On Mon, 17 Jun 2013 11:43:53 -0400, Pat wrote:

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
That behavior is normal. The DC-DC converter almost certainly has a
capacitor across the input, and certainly has a capacitor across the
output. There will be a high initial current draw when power is
applied. Aftehr that it draws enough to provide the required output
current plus a small amount for the circuitry. With no load it's
output will rise until an overvoltage clamp shuts down the control IC.

I'd use a 5A slow blow fuse in the input line with a switch to control
the lights.

PlainBill