Ohms law calculation issue in low voltage troubleshooting

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1idejim

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I troubleshoot irrigation systems so I am constantly comparing resistance - voltage - amperage to determine the well being of my field wiring. My issue is when measuring these values I receive different values than when I calculate the values using Ohms Law.

As an example, my in field measurements are:

R= 23.3 ohms
E= 25.8 vac
I= 188.5 mA

Now, if I apply Ohms Law E\R=I, I receive 1.107296 as the answer.

What might I be doing incorrectly or what could I be misunderstanding?

Thank you
 
It would help if you would post your method of making these measurements.
1. Did you measure the voltage at the same time you read the amperage. The reason I asked is if the voltage
level fell when the load was added, you might have a bad connection at you voltage source.
2. Problem with meter
3. The source may not be able to put out 1 amp. Just because ohm said it should, doesn't mean is can.
4 When you measure the resistance of the circuit with no load, the measurement can be different if there is a bad connection. The
bad shows up when load is added.
Just a few thoughts.
 
Can you sketch a circuit diagram and show the components and where you are measuring the values.

I would guess the V and I values you are measuring are correct, but your not including all the impedance.
 
1idejim said:
I troubleshoot irrigation systems so I am constantly comparing resistance - voltage - amperage to determine the well being of my field wiring. My issue is when measuring these values I receive different values than when I calculate the values using Ohms Law.

As an example, my in field measurements are:

R= 23.3 ohms
E= 25.8 vac
I= 188.5 mA

Now, if I apply Ohms Law E\R=I, I receive 1.107296 as the answer.

What might I be doing incorrectly or what could I be misunderstanding?

Thank you
Click to expand...

Ohm's Law is still in effect. V (or E) does indeed equal I X R (or Z), with a couple of caveats.

It is important that you make your measurements of voltage and current under the same conditions, i.e., don't measure the voltage open circuit and the current under load. Another thing is that AC resistance (impedance, or Z) can be very different from DC resistance if there are any reactive (capacitive or inductive) elements in your circuit and it will vary with the frequency of the applied AC voltage.
 
Last edited:
ggunn said:
Ohm's Law is still in effect. V (or E) does indeed equal I X R (or Z), with a couple of caveats.

It is important that you make your measurements of voltage and current under the same conditions, i.e., don't measure the voltage open circuit and the current under load. Another thing is that AC resistance (impedance, or Z) can be very different from DC resistance if there are any reactive (capacitive or inductive) elements in your circuit and it will vary with the frequency of the applied AC voltage.
Click to expand...

Correct.

I would hazard to guess that there are solenoids or relays in this system, so impedance is probably the factor needed, not resistance.

So, if you have 25.8 volts AC under load with 1.885 mA of current, you have 137 ohms of impedance. The DC resistance may help estimate the inrush current, but for coils fed by an AC source it's not much good for anything other than looking for shorted or open coils.
 
131004-1412 EDT

1idejim:

First, use the correct units.

25.8/0.188 = 137 ohms.

As others have pointed out if your measurement is with 25.8 V 60 Hz, then there may be a reactive component, inductance, that makes the AC impedance much greater than the DC resistance.

.
 
Thanks to you all.

First off, the meters are fine and I am performing the measurement tests correctly.

Yes, I am measuring the condition of the field wiring and solenoids for the valves.

While i have very few issues troubleshooting, I was wondering what I was missing, now I know and again I thank you.
 
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