Electrical question????? - Fiberglass RV


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Old 08-14-2012, 02:52 PM   #1
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Electrical question?????

I have several 12v accessories that have 120v adapters, for example a radio. My question is when I find a 12v adapter that fits the radio do I have to worry about the fact it uses 500 m/a on the 120v adapter? In other words will it harm the unit if there is a lot more milliamps available? Is it like a/c where your draw of amps is automatic, dependent upon the needs as long as it is 120 volts? I can plug in a 1 amp or a 12 amp appliance and both will work. Is it the same for 12 volts?
Thanks for any help.
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Old 08-14-2012, 03:21 PM   #2
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I believe that parties representing both AC and DC have agreed to the terms of Ohms's Law.

No problem.
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Old 08-14-2012, 03:28 PM   #3
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Thanks Tom,
While I have you, I notice some of my adapters say 9v others 12v, both have the cigarette type plug. Is this a concern?
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Old 08-14-2012, 03:31 PM   #4
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Jim

As long as the adapter is really the same voltage and is able to supply the current required by the device AND is wired like the device needs then yes it is OK to use any mating adapter.

I say it this way because some devices will have DC adapters but some will be AC step down adapters and you need to be certain that just because it says 12volt that it is dc or ac as the device meeds.

Also I have seen devices wired with the center pin being the positive wich is sort of the standard but every once in a while you will see one wired the opposite way.

If you make a mistake and let the smoke out well....

Does this answer your question and why do you ask exactly?

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Old 08-14-2012, 03:44 PM   #5
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Originally Posted by cpaharley2008 View Post
In other words will it harm the unit if there is a lot more milliamps available?
Thanks for any help.
As long as there is enough current available ( the rating), the device will draw what it requires. In other words, the voltage is set by the source, the current is set by the device. Ohm's law only works for linear ( purely resistive) devices. Most things you plug in are not linear. Raz
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Old 08-14-2012, 03:48 PM   #6
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Thanks everyone, I have accumulated over the years a dozen 12v adapters, some for gadgets I no longer have. I'm trying to outfit the new rig with a nice am/fm radio as well as a weather alert system, all of which operate off both 120 and 12v. I'd rather not carry both adapters but limit myself to 12v only.
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Old 08-14-2012, 04:19 PM   #7
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Whatever device you plug in to the 12V adapter will self limit the current it draws so there should be no problem with having excess current available. It's the same thing as when you plug your electric fan into a wall socket. Your house may have 50 or 100 amps available from the pole but your fan still only draws an amp or two.
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Old 08-14-2012, 04:59 PM   #8
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Thanks Tom,
While I have you, I notice some of my adapters say 9v others 12v, both have the cigarette type plug. Is this a concern?
To be clear on these adapters - do they have a cigarette lighter plug on one end and a small round plug on the other? And they are labeled either 9 volt or 12 volt?

If so, you would need to use the correct adapter with each appliance - i.e. 12 volt adapter for 12 volt appliances and 9 volt adapter for 9 volt appliances. I'm pretty sure the adapter has an internal device to make sure that the output is the correct voltage for its appliance when plugged into a 12 volt power source.
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Old 08-14-2012, 05:01 PM   #9
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Polarity may still be an issue.
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Old 08-14-2012, 05:12 PM   #10
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To be clear on these adapters - do they have a cigarette lighter plug on one end and a small round plug on the other? And they are labeled either 9 volt or 12 volt?

If so, you would need to use the correct adapter with each appliance - i.e. 12 volt adapter for 12 volt appliances and 9 volt adapter for 9 volt appliances. I'm pretty sure the adapter has an internal device to make sure that the output is the correct voltage for its appliance when plugged into a 12 volt power source.
Yes, cigarette and round plug other end. So if one radio is 9 v use the 9v plug and other radio is 12v use that one but not the reverse?
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Old 08-14-2012, 05:15 PM   #11
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The 12 VDC appliance will probably just not work on 9 VDC. The 9 VDC appliance may smoke on 12 VDC.
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Old 08-14-2012, 05:19 PM   #12
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So how does a 9v cigarette plug reduce the voltage from 12volts?
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Old 08-14-2012, 05:22 PM   #13
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So how does a 9v cigarette plug reduce the voltage from 12volts?
It would use a DC to DC converter. They used to be very expensive, but less so now. The ones that I have dealt with chop the DC into a dirty AC, then use a transformer and rectify it back to DC. Hopefully they are more solid state now.
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Old 08-14-2012, 05:23 PM   #14
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There may be a converter or adapter inside it to reduce the voltage while letting you connect directly to 12vdc.

That is why I mentioned being certain about the voltage ad orientation of the leads even if they look identical.

Better safe than letting smoke out!
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