DC or AC ammeter, or both?

Anything electric, AC or DC

Postby Larwyn » Tue Aug 26, 2008 5:37 pm

Cliffmeister2000 wrote:
NightCap wrote:Do the inexpensive amp meters go inline? For example cut the red wire and insert gauge? Is there a wiring diagram for guages? I have not seen one in my travels through T&TTT.


As I remember it, an ammeter like this is really a voltmeter that measures the voltage drop across a shunt. A shunt is a very exact, very low resistance resistor. The meter is calibrated with Ohm's law, which can be represented as (p=ie, p=i(squared)r, i(squared)r=ie, ir=e, i=e/r (I couldn't figure out how to superscript the 2 to make it represent "squared", so I just wrote squared)

p=power (watts)
i=current (amps)
e=electromotive force (volts)
r=resistance (ohms)

So, with a known resistance, you measure the voltage drop across that resistance, divide the voltage drop by the resistance and you get amps.


It matters little, but I believe an analog ammeter is actually an ammeter. The shunt or resistor is in parallel with the meter to act as a current divider and scale the meter according to the incrementation of the meter scale. A voltmeter is also an ammeter only calibrated to read volts and scaled with series, rather than parallel resistors. In my experience most analog meter movements in common use are actually 1milianmp full scale ammeters with external circuitry to make the readings correspond with the meter labeling and incrementation. The ammeter is placed in series with the current to be read, while the voltmeter is placed in parallel with the voltage to be read. :beer:
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Postby brian_bp » Tue Aug 26, 2008 8:37 pm

Larwyn wrote:...It matters little, but I believe an analog ammeter is actually an ammeter. The shunt or resistor is in parallel with the meter to act as a current divider and scale the meter according to the incrementation of the meter scale. A voltmeter is also an ammeter only calibrated to read volts and scaled with series, rather than parallel resistors. In my experience most analog meter movements in common use are actually 1milianmp full scale ammeters with external circuitry to make the readings correspond with the meter labeling and incrementation. The ammeter is placed in series with the current to be read, while the voltmeter is placed in parallel with the voltage to be read.

Yes, exactly. :thumbsup: Well put.

How it works:
A conventional analog meter has a coil of wire mounting on the needle shaft, and sitting in the field of a permanent magnet. A spring holds the needle towards zero, and the amount of current determines how much magnetic force is created to apply a torque against the spring and move the needle up. Tiny spring, tiny forces... tiny current required.

Digital meters, on the other hand, are fundamentally voltmeters, and are made to read voltage or current in various ranges by similar resistor techniques.
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Postby ARKPAT » Wed Aug 27, 2008 12:31 am

Amps=Current flow is measured IN SERIES (inline - or remote inline) with the circuit.

Volts=measures the pressure (voltage - in parallel with the battery or source) of the circuit.



Larwyn Posted: Tue Aug 26, 2008 5:37 pm Post subject:

--------------------------------------------------------------------------------

Cliffmeister2000 wrote:
NightCap wrote:
Do the inexpensive amp meters go inline? For example cut the red wire and insert gauge? Is there a wiring diagram for guages? I have not seen one in my travels through T&TTT.


As I remember it, an ammeter like this is really a voltmeter that measures the voltage drop across a shunt. A shunt is a very exact, very low resistance resistor. The meter is calibrated with Ohm's law, which can be represented as (p=ie, p=i(squared)r, i(squared)r=ie, ir=e, i=e/r (I couldn't figure out how to superscript the 2 to make it represent "squared", so I just wrote squared)

p=power (watts)
i=current (amps)
e=electromotive force (volts)
r=resistance (ohms)

So, with a known resistance, you measure the voltage drop across that resistance, divide the voltage drop by the resistance and you get amps.



It matters little, but I believe an analog ammeter is actually an ammeter. The shunt or resistor is in parallel with the meter to act as a current divider and scale the meter according to the incrementation of the meter scale. A voltmeter is also an ammeter only calibrated to read volts and scaled with series, rather than parallel resistors. In my experience most analog meter movements in common use are actually 1milianmp full scale ammeters with external circuitry to make the readings correspond with the meter labeling and incrementation. The ammeter is placed in series with the current to be read, while the voltmeter is placed in parallel with the voltage to be read.


:thumbsup:

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Postby Larwyn » Wed Aug 27, 2008 8:21 am

ARKPAT wrote:Amps=Current flow is measured IN SERIES (inline - or remote inline) with the circuit.

Volts=measures the pressure (voltage - in parallel with the battery or source) of the circuit.



