24V to 12V

tgeewe · #1 (**permalink**) 01-19-2008

Hi.

Do you think I can use a plain DC adapter to use my 24V boat system with 12V electronics.
I figured that the NMEA ports would still get a 24V signal (and send a low 12V one on the other end). Is that right?

What are my options? What problems are there?

Thanks,
Gabor

artbyjody · #2 (**permalink**) 01-19-2008

You can you will need an Voltage Convertor to drop the 24V to 12V...see
Voltage Converter

its rv based but it is the same principle...

tgeewe · #3 (**permalink**) 01-19-2008

Thats all right. Thats the DC adapter I was refering to (is it?)

But what about the NMEA outputs. Will that have the voltage same as the battery voltage.

I mean the NMEA will be coming from (and also going to) a device using 24V as the power.

artbyjody · #4 (**permalink**) 01-19-2008

Quote:

Originally Posted by tgeewe

Thats all right. Thats the DC adapter I was refering to (is it?)

But what about the NMEA outputs. Will that have the voltage same as the battery voltage.

I mean the NMEA will be coming from (and also going to) a device using 24V as the power.

No you would rewire the NMEA 12 volt input from the Voltage Convertor (which would be 12VDC) not from the battery (24 VDC)..There is no NMEA 24DC that I know of... its all 12VDC based...and if there is then what was the question again?

tenuki · #5 (**permalink**) 01-19-2008

(EDIT: True, but not the point, keep reading, I'm clueless right now, still thinking only of NMEA 0183.)

Device voltage is independant of NMEA voltage.

NMEA talker ports better be meeting the electrical characteristics of EIA-422-A, which specifies 0 - 5 volt balanced. Listener ports are designed to work with that + have optical isolation.

You may have many devices that talk NMEA, and regardless of the voltage that device is powered by (220AC, 18vDC,whatever) the NMEA ports will all talk together.

artbyjody · #6 (**permalink**) 01-19-2008

Quote:

Originally Posted by tenuki

Device voltage is independant of NMEA voltage.

NMEA talker ports better be meeting the electrical characteristics of EIA-422-A, which specifies 0 - 5 volt balanced. Listener ports are designed to work with that + have optical isolation.

You may have a device that talks NMEA, and regardless of the voltage that device is powered by (220AC, 18vDC,whatever) the NMEA ports will all talk together.

Do you have a current example? Out of all the major players it seems 12VDC is standard....not debating there are others but as for NMEA - 12VDC seems standard and would like to know which are not..you can't mix its not like comparing things to amps...

tenuki · #7 (**permalink**) 01-19-2008

(EDIT: this is only partially correct, still not getting the point. See correction in next post)
You can use any NMEA certified device as an example I think.

The NMEA _device_ can be powered by whatever voltage it wants, that is what you are confusing the issue with...

But on the NMEA _bus_ it should be 0/5 volts signal in the case of NMEA 0183 and +/- 2.5 volts signal in the case of NMEA 2000. (essentially the same thing, just different style of differential detection circuitry)

Code:

          12v             5v           24v
battery ------- Device ~~~~~ Device ------ different battery bank

-- = powerlines
~~ = NMEA bus

btw, my only experience with NMEA to date was using a serial port sniffer to reverse engineering a certain locked vendors non standard protocol extension so that I could write some software to read it, so I know far more about the structure of the application layer and data link layer protocols than the electrical layer specifics, but hey, this is designed to go over RS232 originally and I do know how to interface to that....

tenuki · #8 (**permalink**) 01-19-2008

Ah ha, I see our disconnect and my ignorance.

Quote:

Originally Posted by tenuki

(just different style of differential detection circuitry)

NMEA 2000 (which I was unfamiliar with) requires a reference voltage on the bus, this is what you are talking about, not the signaling voltage which is what I'm talking about.

The answer to the OP is the spec requires 9-16 volts as the reference power. This means 24 volt systems require a step down DC converter to 12v if you are going to use them to power the bus.

Right on Jody!

tenuki · #9 (**permalink**) 01-19-2008

Here's a good install guide that mentions the 24->12 converter requirement to power the bus.

And for good fun the schematic (NMEA 2000 interface) that knocked some sense into my thick skull:

tenuki · #10 (**permalink**) 01-19-2008

OK, Quick Tenuki post count for this thread...

2 inaccurate posts (my first two)
2 posts with marginally useful info (my second two)
1 totally nonsense post. (this one)

hmnn, I'm skewing a bit, usually its more like 5-1-20, I'll have to work on that...