Ammeter vs. Voltmeter

[Editor_Reid]

DISCLAIMER: I am no mechanic! (Or electrician.)

Speaking with an electrician friend the other day he told me that he prefers voltmeters over ammeters (for automotive installation) because all of the car's power has to pass through an ammeter, exposing the entire electrical system to shorts and other problems. This didn't ring a bell with me, but I was not equipped to differ.

Just wanted to ask for comments and experiences from those who have installed voltmeters or ammeters in their Healeys; which? why? problems?

Thanks.

 

[dklawson]

This is not a question unique to Healeys, it applies to all vehicles.

Your friend is correct that all of the current from the alternator/generator needs to pass through the ammeter to give you an indication of the charging system condition. This dictates that those heavy gauge wires pass through the firewall and along the back of the dash. (The obvious exception is the power to the starter motor which never passes through the gauge).

By comparison, the volt meter requires only a simple ground and "hot" connection using light gauge wires. Current flow through the gauge is only what is required to move the needle/pointer. The volt meter can tell you whether or not the generator/alternator is working and that's all you really need to know.

Ammeters may look cool as they swing from + to - in the direction of current flow, and they may look "period correct" for a car which only has a 22 Amp Lucas generator, but they still aren't that safe. They certainly aren't the thing to use if you've moved up to an alternator.

 

[Keoke]

/ubbthreads/images/graemlins/iagree.gif

But ,why not use one of each then you can really tell Watts going on. On the otherhand, a shunt type ammeter will eliminate the high current concerns.---Fwiw--Keoke

 

[Michael Oritt]

[ QUOTE ]
why not use one of each then you can really tell Watts going on

[/ QUOTE ]

Ohm my god--it's only Thursday--save your puns for a Faraday!

 

[dklawson]

Keoke, refresh my memory. Don't shunt gauges use big resistors that you measure the voltage drop across? How does their accuracy compare to a "traditional" inline ammeter?

 

[Dave Russell]

Hi Doug,
The shunt would likely be around 0.05 to 0.1 ohms at 50 to 100 watts. External shunt type meters are used in nearly all high accuracy applications. With the standard dash gage the shunt is inside the meter. A remote shunt in the main line with small wires going to the panel meter would solve the problem. However few automotive applications use remote shunts so it would be hard to find & probably costly.

Besides, as you say, nothing to gain & much to lose.
D

 

[JamesWilson]

The theory is simple enough.

The idea is to put a relatively small resistor (the shunt) in the circuit you wish to measure, the meter in line with a relatively large resistor then connects across that resistance. The ratio of the two resistors splits the current between the main circuit and the guage. This then limits the amperage to some acceptable low level that the guage can handle, and the user may comfortably have in the car's interior.

The accuracy is limited by the inherent accuracy of the guage and by the production tolerances of the resistors and the length and resistance of the wiring used for the connections. Given that the resistances don't change a meter once calibrated for their effects will be accurate regardless of where the shunt is placed- in the housing or elsewhere.

https://www.allaboutcircuits.com/vol_1/chpt_8/4.html

describes an automotive meter in which the shunt resistor is housed within the guage's case. It would be simple to remove that to another location and then wire the guage to it. The guage could then be calibrated to allow for the wire resistance and the other resistances.

The shunt resistors are "big" in size for handling large currents, but should be small in terms of the resistance introduced into the circuits.

I hope this is clear.... /ubbthreads/images/graemlins/confused.gif

 

[Keoke]

Thats the way its done JW. Excellent reference--Keoke /ubbthreads/images/graemlins/thumbsup.gif

 

[Keoke]

Michael, I am truly sorry I thought it was friday since I don't have to go to work in the munin.---Keoke /ubbthreads/images/graemlins/laugh.gif

 

[Ed_K]

Basil, This thread is a good example of your " efficient " method of warehousing technical data. I'll explain; The question is asked, several members offer possible solutions/ concepts. And finally JW posts a link to another server. The other server has the cost of maintaining the data files. Your server doesn't require as much storage capacity or
network bandwidth as it would if it also HELD all of these data reference files. Your LBC forum performs the same function as a card catalog for a library. Someone else had to pay for building and maintaining the library. Actually you have hit apon a great model for a technical library....
.... A voltmeter tells you everything you need to know about the state of your electrical system. If the voltmeter reads above battery voltage, the system is charging, if it reads below the battery voltage, it is discharging. Voltmeters are
cheap and readily available in all auto parts stores and online, many color choices and light options. In M.H.O.

 

[Keoke]

Well Basil , What are you going to say? ED presented a very interesting approach to your technical Data base without interrupting the forum. Further reduction in repetition and redudancy can be achieved by assembling it using set theory. The only down side I can visualise is if the referenced web site were to fold at some later date---Keoke

 

[JamesWilson]

[ QUOTE ]
The only down side I can visualise is if the referenced web site were to fold at some later date

[/ QUOTE ]

And even that potential problem is minimized by simply knowing such general technical information is available so that a Google search on "ammeters" should turn up other descriptions of their operation and basic theory. It only took me a couple of minutes to find a source to support what I learned thirty+ years ago. This isn't rocket science or particularly sophisticated.

I accept that other issues may be more complex and less commonly found. But the BCF does provide a useful way of pointing people to other sources.

 

[Keoke]

I accept that other issues may be more complex and less commonly found. But the BCF does provide a useful way of pointing people to other sources.

[/ QUOTE ]
/ubbthreads/images/graemlins/iagree.gif
It will take a lot of work to put the file together. I guess Basil is thinking up a simple way to do it.---Keoke /ubbthreads/images/graemlins/laugh.gif