"Pat" wrote in message
...
On Tue, 18 Jun 2013 08:12:54 +0100, "N_Cook"
wrote:

Pat wrote in message
. ..
Does anyone here have any experience with dc to dc converters? I need
a unit that takes approximately 48 volts input (Four 12V Lead-Acid
batteries in series) and provides 12 volts out at a few amps. This
will be used on a pontoon boat to supply 12V for navigation lights
from the 48 V battery bank that drives the electric motor.

I found a module on the Internet that can handle 10 A output at 12
volts and only costs $25 (USD). Many no-name websites offer it
anywhere from $22 to $40. All use the same exact picture. I picked a
site that seems less scary than others and ordered one. It arrived
very quickly.

At first, it seemed defective, but after playing around a little, I
think I figured it out.

Now to my questions:
1) It is normal for such a device to draw high current when first
connected? It draws quite an arc when the 48V line is connected but
then only draws 31 mA with no load. Removing power for just a second
starts that process over - ie, draws an arc with a loud snap when
connecting it. I was using a switch when I first tried it and heard
the snap from inside the switch. I then changed to a jumper wire and
could see the arc.

2) Do these things normally have a minimum load spec? With no load,
the voltage starts at 12.34 V and then drops to 4.7 V or so over a
period of 30 to 40 seconds. It then jumps back to 12.34 and starts
the process over. The instructions don't mention a minimum load, but
I tried adding a lamp that draws a few amps. The output was then
steady at 12.31 Volts. I don't have a scope, so I have no idea what's
going on over timeframes shorter than a second or so.

Anyone else ever buy one of these things? Is the above behavior
normal? I guess you get what you pay for and $25 isn't much. It is
nicely potted so water resistance isn't an issue. But that also means
I can't take it apart to see what's inside. The label says "TOBSUN
THJ4812C120Z 120W DC-DC CONVERTER".

Pat

Could you not rewire the boat lighting so 4 lamps are in series, with some
sort of LED failure indicator or something to show which lamp/wiring has
failed , when it does. One for red, one green and 2 riding lights, about 4
are used together , never separately. Then for at anchor. and just one
light. then a 48V one or 2 or 4 24V or 12V smaller wattage in one housing.

Thanks to all for your comments and suggestions. I'll do some testing
to see what minimum load will keep the output at 12V. I may also add
a small resistor in series with the input to keep the inrush current
down a little. I realize that will lower the overall efficiency, but
I hate that snaping sound the switch makes with nothing limiting the
inrush. Regarding the suggestion of putting the lights in series, I
thought of that, too. However, the lights (4 of them total) are not
even close to being matched loads. The red and green only draw 60 mA
each. The anchor light draws 175 mA. The spot light used for docking
draws 1.75 A all by itself. (All these are LEDs with their own
internal current limiting resistors).

Anyway, thanks again for your comments.
Pat


Given that the currents of the running lights are so low, and the one that
draws a bit more is only used for docking, I think that you are making
things needlessly complicated for yourself. The original suggestion that
someone made of just tapping the feed from one battery up the stack, is the
perfect and sensible solution. With a total current draw of less than 300 mA
(excluding the docking spotlight) this will not cause any significant
'unbalancing' of a high current-capacity lead acid stack at all. If it did,
then there would be something seriously wrong ...

Every time you add any kind of electronics to a simple wiring scheme, you
are introducing a further layer of potential unreliability. Bear in mind
also, that this is exacerbated in a marine situation by the intimate
presence of corrosive seawater. Any kind of simple electronic 'dropper' from
48 down to 12 volts, is going to dissipate significant power and run hot
unless it is a switch-mode type, in which case, it ceases to be simple,
opening the way for even lower reliability. If you really *don't* want to
use a single battery solution, then the other simple way to do it, given
that you've now told us that the nav lights are LEDs with an appropriate
internal resistor for 12 volt operation, is to hang the lights across the
full 48 volts, but quadruple the series resistor. Or if you can't easily get
to the resistor to replace it, add an external one of three times the value
in series with each light.

Arfa