Larwyn Posted: Tue Aug 26, 2008 5:37 pm Post subject:

--------------------------------------------------------------------------------
It matters little, but I believe an analog ammeter is actually an ammeter. The shunt or resistor is in parallel with the meter to act as a current divider and scale the meter according to the incrementation of the meter scale. A voltmeter is also an ammeter only calibrated to read volts and scaled with series, rather than parallel resistors. In my experience most analog meter movements in common use are actually 1milianmp full scale ammeters with external circuitry to make the readings correspond with the meter labeling and incrementation. The ammeter is placed in series with the current to be read, while the voltmeter is placed in parallel with the voltage to be read.


:thumbsup:

Pat


I think you read less than all of my post and confused the shunt wiring with the meter application.

The ammeter with its associated shunt/resistor is placed in series with the load, however the shunt itself is in parallel with the ammeter to serve as a current divider which is used to scale the meter.

The voltmeter with it's associated scaling resistor is placed in parallel with the load, however the scaling resistor itself is in series with the meter movement for scaling purposes.

I think we said the same thing. I just covered more territory than you did.... :D
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Postby wlooper89 » Fri Aug 29, 2008 4:53 pm

The recent posts about meters have been very good and accurate. I would like to add a simple description in layman’s terms about the meters. Please correct me if this is not generally correct. The analog voltmeters and ammeters, both A/C and D/C, measure voltage by means of an internal coil that moves the meter needle by means of a magnetic field.

In the case of a D/C voltmeter the two wires from the meter are connected to a positive and negative power source, possibly two battery terminals. I believe this may also be described as connected in parallel with the main load.

A D/C ammeter uses a very low resistance shunt that can be either external or internal. In either case the line to be measured is cut and the ends are attached to the two terminals of the shunt or meter. Most of the current passes through the shunt unimpeded, but there is a very slight voltage drop that is enough to power the coil and move the meter needle. This voltage is sent to the meter coil either by external wires from the shunt or internally in the meter. In this case the shunt is connected in series with the line being measured and the wires to the meter, external or internal, are in parallel with the shunt.

An A/C ammeter uses the same type of coil to move the meter needle, but in this case the power comes from another coil that is placed around the A/C wire being measured. The alternating current produces a small current in the coil around the wire by induction, enough to cause the coil in the meter to move the needle.

I hope this sheds some more light on the way ammeters and voltmeters work. It is a bit late in the day so I will visit this post again and edit if necessary.

Thanks, Bill
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Postby Larwyn » Fri Aug 29, 2008 7:56 pm

wlooper89 wrote:The recent posts about meters have been very good and accurate. I would like to add a simple description in layman’s terms about the meters. Please correct me if this is not generally correct. The analog voltmeters and ammeters, both A/C and D/C, measure voltage by means of an internal coil that moves the meter needle by means of a magnetic field.

In the case of a D/C voltmeter the two wires from the meter are connected to a positive and negative power source, possibly two battery terminals. I believe this may also be described as connected in parallel with the main load.

A D/C ammeter uses a very low resistance shunt that can be either external or internal. In either case the line to be measured is cut and the ends are attached to the two terminals of the shunt or meter. Most of the current passes through the shunt unimpeded, but there is a very slight voltage drop that is enough to power the coil and move the meter needle. This voltage is sent to the meter coil either by external wires from the shunt or internally in the meter. In this case the shunt is connected in series with the line being measured and the wires to the meter, external or internal, are in parallel with the shunt.

An A/C ammeter uses the same type of coil to move the meter needle, but in this case the power comes from another coil that is placed around the A/C wire being measured. The alternating current produces a small current in the coil around the wire by induction, enough to cause the coil in the meter to move the needle.

I hope this sheds some more light on the way ammeters and voltmeters work. It is a bit late in the day so I will visit this post again and edit if necessary.

Thanks, Bill


You are close and the details are not that important to connecting the meters. Your understanding of the connections will make the meter work. If we are speaking of analog meter movements, they are in fact ammeters, whether applied as ammeters or voltmeters the movement itself is an ammeter. The external circuitry in conjunction with the labeling of the meter scale will cause the meter to indicate either amps or volts. The external coil that you place around the wire for the AC ammeter Current transformer) mostly serves to decrease the accuracy of the meter at the power levels at which you are working. It is no problem to find an AC ammeter that simply connects in series with the load just as the DC ammeter does for better accuracy. Of course extreme accuracy is not that important in this application so go for whatever you decide..... :thumbsup:
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Postby Joe G » Fri Aug 29, 2008 8:18 pm

This is getting to be a fairly long thread, and I'll admit that I haven't read every single post, but as far as AC/DC ammeters are concerned, I would use neither.

