In a message dated 98-06-11 07:53:55 EDT, dchilds@epri.com writes:
> I was thinking of putting an ammeter in the Spit. What range should I
> get? 30-0-30 or 60-0-60. What in that little car would pull more than
> 30 amps? One of the biggest loads I can think of would be the lights.
> I would guess that the lights are 55 to 60 watts so lets assume 100
> watts to be safe. 100/12*2=17 amps. What is the general experience
> here?
Dave,
For a Spitfire, 30-0-30 is fully adequate. If the ammeter is wired correctly,
it reads ONLY the current going into or out of the battery, with a few
exclusions, and NOT the current being supplied to the rest of the car from the
generator or alternator. In all cars, the current to the starter is excluded.
Some cars also exclude the lights and/or the horns.
There is a lot of controversery over the relative merits of an ammeter vs a
voltmeter. Given all the data, you may want to reconsider and use a voltmeter
instead. Following is a brief summary of the pros and cons of each:
1) An ammeter gives a direct indication of the charge/discharge current. A
voltmeter only says that you should be getting a charge if the voltage is
sufficient, and that you are probably getting a discharge if the voltage is
not sufficient. Most of the time, that is correct.
2) An ammeter responds instantly to changes, whereas a voltmeter, being a
thermal device, responds rather slowly.
3) An ammeter is MUCH harder to install in a car that didn't come equiped with
one than a voltmeter, from a wiring standpoint. There will need to be a fair
amount of rewiring done. A voltmeter only requires one lead to be connected
to the most convenient green wire, and one wire to ground. That is in
addition to the wires required for the illumination bulbs, which will be the
same for either one.
4) The leads to an ammeter are typicaly not fused. You are bringing into the
car additional direct connections to the battery, and connecting them to
exposed terminals on the back of the meter. A short here will try to carry
the total capacity of the battery till the leads burn in two. A voltmeter
uses a very small wire, and is fused. A short on one of the voltmeter wires
will just blow a fuse and no damage done.
>From an operator interface* standpoint, I prefer the ammeter. From a
practical standpoint, I prefer the voltmeter. Either one, if installed
properly, will do just fine, and either one is very preferable to having
neither.
* forgive me the use of this term, but I spent several years of my life
studying how human beings interact with instruments and controls in order to
optimize the design of control rooms for nuclear power plants - old habits die
hard!
I have drawn up a diagram, in jpg format, showing how an ammeter circuit
works, along with a brief description. If you are interested, let me know and
I will send it to you. I can't send attachments to the list. I intend to
send this to Ken Streeter to post on the VTR web site as soon as I can find
the time (along with several other things I have promised Ken. I'm working on
them, Ken, I promise!).
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74
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