Bob Dolph Answers Questions on Phone Circuits, Mount-Sealed Batteries

Q: I was checking your April 2003 “Tech Talk” on phone circuits and had a quick question? What is the voltage and current on an “on-hook” and “off-hook” telephone circuit? How do I read this with a voltmeter?

A: First, I want to clarify that if you have a copy of the April 2003 “Tech Talk,” the status of the off and on-hook voltages was backwards. This was not caught before the magazine went to print. However, we did print a correction statement in a following “Tech Talk” column.

To set the record straight; when you have an “on-hook” (phone in cradle) condition, the circuit is open so your voltage will be 48–51VDC. Placing your red probe of your digital volt meter (DVM) on the tip (green) and the black probe on the ring (green) you should see a positive 48-51VDC.  Since the circuit is open, current flow should be zero or very low.

A typical “off hook” phone will show about –20V on the tip and –28V on the ring or an 8V differential between the two with about 200–300 ohm load from the telco equipment will provide around 20–50ma current depending on the phone line resistance and phone equipment load. Current flow will be from tip to ring through the circuit.


Q: I have heard that it is unsafe to mount seal red “gel-cell” lead acid batteries on their side in an alarm cabinet. Is this true?

A: For the most part, this is false. Most manufacturers will state that their sealed batteries can be mounted on their sides. These batteries typically have small vents near the terminals that should not be covered. The only times that I have seen a sealed battery leak from the vent is when the charging circuit was slightly overcharging the battery. This is, however, what the vents are designed for.

Below is an example of a battery manufacturer’s specification:
“These rechargeable batteries are lead-lead dioxide systems. The diluted sulphuric acid electrolyte is suspended and, thus, immobilized. Should the battery be accidentally overcharged producing hydrogen and oxygen, special one-way valves allow the gases to escape, thus avoiding excessive build-up. Otherwise, the battery is completely sealed and is, therefore, leakproof and useable in any position.”

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