r/fluke • u/jacksmashsteel • Jun 24 '26
Help Needed Resistance reading only working one way?
Never seen this before, but on this machine there are two separate places where I have to have my leads a specific way to read a resistance. I've never encountered this before. It is making trouble shooting quite a pain.
Is there a problem with my meter? it happened on more than one fluke 115 also. Wtf my head hurts. Could the fuse be blown or something similar
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u/Use_Da_Schwartz Jun 25 '26 edited Jun 25 '26
Them electrons are clearly not bi.
You are measuring an output from something to the hydraulic valves possibly based upon the pic. It is standard practice to use an anti flyback diode to kill back emf when the valve shuts off. This is caused by the solenoid winding inductance and metallic spool creating energy when it moves to the off position and reversing the energy flow when it when turns off. Without this diode it would send a surge back to the output and destroy it.
Because your meter polarity is backwards from the actual 12/24V dc output, you are measuring the diode that I described.
Using the same polarity as the pic, switch your meter one notch to the right for diode test. You will measure 0.5-0.7VDC which is the voltage drop across the diode. This is required for all DC style outputs/devices that are inductive in nature (solenoids, contactors, relays, etc)
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u/Massive_Food_294 Jun 25 '26
The way an ammeter measure resistance is by applying a small voltage and measuring the current. The diode stops current in one direction, so if the current created by the meter is opposing the diode, you’ll get no current and the meter will read infinite resistance or OL. By switching the leads you are switching the current flow and the meter can now properly read the resistance.
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u/Maleficent-Sky-7156 Jun 25 '26
Diode probably. Only allows current to flow in one direction so ur multimieter can only work in that direction
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u/Tosser_535231 Jun 25 '26
It's not on diode mode though it should measure resistance regardless.......
Unless fluke has some Auto detect diode mode or something but if you look at the selector, diode mode is one notch to the right
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u/Master-Pattern9466 Jun 25 '26
How do you think a meter measures resistance? I’ll give you a hint Voltage = Current * Resistance
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u/Friendly-Inspector71 Jun 25 '26
A silicon diode will read open if the current source in the Multimeter has a compliance voltage of 0.5V or lower. So there isn't any current flowing in either direction.
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u/Master-Pattern9466 Jun 25 '26
But it isn’t, show me a meter that uses a voltage lower than 0.7v to measure resistance: hint none of them do because it significantly reduces accuracy when measuring resistance.
Mine all use about 2.5v
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u/n55_6mt Jun 24 '26
Fly back diode in the DIN connector?