Peter Bennett wrote:

On Thu, 21 Apr 2005 09:22:27 -0700, Keith Hughes
wrote:


2. If the 12V device (VHF IIRC) is a significant load, you can always
build a simple voltage divider (the VHF should be input voltage tolerant
enough to handle the voltage sag during transmission) and run from both
batteries. The overall battery draw will, of course, be somewhat larger
since you'll dissipate heat in the dropping resistor. If multiple
devices are used, however, this approach quickly becomes problematic.

A series voltage dropping resistor may work for some devices where the
current draw is constant, but it _will not_ work for a VHF radio. The
radio will only draw a few hundred mA on receive when squelched,
somewhat more when actually receiving, and a few amps when
transmitting - there is no way a simple resistor can keep the supply
voltage to the radio within acceptable bounds with that current
variation.

Keith mentioned a voltage divider. That's not a simple resistor.

Even a voltage divider is unlikely to be very satisfactory either,
since in order to keep the voltage within acceptable range for the
VHF, the resistance values will probably need to be so low, you'd
be dissipating as much power in the divider all the time as the
VHF on full-power transmit.

One possibility might be to use a zener diode in place of a simple
series resistor.

At the end of the day, having different-voltage devices aboard is
always going to be a pig's breakfast unless you have proper
fully independent systems, with two alternators, one for charging
the 24V batteries, and one for the 12V ones, or a charge controller
capable of charging a 12V battery from the 24V alternator.

On Thu, 21 Apr 2005 21:46:09 GMT, Ronald Raygun
wrote:

Peter Bennett wrote:

On Thu, 21 Apr 2005 09:22:27 -0700, Keith Hughes
wrote:


2. If the 12V device (VHF IIRC) is a significant load, you can always
build a simple voltage divider (the VHF should be input voltage tolerant
enough to handle the voltage sag during transmission) and run from both
batteries. The overall battery draw will, of course, be somewhat larger
since you'll dissipate heat in the dropping resistor. If multiple
devices are used, however, this approach quickly becomes problematic.

A series voltage dropping resistor may work for some devices where the
current draw is constant, but it _will not_ work for a VHF radio. The
radio will only draw a few hundred mA on receive when squelched,
somewhat more when actually receiving, and a few amps when
transmitting - there is no way a simple resistor can keep the supply
voltage to the radio within acceptable bounds with that current
variation.

Keith mentioned a voltage divider. That's not a simple resistor.

Even a voltage divider is unlikely to be very satisfactory either,
since in order to keep the voltage within acceptable range for the
VHF, the resistance values will probably need to be so low, you'd
be dissipating as much power in the divider all the time as the
VHF on full-power transmit.

One possibility might be to use a zener diode in place of a simple
series resistor.

At the end of the day, having different-voltage devices aboard is
always going to be a pig's breakfast unless you have proper
fully independent systems, with two alternators, one for charging
the 24V batteries, and one for the 12V ones, or a charge controller
capable of charging a 12V battery from the 24V alternator.


Come on guys, get back to the proper way to do the job and the
simplest. The 24 to 12 volt converter.

Resistor dividers will draw more power than the radio. A zener has to
be very large for the job at hand. Again it will consume as much power
as the radio.

regards
Gary

Gary Schafer wrote:
On Thu, 21 Apr 2005 21:46:09 GMT, Ronald Raygun
wrote:


Peter Bennett wrote:


On Thu, 21 Apr 2005 09:22:27 -0700, Keith Hughes
wrote:



2. If the 12V device (VHF IIRC) is a significant load, you can always
build a simple voltage divider (the VHF should be input voltage tolerant
enough to handle the voltage sag during transmission) and run from both
batteries. The overall battery draw will, of course, be somewhat larger
since you'll dissipate heat in the dropping resistor. If multiple
devices are used, however, this approach quickly becomes problematic.

A series voltage dropping resistor may work for some devices where the
current draw is constant, but it _will not_ work for a VHF radio. The
radio will only draw a few hundred mA on receive when squelched,
somewhat more when actually receiving, and a few amps when
transmitting - there is no way a simple resistor can keep the supply
voltage to the radio within acceptable bounds with that current
variation.

I did not recall, for sure, what the device was. That's why I hedged.
Whatever the device, you'd have to look at the full load/idle ratio and
see if a voltage divider makes sense. In the OP's case, I'm not sure.

Keith mentioned a voltage divider. That's not a simple resistor.

Even a voltage divider is unlikely to be very satisfactory either,
since in order to keep the voltage within acceptable range for the
VHF, the resistance values will probably need to be so low, you'd
be dissipating as much power in the divider all the time as the
VHF on full-power transmit.

One possibility might be to use a zener diode in place of a simple
series resistor.

At the end of the day, having different-voltage devices aboard is
always going to be a pig's breakfast unless you have proper
fully independent systems, with two alternators, one for charging
the 24V batteries, and one for the 12V ones, or a charge controller
capable of charging a 12V battery from the 24V alternator.


True, but do-able. And relatively cheaply compared to off-the-shelf
solutionsa.

Come on guys, get back to the proper way to do the job and the
simplest. The 24 to 12 volt converter.

I would certainly agree. Shouldn't be too hard to find a 12VDC output,
12-12VDC regulated power supply.


Resistor dividers will draw more power than the radio. A zener has to
be very large for the job at hand. Again it will consume as much power
as the radio.

Hmmm...where have I hear that before??? :-)

Keith

In article ,
Gary Schafer wrote:

Come on guys, get back to the proper way to do the job and the
simplest. The 24 to 12 volt converter.

Resistor dividers will draw more power than the radio. A zener has to
be very large for the job at hand. Again it will consume as much power
as the radio.

regards
Gary

As I posted very early on, one could use a 24-36Vdc/12Vdc Switching
Converter of an appropriate current rating, to charge a small 12Vdc
Group 12, AGM, or Gellcell battery, and then run a 12Vdc buss for all
the loads aboard that require that range of power. This is entirely
consistant with The Fishing Vessel Safety Act, and SOLAS Requirments
for Reserve Power for Required Communications Systems. A lot of the
older Coastal Freighters af less than 1600 Tons that trade in the
gulf of Mexico, and in the North Pacific and Gulf of Alaska, were
32Vdc powered vessels, and the SOLAS requirments were meet using this
type of system.


Bruce in alaska
--
add a 2 before @