At 11:06 AM 9/7/2006, JC Smith wrote:
>I'm going to be running some DC powered equipment on my tower. Preamps at
>least, maybe a downconverter and perhaps a 1.2 GHz brick. I've been
>planning to run power to the tower and install a DC bus in a raintight box.
>I have a number of surplus power supplies that have terminals labeled +s
>and -s (sense). They are currently jumpered to the + and - output terminals
>but if the jumpers are cut and two separate pairs are run to the load, the
>power supply is supposed to adjust the voltage at the output terminals to
>maintain the correct voltage at the load (compensate for the loss in the
>connecting wires).
yes.. that's how it works (although, there are typically some limits
on how much compensation you can get... 1.4 Volts (two diode drops)
might be as much as you get (because they internally connect the
sense and out with a diode to limit the output voltage in case you
inadvertently lose the sense wire)
> Does anyone have experience with this? I don't really
>want to run super-heavy gauge wire to the bus box and this looks like a good
>alternative. Will the power supply adjust over its fill range to
>compensate? Some of them are variable from 0 - 32 Volts; that could allow
>the use of fairly small wire, but there must be some limits on how this
>system works. Unfortunately, I have no manuals for the supplies, but I am
>going to look for them.
>
>Anyone with experience with using the +s and -s feature of these supplies?
Something to watch out for: Those sense leads go into the error
amplifier in the voltage regulator, and may be quite RF
sensitive. There may also be a limit on the capacitance or
inductance of the sense leads (to prevent the regulator from
oscillating). They're really not designed to deal with tens or
hundreds of feet of wire.
What you might want to look into is running AC power from a "class 2
current limited" source (e.g. a landscape lighting transformer or
similar) at 24VAC and then rectifying and regulating locally.
Another approach is to use a constant current source at the sending
end and a constant voltage regulator at the receiving end. Something
like a 7812, for instance. You can also buy commercial devices
intended just for this kind of application (e.g. they run off the
loop current in a 4-20mA measurement system), but they're pretty low
power, as a general thing.
If you desperately want to use 110V supplies (maybe you've got a
bunch sitting around in a box that you got cheap).. you can send 24
or 40VAC up, and then use a transformer to make 120VAC to run the
PS. This scheme gives you galvanic isolation, which is nice on long
runs. You can find 50-200 VA sized low voltage transformers
surplus. Something that is VERY common as a surplus item in decent
sizes is a 480:240:120V transformer. They're used to get power for
120V stuff from a 480V line in a factory that's wired 480 delta. You
can run 120VAC into the 480V winding and get 30VAC out, which is a
nice low voltage to run out to the tower.
Or run a 24VDC supply and use a wide input range DC/DC converter at
the point of use on the tower. 9-36V inputs are fairly common, so
you could tolerate 10-15V of drop and still work just fine. You can
get a wide variety of power levels from a few watts to 100W or so
from Mouser or DigiKey. The modular supplies are fairly RF quiet,
and you can always put them in a box with filters. Surplus places
will often have the modules at a good price, although older surplus
might require more RF filtering.
You might also find a good deal on power supplies designed to run off
a -48VDC bus (widely used in the telecom industry).
http://www.allelectronics.com/cgi-bin/item/PS-526/480/SWITCHING_POWER_SUPPLY,_40-60_VDC_INPUT_.html
is a typical sort of 300W power supply (5V@25A, 12V@10A, -12@1A, from
a 40-60V bus) and runs $19 each.
>Thanks & 73 - JC, K0HPS
Jim
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