Since it's not very likely that a TD or TTT would ever draw enough current to trip a 30 amp AC circuit breaker; and a DC ammeter tells you very little about your battery's state of charge or reserve capacity, I believe that neither one would be useful while camping. They would be more of a distraction than anything. Just another thing to needlessly worry about.

If you want to get fancy, I would install a milliamphour meter. That would give you far more useful information than either an AC or DC ammeter.

Just my $.03 worth (inflation, don't ya know).
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Postby Larwyn » Fri Aug 29, 2008 8:32 pm

Joe G wrote:This is getting to be a fairly long thread, and I'll admit that I haven't read every single post, but as far as AC/DC ammeters are concerned, I would use neither.

Since it's not very likely that a TD or TTT would ever draw enough current to trip a 30 amp AC circuit breaker; and a DC ammeter tells you very little about your battery's state of charge or reserve capacity, I believe that neither one would be useful while camping. They would be more of a distraction than anything. Just another thing to needlessly worry about.

If you want to get fancy, I would install a milliamp hour meter. That would give you far more useful information than either an AC or DC ammeter.

Just my $.03 worth (inflation, don't ya know).


Joe,

I cannot say that I disagree. I have no meters installed in my TD. I do think that the "electrical guru's" on this group have instigated some to feel that they need to keep and eye on the "amps per circular mill" so they do not warp the space time continuum. Others simply want to keep an eye on current flow. No problem, each to his own. :thumbsup:

The milliamp hour meter might be a good way to keep an eye on battery level, but would a second battery not be more useful, for about the same money??? :thinking: :D
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Postby wlooper89 » Fri Aug 29, 2008 9:21 pm

Certainly I am not an electrical expert, but keeping an eye on the space time continuum is a must! That was funny. I love voltmeters and ammeters! :D

Bill
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Postby Larwyn » Fri Aug 29, 2008 9:41 pm

wlooper89 wrote:Certainly I am no electrical guru, but keeping an eye on the space time continuum is a must! This is a very important discussion and long live voltmeters and ammeters. :D

Bill


No offense meant toward you Bill, hope you did not perceive otherwise.
:thumbsup: :thumbsup:
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Postby wlooper89 » Fri Aug 29, 2008 9:53 pm

Larwyn, in no way would I take offense to anything you might post. I have learned a lot from you.

Best regards, Bill
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Postby 2bits » Fri Aug 29, 2008 9:58 pm

I apologize in advance...

How about a dummy light??? :roll:
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Postby jplock » Sat Aug 30, 2008 12:04 am

I have found it handy to have a analog DC ammeter and digital DC votmeter on the power panel of my TD. The Voltmeter allows to show the sate of charge on the battery, and the ammeter show the current draw off the battery the more amps being drawn the faster the battery runs down. say for example if you have a 70 amp hour battery and you are drawing 10 amps you could draw 10 amps for 7 hours.
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Postby wlooper89 » Mon Sep 01, 2008 3:52 pm

That is a very nice looking panel. I also have a digital voltmeter and have ordered three analog ammeters. One of them is on backorder but hopefully they will all arrive soon. I know I am going overboard on meters. The new ones will be for A/C load, an automotive type that will show battery charge or discharge and one similar to yours that will show trailer D/C load. Photos to follow. :thumbsup:

Thanks, Bill
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Postby wlooper89 » Wed Sep 10, 2008 9:44 am

mechmagcn wrote:
Sonetpro wrote:
I would be more worried about that propane tank you have in there with all of the relays and switches you have in there that can cause a spark. :thinking:

My thoughts exactly, I don't want something as explosive as propane and all of those electrical connections in the same box :o


My new meters have arrived and finding a place for them is a challenge. It looks like the propane tank will have to leave the electrical compartment to be transported outside on the trailer tongue. I was beginning to share your concerns anyway about having the tank in the electrical compartment, even though it is removed from the compartment for use. Looking at the manufacturer web site I learned the tank can give off gas when the valve is closed if high temperature causes a pressure relief valve to open. :o

In place of the tank will be a small panel to hold all the meters and two switches to control selection of converter or battery voltage and meter backlights. :) The space under the panel will be occupied by a pedestal of the same shape that will support the meter panel and house wires and connectors.

Thanks everyone for good advice!

Bill
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