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Re: [TowerTalk] "bias Tee" from MFJ and others

To: "JC Smith" <>, <>
Subject: Re: [TowerTalk] "bias Tee" from MFJ and others
From: Jim Lux <>
Date: Thu, 07 Sep 2006 12:24:02 -0700
List-post: <>
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).,_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



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