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View Full Version : Voltage drop help? For the smart people







kengineering
15-08-2016, 04:14 PM
I have a simple self contained box. 120V Condensing unit, dixel control. Customer says unit was running.
Tech says 120V into controller but only 50V out?

Tayters
15-08-2016, 05:01 PM
Is that a switched voltage to run solenoid valve or direct to comp?
Either way, guessing it comes out of a relay with dirty contacts.

kengineering
15-08-2016, 05:34 PM
direct. Funny thing is, line in to the controler 120V. Line out, with compressor disconnected, 120V. with compressor connected 50V.

cadwaladr
15-08-2016, 05:54 PM
Controller relay faulty,make sure the replacement can handle the load or fit a mini contactor

monkey spanners
15-08-2016, 07:00 PM
Think of it like a dripping tap/fawcet, put your finger over it and you got pressure but no flow, finger away and you got flow and no pressure.

Tayters
15-08-2016, 11:20 PM
direct. Funny thing is, line in to the controler 120V. Line out, with compressor disconnected, 120V. with compressor connected 50V.

Remember that voltage is dropped across a resistance. Comp disconnected then largest resistance is between line out and neutral (open circuit in this case). When comp is connected you have say 100 ohms resistance on the contacts but only 2 ohms in the compressor so more voltage dropped across the relay.

Let's not get ahead of ourselves though. Perhaps the bad connection is further back, from a fuse/breaker/controller connection/etc. Next step has to be to link out the controller to prove the complete circuit and not in this case but always worth confirming the neutral connection is good instead of relying on checking to earth.
Google potential divider and it should explain a bit (http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0ahUKEwjFkZj_uMTOAhVjFMAKHeNQDDIQFggkMAI&url=http%3A%2F%2Fwww.bbc.co.uk%2Fschools%2Fgcsebitesize%2Fdesign%2Felectronics%2Fcomponentsrev4.shtm l&usg=AFQjCNElzMSgeeuRTEDBv2OtQ5Cl83rbkQ&sig2=07B9pRLTh4Y9Pj6gpKeLXg)

Me smart? Oh no! Spent enough time tracing wiring faults in cars which taught me to check circuits under load, not just with wires disconnected - Ford Fiesta rear wiper motor taught me that lesson!

Cheers,
Andy.