The BSA A7-A10 Forum

Technical (topic titles must be descriptive) => Lucas, Ignition, Charging, Electrical => Topic started by: Hubie on 17.06. 2010 08:38

Title: Ammeter
Post by: Hubie on 17.06. 2010 08:38
Just a question re ammeters.  Now that I have converted to 12 volts, are there 6 volt and 12 volt ammeters?  Do I need to change mine?


Title: Re: Ammeter
Post by: MG on 17.06. 2010 08:42
Yep, there are 12V ammeters available.
But actually it's no problem to keep the 6V version, you just have to live with the smaller needle deflection due to the reduced current.
Title: Re: Ammeter
Post by: Hubie on 17.06. 2010 10:46
Thanks mate,

You have confirmed my suspicion.  I thought all was working fine considering my indicators would flash faster under revs and slow down at idle, a clear sign the chargins system is working.  I might go ahead and get one just because I am anal!


Title: Re: Ammeter
Post by: groily on 17.06. 2010 15:42
Quite often, ammeters are highly dubious anyway, depending on what you've got. I don't reckon it's anal at all to have the only important instrument working right and clearly!
Lots of cheap ammeters flicker for lack of damping on the needle, some of them the needles tend to touch the face after a few vibes and stop operating smoothly, some of them the terminals become loose on the back and they just fall apart.
If you're really going to buy one, suggest you spend the extra it takes to get a good one. My favourites are the larger Miller ones with white faces and well-spaced -8 0 +8 scales which I can read without my blasted specs on (they also come in black, like Fords did), but they won't fit a BSA headlamp without butchery. Original Lucas ones are fine, with damped needles and a choice of scales (unless you want to have whatever scale the thing's supposed to have had as original).
I think there are other threads here on this, but you definitely get what you pay for.

And of course, per MG's comment, with 12v systems there are only half the amps floating around for any given load so a typical -12-6 0 +6+12 scale and wobbly needle isn't always helpful. But whichever voltage, with a decent regulator and charged battery, you shouldn't be seeing charge rates of more than an amp or two except just after sitting at tickover with heavy loads on. More is not Better - lots of amps says either the battery is weak or that, owing to poor regulation, the dynamo's field coil is being over-served, the armature unnecessarily loaded and the battery over-charged. Badly-set regulators have given dynamos a bad name for half a century.
Title: Re: Ammeter
Post by: Brian on 17.06. 2010 17:36
I have bought a few ammeters over the last few years, some good, some not so good. This particular one is very good, I have had it in the bike for about two years now and it works perfectly. Have a look.
Title: Re: Ammeter
Post by: Triton Thrasher on 17.06. 2010 20:21
One of the few good components on an Indian Enfield is the ammeter.  Bit of a tight fit in the headlamp but mine's been working for years and years in the Triton.
Title: Re: Ammeter
Post by: trevinoz on 17.06. 2010 23:01
There is no such thing as a 6 or 12 volt ammeter.
Generally ammeters don't care what voltage is in the system as they only read current.
You will find that bikes have 8 or 12 amp instruments.
Generally the 8 amp unit is used with a dynamo and 12 amp with an alternator.
Title: Re: Ammeter
Post by: MG on 18.06. 2010 10:15
many suppliers sell 6 and 12V versions. Having never used a 12V system, my guess was that the 12V ammeter would have a different dial and characteristic, so having the same needle deflection at half the amps, compared to the 6V version.
Yet all these seem to have a -12A to +12A dial, so I suppose my guess about the different characteristics was simply wrong, as then the 12V versions would give false readings. (

Unless there is a bulb in there for lighting the dial, where would the
sense in stocking 2 different voltage versions be?
Did I overrate the suppliers and they do it just because of plain ignorance?

If so, sorry for having misled you, Hubie. Stick with your original ammeter, no sense in buying a 12V version if it is just the same as the one you have.

Title: Re: Ammeter
Post by: Triton Thrasher on 18.06. 2010 11:57
The ammeters meaeure current, rather approximately, no matter whether 6 or 12V.  6V circuits pass double the current of 12V systems, for the same power of generator or lamps.  That means sometimes an ammeter in a 12V bike has the wrong displayed range for a 6V system and vice versa.

