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Old 02-21-2014, 08:45 PM   #1
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12vac-12vdc switch goes to this, what for?

So we were looking at my electrical today (new to me boat) and found this box, which appears to do nothing. It was wired to the ac-dc switch on the main panel. Anybody know why its there and what it might do?
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Old 02-21-2014, 08:52 PM   #2
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Not positive, but it looks like a 12 volt power supply for at the dock boat power.

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Old 02-21-2014, 09:01 PM   #3
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It's pretty hard to tell what it might do from here, but what happens when you flip the switch? How is it connected into the boat's electrical system? What's inside the box?

Did the surveyor have anything to say or write about it?
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Old 02-21-2014, 09:11 PM   #4
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Nothing happens when you flip the switch, didn't look in the box yet. Didn't have a chance to trace all the wires yet. It was hooked up to the elec system, we think the "box" is dead and just wondered what it was so we could figure out what it did. We are probably going to rewire this entire boat, or most of it anyway. At least we will know why things are the way they are that way.
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Old 02-21-2014, 09:13 PM   #5
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Nothing happens when you flip the switch, didn't look in the box yet. Didn't have a chance to trace all the wires yet. It was hooked up to the elec system, we think the "box" is dead and just wondered what it was so we could figure out what it did. We are probably going to rewire this entire boat, or most of it anyway. At least we will know why things are the way they are that way.
I don't think the builder or a previous owner would have installed a box and switch that does nothing. Have an electrician figure it out. It could be important.
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Old 02-21-2014, 09:17 PM   #6
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Well I can believe that. Funny, I'm sure it did something at some time, but the PO had a picture of it stapled to a piece of legal paper with a big question mark on it haha.
Again, thanks for your input.
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Old 02-21-2014, 10:07 PM   #7
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Can't think of why you would need 12v AC unless it's to power a transformer? It might be an inverter which provides 12vac for a particular electronic device. Maybe an SSB? Seems to me we used to have dynamotors in our aircraft to provide 24v AC to run an HF radio as there was nobody to talk to on VHF and at that time a Dynamotor was the only way to generate AC from DC.
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Old 02-22-2014, 12:00 AM   #8
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I bet there is a 120v to 12v transformer in the box, and the switch selected the source from 12vdc batt or from 12vac from transformer. Who knows what the load was, maybe a blower, fan, water pump??? Some DC motors will run on ac or dc.
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Old 02-22-2014, 12:26 AM   #9
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Also 12VDC incandescent lighting can be run from 12VAC. There was a thread several years ago and the question was what do these two transformers do. That's what they did, run the 12VDC lighting when plugged in at the dock.

This may be some version but a little nicer looking, ignoring the rust.
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Old 02-22-2014, 12:41 AM   #10
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It sure looks like a power supply to me. I worked on a lot of that stuff in the military. Just put it to one side or the other and throw a meter across it. It may still work!
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Old 02-22-2014, 02:34 AM   #11
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Correction - dynamotors convert DC to AC.

Senior's moment.
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Old 02-22-2014, 07:42 AM   #12
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Or low voltage DC to high voltage DC.
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Old 02-22-2014, 09:05 AM   #13
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Also 12VDC incandescent lighting can be run from 12VAC. There was a thread several years ago and the question was what do these two transformers do. That's what they did, run the 12VDC lighting when plugged in at the dock.

This may be some version but a little nicer looking, ignoring the rust.
If I had to guess, that would be my guess but I am at a loss as to why someone would run 12 volt lighting from a transformer when 12 DC is available from the batteries. If you're on shore power, the charger would make up the difference and then some.


The terminals on the top lead me to believe that this wasn't originally built as something for a boat, it looks more like something a technician would have on a bench for testing equipment.
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Old 02-22-2014, 10:01 AM   #14
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If I had to guess, that would be my guess but I am at a loss as to why someone would run 12 volt lighting from a transformer when 12 DC is available from the batteries. If you're on shore power, the charger would make up the difference and then some.


The terminals on the top lead me to believe that this wasn't originally built as something for a boat, it looks more like something a technician would have on a bench for testing equipment.
if I think about it years ago they had a way of converting 120 to 12 volts dc and you used that instead of using the batteris for n12 while at the dock.that way you did not have to run the batt charger for 12 volts.back in the day the chargers were not as good as they aare today
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Old 02-22-2014, 10:18 AM   #15
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Reminds me of other boats where electrical engineers were owners at some point. There is no limit to the creative yet random mix of strange electrical and electronic devices they will add to the boat systems. Then after they sell the boat, someone has to try sort out what they did.
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Old 02-22-2014, 09:58 PM   #16
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if I think about it years ago they had a way of converting 120 to 12 volts dc and you used that instead of using the batteris for n12 while at the dock.that way you did not have to run the batt charger for 12 volts.back in the day the chargers were not as good as they aare today

That is what I suspect happened. Now chargers are so good that they act as serious battery chargers AND power supplies for large loads that really involve the batteries not at all.

This and the older thread involved older boats that someone discovered these xfrmrs and then wondered what it was. Ski's point is dead on.
Some one is going to find one of those on my old fridge and wonder what the??? although I did leave an explanation and schematic.
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Old 02-24-2014, 09:40 AM   #17
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Reminds me of other boats where electrical engineers were owners at some point. .
Or thought they were electrical engineers.
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