intercom power supply

[waynehoskins]

Originally Posted by wyatt20019

so what do i need on top of this? AC/DC POWER SUPPLY ADAPTOR 30 VOLT 30V 30VDC 500MA - eBay (item 270261524815 end time Aug-31-08 18:24:05 PDT) and how do i do it?

That's not going to be regulated, but it at least may have some amount of filter capacitor in it. That might work directly as-is. Failing that, it's easy to build up a 317 and the other components of the regulator circuit onto a small board or some such.

Another thing you will want to do is terminate the intercom bus at that same connector with whatever resistor value they suggest (200 ohms or something like that).

[wyatt20019]

thanks, how should i wire it to an xlr head?

[dramatech]

check mpja.com. They have lots of 24v dc supplies and their prices are the best I have found. I have purchased several from them to make scroller power supplies. They sell both switching regulated and linear regulated supplies. For scrollers, the switching units are best. For intercoms, the linear are best.

[waynehoskins]

Originally Posted by wyatt20019

thanks, how should i wire it to an xlr head?

pin 1 ground
pin 2 dc
pin 3 intercom bus (200-ish ohm terminating resistor)

And it's "XLR connector", by the way.

[wyatt20019]

what do i ground it too? and could i use a DMX terminator on the last beltpack?

[waynehoskins]

Originally Posted by wyatt20019

what do i ground it too?

You're right, ground isn't quite the right word. That's actually DC common and signal common.

Originally Posted by wyatt20019

and could i use a DMX terminator on the last beltpack?

While that would work, you really want the terminating resistor to be at the power supply. Its function is different to that of a DMX terminator. While a DMX terminator serves to suppress any reflections from the end of the transmission line, the purpose of the intercom-bus terminating resistor is to load the bus at a static level. Without it, the bus level will be very very high and will be very sensitive to the number of devices on the bus: when you add another pack, the bus level will drop noticeably. This resistor, which could be more properly called a loading resistor, is of significantly lower value than the impedance of a pack, and thus sets the operating level of the bus at something equivalent to 30 or 100 devices. You don't want this loading resistor to be a removable device like a DMX terminator.

[wyatt20019]

so the by the "ground" you mean the wire that isnt hot? and where can i find the resistor/terminator.

[waynehoskins]

Originally Posted by wyatt20019

so the by the "ground" you mean the wire that isnt hot?

Well, we don't really use the term "hot" to describe DC. "Hot" is used to describe a particular conductor of an AC circuit or the "true" of a balanced signal. Your cable coming out of the brick there has two wires. One corresponds to the big "plus" symbol on the brick, the other to the big "minus" symbol. The "plus" wire is positive with respect to the "minus" wire; the "minus" wire is negative with respect to the "plus" wire.

For a positive power supply circuit, we take the "minus" wire as common (this is also a "negative ground" arrangement like your car), "common" being the signal that measurements are referenced against. We also call this "ground", though "ground" can mean a bazillion different things.

The wire that's positive with respect to ground is usually given a label like "+30" or, in the case of a single-voltage power supply, simply "power" or "dc".

To find which wire is which, first make sure you have a DC voltmeter, then chop off the end of the DC cable. Separate the conductors of the cable a ways and strip them. Plug in the power supply. Put the red probe on one wire and the black probe on the other. Look at the meter. If the reading has a leading minus sign, then the wire your red probe is touching should be used as ground. If the reading does not have a leading minus sign (that is, no sign or a plus sign), then the wire your black probe is touching should be used as ground.

Originally Posted by wyatt20019

and where can i find the resistor/terminator.

Radio Shack will have something close enough, though they'll overcharge you.

But based on your electronics knowledge, I'd really caution you to simply buy a proper CC power supply. Power supply circuits are simple, but it takes more knowledge and experience than you've got, no offense. "Which wire is which?" and "What's ground?" are huge red flags that you shouldn't be poking around inside PS circuits.

[icewolf08]

Originally Posted by wyatt20019

we have some portacom units but no power supply, would a clear com power supply work? like this one

NEW CLEAR COM PK-5 INTERCOM POWER SUPPLY - eBay (item 150285258622 end time Aug-26-08 15:07:52 PDT)

For all the suggestions here, why not just get the Porta-Com Base station. The PC-100 base station supports 2 channels of communications, and for about $350 it is worth getting the real thing rather than kloodging something together. I am sure that you can get it at other places, but I found it for sale from B&H Photo & Video, who are very reliable. Link

[icewolf08]

More info for you. If you are insistent on building your own power supply, it does sound like Porta-Com can interface with Clearcom, so things like termination would be the same. As people have mentioned, the pinout for the system is:
1- common
2- +30VDC
3- audio

Termination for this system requires one 10K Ohm resistor across pins 1 and 3, AND in parallel with that: a 200 Ohm resistor in series with a 1uF capacitor.