I think someone already said that.
Title: Re: Ammeter
Post by: MG on 18.06. 2010 13:32
That's what I was talking about TT.

For a bike with 60 Watt dynamo running at 6V the ammeter should have a range of +/-12A (with some reserve), the appropriate 12V version would have a range of +/- 6A then.
Yet most 12V versions I have seen have a +/- 12A dial, which either means that they are simply the same as the 6V ones, resulting in a rather small needle deflection when used with 12V, or the internals are different and the dials are wrong, with the ammeter showing 12A while in fact 6A are flowing.
 ???  *dunno*

Title: Re: Ammeter
Post by: a101960 on 18.06. 2010 13:56
If you do opt to replace the ammeter go for the damped version. At about £20 (that is almost twice the cost of the cheap non damped version). I bought, and fitted a damped ammeter to my bike, and it was worth every penny. The pointer gives a smooth reliable indication as opposed to the nasty cheapo ammeter that was fitted when I bought the bike. The pointer just oscillated all over the place. If the charging circuit had been faulty in any way I would never have known. That is how useless it was. The old adage as always applies: You get what you pay for!

Title: Re: Ammeter
Post by: groily on 18.06. 2010 17:25
Is what I thought you were talking about MG (and TT). Obviously the volts don't matter, but try reading a typical 40-0-40A or worse 60-0-60A car instrument on a 12v motorcycle charging a good battery at 0.5A with a decent regulator. I fitted such to my pre-war Riley when I swapped the 120W Rotax dynamo for a 600W scrap-yard alternator with appropriate wiring mods in nocturnal despair, and it's a fat lot of good. But the original 12-0-12A instrument would have been up against the stops on start-up after a lay off. It's all about usable scales, not about the principles involved.
Title: Re: Ammeter
Post by: Triton Thrasher on 18.06. 2010 17:44
[quote author=groily link=topic=2824.msg But the original 12-0-12A instrument would have been up against the stops on start-up after a lay off. It's all about usable scales, not about the principles involved.

If the ammeter doesn't actually go on fire, it probably doesn't matter if the needle is up against the stops for a few seconds, except I wouldn't like to charge a battery at 60A.
Title: Re: Ammeter
Post by: trevinoz on 19.06. 2010 01:03
You blokes are getting confused.
The dynamo system only uses a 8-0-8 ammeter because that is about the limit of output of the unit whereas the alternator is allegedly capable of delivering 10 amps or so.
Again, the ammeter is not dependent on the system voltage in any way. It measures current only.
You could use the same meter on a 240 volt system as long as it is in the expected current range.
Title: Re: Ammeter
Post by: lawnmowerman on 19.06. 2010 09:20
Bang on Trev - it is an ammeter and measures current flow - voltage does not come into it at these levels. All you are interested in is whether current is flowing in or out of the battery. When the ammeters are calibrated after manufacture the current flow should measured with an accurate ammeter in series and the readings should be the same. Ideally the scale of the ammeter should be more than the main fuse rating as a dead short will cause full scale deflection of the needle and possible damage although this will only be momentary until the fuse blows so generally the scale of the ammeter should reflect the output in amps of the charging system.
You could equally fit a voltmeter and monitor the battery voltage - it should read 12-13v on a fully charged battery (depeding on type and condition) but on start up the dynamo or alternator should raise the voltage to just over 13v or whatever the voltage regulator cuts out at. 6v systems will obviously be half these figures. The difference in potential difference (voltage) will cause current to flow through the battery and charge it until it reaches a fully charged state.
The overall limiting factor is the output in Watts (volts x amps) of the dynamo or alternator.

Sorry if this has confused anybody still further!

Title: Re: Ammeter
Post by: muskrat on 19.06. 2010 11:01
G'day all,
               all this talk reminded me I had a new one for the '51, 6 volt with dvr2 to 12 volt. I found it, a Wassel 8-0-8. Wired it backwards first, Doh. Motor off, headlight on = -4. Motor idle = -4, motor rev = +4, motor rev plus headlight on = +2. So I can sleep tonight.
Title: Re: Ammeter
Post by: A10Boy on 23.06. 2010 10:19
These things are not precision instruments and only give a general indication of whether the system is charging or discharging. it would probably be more accurate to have a charging light. Riders still need to be aware that if using headlights, it will discharge at anything under about 40mph in top gear.