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Old 08-08-2002, 09:43 PM   #1
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Question Ammeter

I have a 71 Overlander with a univolt.

Does any one know how the ammeter works? It is wired in parallel to the 12 volt circuit.

I am almost certain that the only way to measure amps is to be wired in series to a dc circuit.

Of course one can use an amp meter which measures flux passing thru a coil but I am interested in getting the original ammeter to work but I cannot see how it works theoretically.

I do not get any indication and I have checked both the manual and I have verified that ther are 12 volts on both sides of the ammeter which indicates that it is wired in parrallel.

If there aare any good ol airstream electricians out there, I could sure use a good lesson on DC theory.

Thanks

smily
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Old 08-09-2002, 06:18 PM   #2
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Smily,

If you look at the fuse panel in your Univolt, you will notice a copper plate about 1X3 inches in size. Mine is a 73 Univolt so it might be alittle different. This plate is called a shunt. Your ammeter is wired in parallel to the shunt. Together they are wired in series to your supply as you would expect. Most of the current passes though the shunt and a very small amount passes through your ammeter (The shunt is effectively is a big fat short with less resistance than the meter). You don't want the full current to pass through the meter as it would burn it up. Remember, current follows the path of least resistance. If you look at the shunt, you'll notice there is an adjustable stud that can be slid up and down a slot. This adjustable stud changes the resistance of the shunt and is set or calibrated by Univolt to achieve full scale indication on your meter at full current. It sounds like you may have a bad ammeter. If the meter is bad (or open), then the 12V you are reading across the meter is really the drop across the shunt. This also says your wires are good.

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Old 08-09-2002, 06:43 PM   #3
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Shunt Ammeter

good to hear from somebody who is electrically oriented.

I understand the theory of how the shunt works. I do have the "plate" with the slot in it. I guess the ammeter is a resistor but how does the current return to ground? Through the switch to the PC board maybe?

I do have twelve volts on both the red and the black wire as indicated on the schematic in my owners manual. I have tested both leads at the control panel to ground and there is definitely twelve volts present.

You may be correct that my ammeter is deficient. I have twelve volts on one side of the ammeter but I do not get any voltage on the opposite side, or secondary if you will, even when the switch is depressed. Do you know where I can get a replacement ammeter?

I have the PC board out right now and I am cleaning it up.

I have read somewhere in this electrical section that someone replaced the small resistor below the fuse. I do not see if he had success or what his problem was before replacing but my question is, what is the resistor for and what is the value of the resistor. Mine seems to be faded some what.

Please see my thread about all of my indicators seem to stay on including the "Power on Light" I have checked the grey lead at botht the Univoly and the control panel and there is twelve volts even when the trailer is NOT connected to shore power, (120 volts)

I would welcome a conversation with you. It seems that you are pretty electrically savvy.

Thanks,
smily
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Old 08-09-2002, 08:52 PM   #4
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Ammeter

The meter only indirectly measures amps.

The Ammeter is merely a very sensitive voltmeter that reads the very small drop across the resistance of the shunt. For instance, if resistance of the the shunt is .001 ohms and you are pulling 45 amps, the voltage across the shunt would be .045 volts. The ammeter is marked in amps, but it is really reading volts across the shunt.

Unless you use a very sensitive voltmeter, you should see the same voltage on both sides of the ammeter.

If the shunt is opened, the ammeter will almost certainly be fried.
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Old 08-10-2002, 08:30 AM   #5
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voltmeter????

I cannot understand how any voltmeter can read volts without a complete circuit. If I hold both leads to the negative post on the battery, I will not read voltage. and Likewise with the positive post of the battery.

If I hold on lead on the positive and one on the negetive, I would expect to see voltage indicated.

I am having a hard time understanding how any voltage meter and ammeter can possibly work in parallel to the negative "buss"

I understand that there may be some resistance between the slot wire and the other wire on the copper plate but the plate is Parallel to the negative wire.

Sparky
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Old 08-10-2002, 08:41 AM   #6
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If I understand John's electrical description, I think I do. The best way to describe it would be to say you are reading the difference between to DC sources.

If you were to take 2 diffrent 12 VDC Batteries and check them + to - individually you would read 2 different voltge levels. If you the read + to + on the two batteries your meter would show the difference in voltage between the two. The reading on all voltmeters is based on the reference ground that is attached to the black or - lead. If you raise the reference ground the overall reading will be less.
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Old 08-10-2002, 08:50 AM   #7
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What you are measuring is the voltage drop across the shunt. The greater the current flow, the larger the drop (voltage=current x resistance). Even though the shunt is in one leg of the circuit, there is still a + and - end to it. You are measuring the the difference between the ends, reading it on an ultra sensitive voltmeter marked in amps.

John
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Old 08-10-2002, 09:54 AM   #8
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Good answer 74Argosy

You are right and you described it better than I did. There is a complete circuit for the ammeter. You can draw a complete circle through the shunt and the ammeter ... that is the circuit.

