Understanding MH electrical systems

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JayArr

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There was a standard many years ago where 12V was injected into the coax with the signal to power portable TVs. Take a voltmeter and check the coax connector to see. The 12VDC didn't affect the signal from the antenna and a special TV had a circuit to separate the DC from the signal.

It wasn't terribly popular and never caught on but I did repair a few dozen TVs like that back in my CRT repairing days.
 

Old_Crow

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There was a standard many years ago where 12V was injected into the coax with the signal to power portable TVs. Take a voltmeter and check the coax connector to see. The 12VDC didn't affect the signal from the antenna and a special TV had a circuit to separate the DC from the signal.

It wasn't terribly popular and never caught on but I did repair a few dozen TVs like that back in my CRT repairing days.
Don't the Dish Tailgater systems run voltage through the coax to power the dish?
 

JayArr

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Possibly. There was also a company called Blonder Tongue that ran 12V through the signal coax to power antenna mounted booster amps. That was way back in the 80s. That has nothing to do with the OPs question though. I just thought the 12V thing might apply since there was a coax port and no apparent power port.
 

Tiercel

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I checked the coax cable for voltage but nothing registered. I have some other electrical issues. I'll post later.
 
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Tiercel

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On Monday I tied into the wire nut splice of the sink light. The purpose was to install a 12VDC socket. The light and the socket worked.

0n Wednesday I noticed 3 lights downstream from the sink light did NOT work. I suspected it was due to my !2VDC socket installation.

I checked light switches, bulbs, and fuses. - I re-did the wire nut splices

I then started mapping the DC fuse panel and checking the current at the fuse. Strangely (to me) the circuit with the sink light and 3 dead fixtures, gave a reading of 6VDC at the fuse. When I checked the current at the sink light (same circuit) it was 12VDC -I don't understand that.

Other unrelated things I found curious are:

A reading at fuse 6 and Acc2 showed no current.

Readings at Acc 1 and Acc 3 showed 14.5 at the fuse.

Below is a photo of the updated fuse layout. Any reading other than 12VDC is in red. I also threw in a photo of the circuit of the working light and the beginnings of the non-working lights to the right. You can see the cap of the 12VDC socket I installed hanging down to the left of the working light.
 

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Kirk

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I then started mapping the DC fuse panel and checking the current at the fuse. Strangely (to me) the circuit with the sink light and 3 dead fixtures, gave a reading of 6VDC at the fuse. When I checked the current at the sink light (same circuit) it was 12VDC -I don't understand that.
Something is wrong with your measurement. What you report can't be accurate.
A reading at fuse 6 and Acc2 showed no current.
I suspect that you mean no voltage? Current is in amps.
 

Tiercel

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Correct and correct on no voltage. Sorry for getting loose with my terms.

Something is wrong with your measurement. What you report can't be accurate.
Kirk, I am not as concerned about the circuits showing no voltage because if there is one thing on the circuit and a switch is open then the circuit is open.

My biggest confusion is the 6 volt measurement (taken a dozen times) at the positive/negative sides of the fuse socket while the fuse was removed. Yet voltage measured at the sink light on that circuit registers 12 volts. Also 12 V measured at a wire nut junction on that circuit. Same meter, same settings, minutes apart.

When I started to initially move the wire nuts I was going to tie into at least one wire popped out. They were really sloppy splices so I cleaned them up. All I can figure is that I crossed a wire but I would think that would blow a fuse.

Since the sink light still worked and the installed 12VDC socket worked I never thought to check the other lights down line but I am sure that was what caused the issue.
 
