Parallel resistance confusion ( fire alarm system )

I am troubleshooting a fire alarm system that has strobe circuits. The strobes have a positive and negative terminal. Putting my multimeter across the terminals (+ to +, - to -) I read 79K ohms. The other way (+ to -, - to +) I read nothing. All very normal and typical of what I would expect a fire alarm strobe to do.

An example circuit of 2 strobes are wired in parallel and at the end of the circuit there is a 24K ohm resistor. Reading the circuit in what we would consider standby or monitoring mode (+ to -, - to +) I read the 24K ohm resister perfectly. However in active mode (+ to +, - to -) instead of reading something like 15K I am reading 22K.

Doing the math 1/14.929 = 1/79 + 1/79 + 1/24

I realize I am missing something amazingly obvious as the system works, strobes do their thing, panels are happily monitoring the strobes, but I do not understand where I have gone astray.

Thank you for your time and kind attention,

EEjack

 

Hi

Your multimeter is designed to read pure resistances only with the circuit not powered. The strobe terminals appear to have a diode, or similar non-linear device, in series with the resistance, which will give you a reading that is difficult to predict. Diodes read open in the reverse direction, and some resistance in the forward direction. The reading will vary depending on, the meter design, meter battery voltage, diode type and temperature.

Putting a resister in parallel with this kind of circuit may very well not give you the reading that is calculated.

Regards,
Ifixit

 

You probably have a diode across the strobe to polarize it.
That way the panel can send a small current through it to make sure it's still connected. The resistor at the last device is called the EOL or End Of Line
resistor. The small current is passed by that resistor back to the panel for supervision. What that means is if the voltage goes away, the line is broken,
and if the voltage goes high (no drop across resistor) the line is shorted.
In the standby mode the polarity is reversed from the alarm mode.
I don't follow your math, what are the numbers and what are you calculating for?

 

You are correct about how things are working. I am trying to understand why in a circuit of two strobes ( which each read 79K ohms ) and one end of line resistor (24K ohms) instead of reading 15K ohms I am reading 22K ohms.

In circuits ranging from 2 through 8 strobes my readings through hover in the 22K ohm range. If the strobes acted like true non variable resistors this should not happen. Obviously they are not.

In the many years I have troubleshot systems like this it never occurred to me to explain why ... until an apprentice challenged me to explain it better than I had.