Ian Jackson used his keyboard to write :
Or you could have got it over reading, then put a pot in the smaller ammeter wiring, to enable proper calibration.
That is how I fitted a charge discharge 'ammeter' to my caravan. Just connected a mV meter across the two end of the main positive feed to the caravan battery, with a suitable pot in series. Then tweak the pot to calibrate.
Wasn't there supposed to be a slight disadvantage in that the EHT was slightly lower with the battery earth polarity reversed? I have to admit that I've never understood this, nor bothered to work out why.
I believe that the argument went that the EHT consisted of around 20kV from the voltage induced in the ignition coil secondary, plus 600V back-EMF in the primary*. With the polarity reversed, these voltages subtracted, so the EHT was 1.2kV lower. However, as I said, I really can't see the reasoning behind this. [*This is because the primary and secondary are in series, and the points short the junction of the two windings to chassis as they open and close. It's done this way to get the extra 'free' 600V.]
In message , Harry Bloomfield writes
Quelle finesse! Nah, KISS! Just pass a known maximum current through a wire shunt (14swg?), slide the meter connections (say) with temporary croc clips for FSD, then solder.
By the way, I must see if I can make a fairly high current load using that green PVC covered steel garden wire. Once, for some high current (20 or 30A?) tests, I used a small drum of RS PVC covered multi-strand tinned copper wire, immersed in a bucket of water. It did the job, but at the end of the tests (two hours later), the water was quite hot, and the wire was now useless because the PVC on a lot of the tight turns had fused together. I probably should have unwound the virgin drum, and rewound it loosely.
In message , Rod Speed writes
The real answer is "It might be - or it might not". It depends what's in it. Around here, it's so hard you can nearly stand a spoon up in it, and will almost certainly neutralise the acid. However, I suspect that the incredibly soft stuff in the Manchester area might be OK. Personally, I'd rather trust distilled or de-ionised (about the same price as Evian).
The coil was in effect an autotransformer, where a higher negative voltage would be applied to the centre electrode in the belief that it produced a brighter spark and didn't corrode the electrode as much as when a high positive voltage was applied.
Coils with negative earth generally had the ground of the HT side connected to ground and so lost the additive effect of the primary. The spark was still negative as per the fashion at that time.
That, at least, was the theory given to me. In practice I have no idea if spark polarity made/makes any difference.
and that was what really mattered.
that's bollox. You will maybe get -ve rather than +ve spike, but it will always be in phase with the primary/.
So what you're saying is that it wasn't that the EHT was less (which was what was puzzling me), but that with the reversed polarity, the spark might not be as bright, and would swap the corrosion to the other electrode? Seems plausible.
But if you wanted to do a proper polarity swap, you should really have changed the coil? Again, seems plausible.
In message , The Natural Philosopher writes
For once, I think you're right!
Hence why I used the term generally. Most tap water has an element of hardness or low levels of other soluble salts.
In general they are harmful to the lead acid battery.
I too, would prefer to use distilled, de-ionised or (qualified) purified water.
Agreed.
Or just look up the R per metre for the cable dia
NT
I seem to remember now we did swap the coil wiring around come to think of it;!...
In message , tony sayer writes
I don't recall doing this (swapping the wires to the two spade terminals on each side of the EHT connection). However, this does seem to be the correct thing to do. See
You'll not have bought any new TV in the 50s for 20 quid.
What I meant was it took about the same in average wages then as a large screen TV does now.
Only in your tiny world.
Plenty of car makers are out to save every farthing. If 'your' method was in the least practical, it would have been used.
I must admit that I was rather surprised at the 51.5% of FSD DC offset error I saw about ten hours ago. The meter hadn't been used for over 6 months. Today, repeating the test some ten hours later, it now starts with a -2.6A reading (which has drifted down to -2.3A over the last 5 minutes or so). This level of DC offset error (~1.2%) is more like what I was expecting to see. The ~50% offset I first saw was a rather unexpectedly high figure (rendered rather academic by virtue of the DCA Zero button).
The problem doesn't exist in the case of AC current measurements (a simple HPF eliminates the DC offset in this case) but that's no help in this case where we need to measure DC current. The inclusion of a DCA zero setting button in a DC clampmeter implies that getting DC readings from a hall effect sensor is an aknowledgement of a problem inherent to hall effect sensors in general.
You may be correct in your assumption that a dedicated sensor might eliminate some of the variables that exist in a typical DC clampmeter based on hall effect measurements (it's possible to make a purely electromechanical DC clampmeter but I've never actually seen one).
Having typed that, here's an example shown on this wiki page:
Further googling lead me to info on open loop and closed loop hall effect current sensors along with some data sheets. It does look as though it should be possible to largely eliminate the need for massive zero calibtration correction where a small residual zero error will be swamped by the typical current readings in something like a car ammeter display (but, in the case of vehicle management by microprocessor control, a more precisely calibtrated zero wouldn't come amiss as part of the operational health logging of the various subsystems).
Worthy of note from the data sheets were the +/- 10 and 2 percent tolerances for the half supply voltage offsets on the output terminal. Whilst the pretty linear response of the open loop sensors can be improved to perfection by using closed loop sensing, this does come at the cost of extra complexity and power consumption.
Using closed loop doesn't, unfortunately, address the issue of the zero current output terminal voltage variations. A calibration compensation is still required to null this particular error out for DC measurements.
I haven't googled but I can't see why a matched pair of hall effect sensors, sandwiched back to back couldn't be used to provide a bridged output signal to do away with the need for a routine zero set calibration which would solve the issue seen in clamp meters using cheap open loop hall effect sensors. IOW, it may well be possible to manufacture, in quantity, relatively inexpensive hall effect current sensors that don't require a zero reading reset every time they're called into use (certainly to a sufficient accuracy for use in vehicles).
It really depends on the age of the car. Older ones probably used universal motors to get adequate power from a small size. PM motors are later - as magnets got better. But there was some overlap as dynamos changed to alternators.
Your needle has stuck. Again.
Yes - the slow movement is also useful to counteract vibration etc in a car.
Trouble with any volt meter (which all basically are) is that the car supply varies so much. A decent moving coil voltmeter will have a compensation coil - but is much more expensive to make that a hotwire type. However, if you need a fast reacting gauge - like say for oil pressure - a moving coil is the better type. For things like fuel and temperature, hotwire is fast enough. And one voltage reg can be used for several gauges.
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