DIYbanter - View Single Post - dc-dc converter

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

[48 V to 12 V DC-DC converter]

1) It is normal for such a device to draw high current when first
connected?

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.

I agree with this. The input capacitor will appear as a dead short for
a little while before it starts to charge up; this is why you get the
big arc.

Really big DC-DC converters and DC-AC inverters (several hundred watts
on up) are sometimes switched on in a two-stage process. First, the
battery is connected to the converter/inverter through a smallish
resistor for a few seconds, to charge the input capacitor part way.
Then, the battery is connected directly to the converter/inverter, with
no resistor. This saves a big arc at the switch or relay contacts and
makes the switch or relay last longer.

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.

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.

Also agreed. The actual minimum load to get it to put out 12.3 V all
the time is probably not very much - on the order of a few watts or so.

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

Make sure you have a spare fuse *on the boat* if you do this.

Alternatively, use a 5 A circuit breaker.

Another suggestion along this line:

You might wire a tap to the "lowest" 12 V battery anyway. Run the nav
lights through a SPDT switch, with the lights to the common or wiper
terminal. One end of the switch goes to the output of the DC-DC
converter and the other end goes to your 12 V tap. Most of the time,
you power the nav lights from the DC-DC converter, and your propulsion
pack stays nice and balanced. But, if the DC-DC converter ever fails,
you can flip the switch and still have nav lights to get home with, at
the cost of a slightly unbalanced propulsion pack.

Matt Roberds