entirely on the design (I'd say the HV winding on the alternator
is probably best) Transistorised one's will have less time to charge the
capacitor at high revs - some types have a feedback system that makes the
inverter work harder during the low end of the charging curve, these will
draw more current at high revs.

 

Indeedly so. It was well known in the days of contact/coil ignition that
yer typical car coil would draw 4A at tickover and 2A at normal road
revs.

 

Inductance may not be the only factor.

Tests on a CB200 coil (and several others in the junk box) showed that when
connected across a 12V battery, the curent started at about 8A and slowly
sank to about 4A.

Evidently they wound the LT coil with PTC wire to eliminate burnout if it
stalls at points closed.

Although on cars ISTR you're more likely to find a ballast resistor.
Presumably that limits the current if stopped with points closed, as the
revs increase and coil current falls the volt drop will shift from the
resistor to the coil - improving spark performance at high revs.

 

Well, having looked it up, Im surprised at how slowly the old points
ignition coil current rises: you live and learn.

 

That made me think: as far as I know, I've only ever owned one bike
that had a field coil alternator, and that didn't use slip rings - the
exciter coil was mounted on the engine cover. All the rest have been
PM alternators.

<sand drips out of ears>

Sorry? What?