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Loop Current & Circuit Loss Technical Bulletin

1 Line Modular / Hardwire Loop Current Regulator 1 Line Modular Loop Current Booster  2 Line Modular / Hardwire Loop Current Regulator

Loop Current Tester 1 Line 66 Block Loop Current Regulator 66 Loop Current Regulators on Block with Test Shoe


Click here to go right to Circuit Loss Information
(below) , to fix "Can't Hear" problems

Loop Current is the amount of electrical energy flowing through the telephone and line, as opposed to the voltage which is the force behind the energy. There is a definite correlation between the Loop Current and Line Voltage (Ohm's Law), but the loop current reading is often what indicates the problems in telephony... not the voltage reading. The carbon transmitter used in telephones has been the controlling factor for years, since it needs over 20ma to sound good.

Our Loop Current Tester is easy to use... Just put it on the line and push one button, and read the Loop Current range off the LEDs          This is how you would wire your meter to read Loop Current. Be sure to put it in DC ma mode, and use the correct banana jack for the positive test lead!

Measuring the loop current is easy. Our Loop Current Tester is the easiest way to test. Just plug it into a modular jack on the line (not in-series), push the button, and read the loop current off the LEDs.

Otherwise, you can use a good quality DIGITAL volt-ohmmeter with a DC milliamp scale. An analog meter, except one designed for telephone work, will usually not give you accurate readings. A Fluke, or a few of the Radio Shack meters will do OK - within a couple of ma. Our Network Meter will display loop current, as well as AC and DC Voltages. The connection diagram for a meter (above) shows the meter in-series with the telephone instrument or system. That will give you the most accurate reading with a meter, since it's going through a 600 or 900 ohm instrument (or Butt-set).

As an alternative to connecting the meter in-series with the instrument, you can just short the pair with your meter leads in the ma mode, and read the direct loop current, which will probably be a couple of ma higher than the reading through a phone, but it might get you close enough to make a decision on what to do.

If you don't use our Loop Current Regulator or Attenuator, and try to use resistors to bring the current down, you really need to watch the db loss as you increase the resistance, so you don't get it too low to hear properly. Try to stay above -7db. Below that you'll have a hard time hearing on long distance calls. Generally speaking, for every 100 ohms you put in-series with the tip and ring (you have to keep the line balanced and put the same value resistor on both sides of the line), you'll drop the loop current by 1ma, and the circuit loss by 1db. That means if you want to get down to 30ma from 35ma using 500 ohm resistors, you'll lower the volume of the line by 5db - and you probably won't be able to hear well. Both the Loop Current Regulators and Loop Current Attenuators have special circuitry that lowers the loop current without lowering the db level. I'll talk about measuring db loss a little later.

As I said before, to measure loop current with a regular meter, you need the meter in-series with the telephone line and telephone. To measure voltage, whether the DC line voltage or the AC audio voltage, you need the meter in parallel (across) the telephone line. You don't want to make a mistake in your hook-up as the readings would be meaningless (and could cause you to take an expensive improper action). Make sure you meter is set on DC ma, not AC ma!

To hookup a regular meter to measure loop current in-series: If you have bridging clips on a 66 block, just open one of them, and connect one lead of the meter towards the CO, and the other towards the equipment. It doesn't matter which color lead goes where since the meter will display a + or - that you don't care about (except on an analog meter which will make the needle go backwards and get damaged).

With the line on-hook, you should see almost no current flowing through the meter (it will read maybe .01ma). When you go off-hook with either the phone equipment, modem, or your butt set, you should get a reading of between 23 and 35ma if the line is OK. Repeat and record the readings for each line, since not all of the lines have loop current problems in many cases.
 

Click HERE to see the Longitudinal Imbalance Tech Bulletin , and HERE to see the Troubleshooting Basics Tech Bulletin , which have more fixes to strange problems.


Click HERE to see our products for Fixing Phone Line Problems.