FAQ
TL;DR: To get unknown resistance, measure two values and divide: one unknown needs 2 readings (V and I). As Steve put it, "V3/I = the resistance of whatever V3 is across." [Elektroda, Steve Lawson, post #21671090]
Why it matters: This FAQ helps beginners fix “no reading” setups and choose the right meter to compute R from V and I in basic circuits.
Quick Facts
- You must power the circuit; without a source, V1=V2=V3=I=0. [Elektroda, Steve Lawson, post #21671090]
- Ammeter goes in series with the load; voltmeters go across the component. [Elektroda, Aye Aye Soe, post #21671091]
- The proper voltmeter symbol is V in a circle; drawing it as a wire is a short. [Elektroda, Frank Bushnell, post #21671086]
- Ohm’s law calculator: R = V/I. If V3 is across the part, V3/I gives its resistance. [Elektroda, Frank Bushnell, post #21671086]
- An ohmmeter injects its own test voltage; disconnect external power before using it. [Elektroda, Steve Lawson, post #21671090]
How do I calculate an unknown resistor (R4) from my circuit?
Measure the voltage across the part (V3) and the current through it (I). Then compute R = V3/I. Ensure a proper power source is present, the ammeter is in series, and the voltmeter is across the component. This uses Ohm’s law directly for a single unknown. “V3/I = the resistance of whatever V3 is across.” [Elektroda, Steve Lawson, post #21671090]
Why does my diagram show zero volts and zero current?
Your sketch lacks a power source, so no current flows and all meter readings are zero. Add a supply appropriate for the resistor network and reconnect meters correctly. Without a source, calculations like R = V/I are undefined because both V and I are zero. [Elektroda, Steve Lawson, post #21671090]
What’s the correct way to place the ammeter and voltmeter?
Place the ammeter in series with the load so all load current flows through it. Place the voltmeter in parallel across the component to measure its voltage drop. Reverse placements cause incorrect readings or shorts. Follow this before attempting R = V/I. [Elektroda, Aye Aye Soe, post #21671091]
My voltmeter symbol looks like a wire—why is that a problem?
A voltmeter drawn as a straight connection behaves like a short in the schematic. That bypasses the resistor and ruins the measurement. Use the V-in-a-circle symbol and show connections across the part, not in place of it. [Elektroda, Frank Bushnell, post #21671086]
Is R4 just equal to R3 if R4 = V3/I?
Only if V3 is across R3 and I is the same current through it. In your shared diagram, confusion came from meter placement and missing source. Fix the schematic first, then use R = V/I on the targeted resistor. [Elektroda, Steve Lawson, post #21671088]
Do I need an ohmmeter, or will two meters work?
You can use an ohmmeter with power removed, since it injects its own test voltage. Or use a voltmeter plus an ammeter under normal operation and divide V/I. If the device is nonlinear, prefer the two-meter method at the actual operating point. [Elektroda, Steve Lawson, post #21671090]
How do I avoid shorting the resistor when measuring voltage?
Never replace the resistor with the voltmeter leads. Clip the voltmeter across the resistor’s two terminals. Verify the meter’s internal resistance is high and that your diagram shows the V symbol, not a wire. This prevents bypassing the load. [Elektroda, Frank Bushnell, post #21671086]
What if the component isn’t a linear resistor?
For lamps, diodes, or thermistors, resistance varies with voltage or temperature. Measure V and I at the desired operating point, then compute R=V/I as a dynamic value. Using an ohmmeter off-circuit may give a misleading result. [Elektroda, Steve Lawson, post #21671090]
Can I build a simple divider to show V/I directly?
Yes. Sense current with a small shunt and measure voltage across the load. Feed both into a ratio circuit or microcontroller to compute V/I. The thread suggests the key is the dividing function when two meters are present. [Elektroda, Steve Lawson, post #21671090]
Quick 3-step: measure resistance in an active circuit using two meters
- Insert the ammeter in series with the target part and power the circuit.
- Place the voltmeter across the part and note V3 and I.
- Compute R = V3/I; record value at that operating point. [Elektroda, Aye Aye Soe, post #21671091]
Why did replies appear to predate the question?
The thread noted timestamps seeming out of order due to edits. Re-editing can shift display times, which confused readers. That does not affect measurement methods, only the forum chronology. [Elektroda, Dir Olan, post #21671084]
What basic tool measures voltage, current, and resistance?
A digital multimeter (DMM) measures all three. It can check supply voltage, series current with a shunt input, and resistance when the circuit is unpowered. This is the simplest way to validate V, I, and R relationships on the bench. [Elektroda, Emmanuel Galleto, post #21671079]
Edge case: Why does my ohmmeter give a wrong value on a live circuit?
Ohmmeters inject a test voltage. External power skews readings or can damage the meter. Disconnect the supply before using the ohms range, then reconnect and use V/I if you need in-circuit behavior. [Elektroda, Steve Lawson, post #21671090]
What file types should I use when sharing diagrams on the forum?
Use GIF, JPEG (.jpg), or PNG for reliable uploads. If a diagram fails to appear, convert to one of these formats and repost. PDF may work, but image formats are safer. [Elektroda, Steve Lawson, post #21671081]
Is the formula R = V/I actually correct here?
Yes. R = V/I is correct when the measured V is across the component and I is the current through it. Ensure meters are placed correctly and the circuit is powered, or both values will read zero. [Elektroda, Frank Bushnell, post #21671086]