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Kirk

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I reread your post and I don't really understand where you were measuring. I can tell you that all electrically common points, such as a junction of multiple wires will always be at the same voltage. If you have lights in parallel they will all see the exact same voltage but if they are in series the voltage will be divided among them. For example, if you were to put 3 identical lights in series each one would see 1/3 of the 12V supply or approximately 4V but if the same lights were in parallel they would each see 12V. The problem with series lights is that if one should burn out all of them will turn off. With the series lights, if you measure from ground to the input of the first you would measure 12V, to the input of the second 8V and to the input of the third 4V and from the downstream side of the third to ground would measure 0V. If the bulb were to be removed from the third light the input to any of them would measure 12V to ground.
 

Tiercel

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I made a raw novice blunder. When I tugged on the wire nut connections a couple wires popped out. The original connection was crazy sloppy. I trimmed/stripped wires to redo them. I kept them clustered as a group and was sure I put them back the way they were without analyzing the color code or layout. I had 4 pair - red/black, black/White, stripe/white, stripe/white.

I stuck a positive wire wth the wrong wirenut splice. So the down line lights got no juice.

It is all fixed now. I also got my battery monitor operational but only temporarily mounted until I decide where I want it and if I will buy extra wire.

This monitor will make me intimately familiar with how much current everything draws. It’s the kind of thing I will obsess over. After a few hours I could never go back to the no information monitor that came as standard equipment.
 
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Mark_K5LXP

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Just to throw this in as food for thought, when you measure some arbitrary voltage, i.e. 6V in a 12V system what should immediately pop into your head is high impedance. Your meter is likely hundreds of kilo ohms if not a megohm input impedance which means it could literally measure a fly fart in terms of energy. So when you stick it into a circuit just about any current flow - intentional or unintentional - has the potential (sic) to register a voltage. So in the aforementioned example to see 6V with a meter that is sensitive to millionths of an amp means whatever else was in the circuit was only conducting this miniscule current. In the context of an RV that would more than likely be a 'leakage' current or something electronic drawing standby current. One way to positively test for a power source or ground will be not with a sensitive meter but a device like a test light that draws some nominal amount of current, enough such that it requires a solid connection and not just a weak conductive path. That way you know whatever you're measuring is a 'hard' connection and not just standby current or a 'wet spot' across some terminals somewhere.

Mark B.
Albuquerque, NM
 

Tiercel

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Btw, I get 12VDC at the fuse plug on the DC panel now. Same place I repeatedly got 6VDC before. I will have to scratch my head some to think that through but that’s the story.
 

Tiercel

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Mark, I just saw your post. I appreciate your input but it will take a re-read when I get a break for my brain to process it.
 

Kirk

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So when you stick it into a circuit just about any current flow - intentional or unintentional - has the potential (sic) to register a voltage. So in the aforementioned example to see 6V with a meter that is sensitive to millionths of an amp means whatever else was in the circuit was only conducting this miniscule current. In the context of an RV that would more than likely be a 'leakage' current or something electronic drawing standby current.
I must be getting old or something as I have no idea what you are driving at??? Current flow has nearly nothing to do with measurement of voltage. An open circuit with on current flow at all will always measure source voltage. It is true that a volt meter does actually draw a tiny current but very few meters are sensitive enough to detect the current used by a volt meter and no $50 or less meter will do so.

I have been using my faithful Fluke 77 for years now and it is a pretty sensitive and accurate meter. I had one at work as well as in my personal tool box and it is both accurate and reliable, but is far more meter than most RV folks actually need.
 

Mark_K5LXP

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I'm driving at measuring a voltage drop at a high impedance node in a circuit. Since a DMM represents a high Z load then it doesn't take a lot of current through that load - and all other components in that circuit - to register a voltage. Turns out a Fluke77 has an input Z of 10 megohms, so in order to register a 6V measurement in a 12V circuit means it's 50% of the resistance. It's not hard to picture 10M of stray resistance in any given circuit so one can easily see random voltages with a sensitive meter. Some models of meters have a low Z mode, not sure about the Fluke 77 but using a low Z meter or indicator like a lamp will eliminate "ghost" voltages due to sensitive meters in high Z circuits.

Mark B.
Albuquerque, NM
 
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