Everything has some resistance, even a copper shunt. You wouldn't want a shunt with a lot of resistance because you would get less voltage to the things that matter like lights. Ohms law tells us that E=IR (volts = amps times resistance). The E in this case is the drop across the shunt. In my example, if the shunt has a R (resistance) of .001 ohms and you pull 45 amps, then there will be .045 volts dropped across the shunt and 11.955 volts to the world (assuming an exact 12v source). That .045 volts is what the meter measures.

Every voltmeter has some resistance and allows a very small current to flow (as a previous poster mentioned). The meter has a resistance, in the nature of millions of ohms, and current through the meter is only sufficient to move the needle.

Whiew! Haven't thought much about these things since I retired after designing computers for IBM for 35 years.
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Old 08-10-2002, 10:14 AM   #9
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Thanks

I have worked with it for years, is now simple for me to understand. But it can be a real bear to explain. Makes me appreciate what my instructors had to go through to get a bunch of kids focused to learn and understand something they couldn't see or touch. You must have started about the same time as me, when plates and grids and B+ reigned supreme.

John
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Old 08-10-2002, 12:50 PM   #10
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ET Phone home

Good to be amongst real ET's.

I am merely a glorified electrician that has evolved to electronics and I have some formal training in electrical theory but it has been awhile.

You could say I am one of those young kids in the airstream world but I have always recognized wisdom when I see it. Only one way to get wisdom and I am certain that all of the posters in this thread have much more experience and wisdom than this young whipper snapper.

Thank god!1 and the Airstream forum!!

Thanks
Smily
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Old 08-10-2002, 02:33 PM   #11
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Smile Old timers

You are right. I started with tubes and crystals as a teenager in WW2.

My dad was an electrician and I started dabbling in electronics early, crystal radios and such. My first real electronics training was courtesy of the USAF RADAR school at Keesler AFB during the Korean War (1951-2). All tubes, of course.

I earned my commisision in 1953 and instructed in F-86D Sabres for 3 years till Uncle Sammy decided he had too many pilots. I then used my RADAR training and commission to get on with IBM.

I fixed machines for a couple of years and then transferred to development where IBM filed 30 patents in my behalf. I think 29 patents actually issued. In the heyday of magnetic tape systems, I worked on most ofthe latest and fastest. I was one of the first to put a microprocessor in a computer control unit.

I retired from IBM in 1991, but worked as a consultant for several years.
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Old 08-11-2002, 12:01 PM   #12
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ET Origins

John, Like you, I can thank Uncle Sam for my Elecctronic Training.
Before World War II, I was an enlisted man in the 11th US Cavalry Horse, assigned to the Message Center in Machine Gun Troop. As a Messenger/Radio Maan, I was trained in Semaphore Flag, and we had hand crank electric generators to power an AM radio transmitter/receiver., where I was taught and used Morse Code in communications ( As an aside=with this training, I got my first "HAM" radio license - still licensed with AdvCl KD8VD). Went on into Army Air Corps, learned electronic armament and became a Tail Gunner on a Flying Fortress in WWII (Pacific War) After Commissioning in the Army, I became a Communications Specialist and Cryptographic Custodian in the Signal Corps. after which I got my Doctorate in Environmental Health and the Army Sent me to Post Doctoral = Medical Electronic Instrumentation School to become an Electronic Medical Instrumentation Specialist assigned to the Army Medical Corps in the office of the Army Surgeon General in the Pentagon. Of all my educational experiences in five universities, and three Army Electronics schools, the training I received at the Army's Medical Instrumentation School was the most challenging . That school has proveded me with the necessary talents to cope with automotive and "motor home elecytronics". I keep a small VAC Meter, which I bought at Radio Shack for $30.00 in my glove box in Silver Belle. I would rather have one of the old "Simpson" meters but this one adequately serves my needs. There is a web page associated with the Old "Airstream Bulletin Board" whjich has a number of topics on servicing and maintaining electrical and electronic circuits on trailers and motor homes which I found to be very informative.
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Old 08-11-2002, 12:48 PM   #13
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"Bug Shooting"

Most of my time with IBM was developing machines before the modern age of computer simulation. We put them together, then "shot the bugs" to make them work correctly. I always found that part of the work to be the most exciting.

I remember once having my face in an oscilloscope for a couple of hours straight, chasing a really elusive problem, When I finally saw the problem, I jumped up and cheered. Then, I saw the Director of the Laboratory and about 4 top managers standing around cheering with me. All of them had done the same thing on the way up and knew just how it felt.

I troubleshoot almost by second nature now. It is much harder to explain to someone long distance how to go about it.
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Old 08-12-2002, 08:54 PM   #14
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ET nostalgia

Wow! truly an honor to hear from men of your staure. I am currently employed by a government contractor and to work in Cryptographic communications but I apologize for nor being able to contribute to the greatt feats that you gentlemen have attained.

I am humbled and proud to be associated with this great american tradition and great minds that comprise the membership.

Respectfully, Ken Smillie

AKA Smily
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