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Power Supply Question

B

Brian Bowers

Hi guys. I should probably know the answer to these questions, but I don't - and I am a little embarrassed by that. So I would appreciate some advice on this topic. I am trying to set up 4 cheap Caller ID units for 4 standard POTS lines. The 4 lines are in a hunt group. Each Caller ID unit normally takes 3 AAA batteries in series...so each unit basically runs comfortably on 4.5 volts. I would like to remove the batteries and run these units off of a stand alone power supply, so I am not constantly replacing batteries. Using my VOM, the units seem to consume such a small amount of current, it is immeasureable. However, it looks like each unit would like to get as close to an exact voltage of 4.5 volts to operate correctly. I've got boxes of wall wart power supplies that have acumulated over the years (who doesn't). Even though the few I have found that are rated for 4.5 volts, actually measure more than that, even under the load of these units. If you give these units (3 Caller ID units connected to the external power supply for this) more than 4.5 volts (even 4.6 or 4.7), they tend to light up all of the characters on the LCD screen, making it hard to read the actual Caller ID info. Give them anything less, and they don't want to operate at all. Disconnecting the external power supply and inserting the batteries makes them the most readable. But this would be a pain trying to remember to replace these batteries all of the time, since I would like to install this 4 unit setup in two on-air studios.

What I have also found is with the one power supply that works halfway decently with this setup (I am running it with 3 Caller ID units, and have busied out the 4th listener line), it is only comfortable running 3 of these units at at time. If I add a fourth Caller ID unit, the units characters can be read OK, but they won't operate correctly. What do you think I am doing wrong here? Do the units need to be somehow isolated from each other. I know I have bridged a DC power supply many times before feeding a number of devices, but never really encountered a problem like this. Would a regulated power supply help? If so, that probably wouldn't be cost effective judging from some of the prices I have seen for them. Any ideas or suggestions? Thanks in advance.
 
Ditto what TomT says. The IC power regulators are quite easy to use. You could adjust until you reach the proper voltage and then you're done ;)
 
Thank you. I should heat sink this right? Other questions - they mention R2 as a set resistor, we're talking a simple pot here right? If so, what value would you make it, and at what watt rating? Also R1 is 120 ohms, what wattage would you make that? 1/2 watt good enough? Anyone want to propose a value for C0? Thanks again.
 
I would put a small heat sink on it. I would use 1/2 watt resistors, and a 250 or 500 ohm multi-turn pot for R-2. 500 Ohm probably easiest to find. Mount everything on one of those little Radio Shack project boards, & stick it into a small project box.

Jameco also good source for small parts for projects like this.
 
Giving the voltage regulator a few volts headroom would make it run better, so a 9 or 12 volt supply would be perfect. A 10 turn pot would be the easiest. make sure there is a 10 to 100uf capacitor on each side (input and output side) and things will be quite lovely... if there isn't the current draw of any LED's or such on the regulator, the regulator won't dissapate any appreciable energy on its own, so a TO-92 package IC would be fine.

Good luck and most of all have fun with the project...
 
One thing to keep in mind is that the POTS lines connected to these devices normally have around 48 volts on them. If this 48 volts is not isolated from the circuit in the caller ID boxes then trying to run more than one box on your power supply may cause some wierd things to happen with your caller ID boxes or possabily even with the phone lines connected to them. Such as hum on lines, crosstalk between the lines, lines being busy or more than one line ringing when really only one line is ringing. I use to play with projects like this years ago and have seen some crazy things happen(I caused some crazy problems) with the stations phones.
Hope everything works
 
If the line is not isolated from the power supply, the Caller ID boxes are not compliant with FCC rules nor good practice and the should be removed from your lines immediately!

rew said:
One thing to keep in mind is that the POTS lines connected to these devices normally have around 48 volts on them. If this 48 volts is not isolated from the circuit in the caller ID boxes ...<snip>
Click to expand...
 
Good point... make sure your transformers are bobbin wound and designed for at least 1500V (hi-pot capable up to) to remain compliant... may be best to stick with batteries. Some caller ID units, especially the ones with LCD displays can last a year of more on a set of alkaline batts... If the caller ID unit has a DC jack, you may be fine, as these have built in opto isolation to the ring circuit...
 
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