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another common curiosity

2296 Views 17 Replies 4 Participants Last post by  1idejim
we've talked about the hot common without much resolve so i have another common question to keep us going.

when russ was finding the elusive fried solenoid we decided to separate the common wire shunt that was in a 10 inch can, he had 4 wires to test and we had an extended earth ground as we were getting a major interference (from his friggn bluetooth we found out) so he touched the black lead to the earth ground wire and the red lead to the common wire to get a resistance reading on each wire.

my hope was that we would be able to get a reading similar to a TDR reading.

the wire that the valve was on read 4 or 5 ohms or a short reading.

as he was testing the other 2 wires i could hear the multimeter tone going off and asked him what he was touching and what he was reading...?

i told him that since there were no other valves connected and they were future expansion wires they should read OL for open....they were reading 0.0 and giving an audible tone.....continuity. with what the earth?

i'm not catching something here, and so far no one can give me an explaination that makes sense.........i know what we're reading, but it doesn't make sense yet.

any takers?
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Why not construct a circuit diagram for us?
Where is the valve - where is the controller - etc. etc. etc.

I can wait for a diagram. The 'hot common' phenomenon was simple enough once it was adequately described. Current must have a path to travel, so there aren't any real mysteries here.
The 'hot common' was a result of a zone wire being grounded, so a person touching the common would complete a circuit, and get zapped. Current must travel in a loop, so grounding one lead of a multimeter, and touching the other lead to a wire, and getting a beep, would mean that wire is grounded.
I would think some wet California soils would have enough minerals and salinity to make a very good electrical conductor. The same phenomenon might not happen after six months of drought.
If you really want to delve into this, you need to get a signal source and a load resistance and a current meter, and see what can actually flow through the wire.
Current readings are always the best to take, since it means some actual power is flowing. We all know about observing the usual 24VAC on a point in the field, where a good solenoid is failing to operate. A voltmeter is drawing almost zero power, so it can get a reading through badly corroded wiring that prevents solenoid operation. An ohms reading is a bit better, but current readings are the real deal, even if you have to do some calculations to find a resistance value. (or for solenoid operations, it would be more accurate to call it impedance, because a solenoid is not pure resistance)
1 - 6 of 18 Posts
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