How to Calculate Unknown Resistance R4 in a Circuit Diagram Using Components

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#1 21671079 21 Feb 2014 20:18

Emmanuel Galleto Emmanuel Galleto

Anonymous

Post #1
21671079 21 Feb 2014 20:18

Hi Olan.,

Maybe you could use a MULTIMETER it has voltage, current and resistance reading.
I have here a DIY multimeter circuit http://www.allaboutcircuits.com/vol_6/chpt_3/11.html

Hope it helps.
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#2 21671080 23 Feb 2014 21:13

Frank Bushnell Frank Bushnell

Anonymous

Post #2
21671080 23 Feb 2014 21:13

Now this is clever. Is it just my browser, or does everyone else see this ?
The question was asked 1 minute ago, and the reply is dated two days ago !
Has EEWeb invented a time machine ?
#3 21671081 23 Feb 2014 21:14

Steve Lawson Steve Lawson

Anonymous

Post #3
21671081 23 Feb 2014 21:14

No attached diagram. Typically this is because it was not in one of the three forms:
* GIF
* JPEG [.jpg]
* PNG

Try converting it to one of these and post it again.

PDF might work, too, but not sure.
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#4 21671082 23 Feb 2014 21:15

Steve Lawson Steve Lawson

Anonymous

Post #4
21671082 23 Feb 2014 21:15

Yup, I see that, too -- weird.
#5 21671083 23 Feb 2014 21:26

Dir Olan Dir Olan

Anonymous

Post #5
21671083 23 Feb 2014 21:26

Thanks for your reply, i have attached image to make my question clearer to you. Your contributions are welcome. Thanks
#6 21671084 23 Feb 2014 21:29

Dir Olan Dir Olan

Anonymous

Post #6
21671084 23 Feb 2014 21:29

I re - edited my question that why it appeared like that.
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#7 21671085 23 Feb 2014 21:33

Dir Olan Dir Olan

Anonymous

Post #7
21671085 23 Feb 2014 21:33

Thanks, i have attached JPEG. Your contributions are welcome, thanks
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#8 21671086 23 Feb 2014 21:54

Frank Bushnell Frank Bushnell

Anonymous

Post #8
21671086 23 Feb 2014 21:54

The correct drawing for a voltmeter is a V in a circle.. In your diagram they appear as short circuits around the resistors.

Your equation is wrong. It should read R3 = V3/I

However, no current will flow in the circuit as drawn, because there is no voltage source.
#9 21671078 23 Feb 2014 22:20

Dir Olan Dir Olan

Anonymous

Post #9
21671078 23 Feb 2014 22:20

Hello.

Please I need help on a circuit/component(s) that can calculate Resistance R4 from this circuit diagram attached. Thanks
#10 21671087 23 Feb 2014 22:24

Dir Olan Dir Olan

Anonymous

Post #10
21671087 23 Feb 2014 22:24

Thanks, correction has been made in d voltmeter diagram. But my equation is right, that why i said i need another circuit/component that can calculate Resistance R4=V3/I
#11 21671088 23 Feb 2014 23:35

Steve Lawson Steve Lawson

Anonymous

Post #11
21671088 23 Feb 2014 23:35

I'm not sure what you're asking. There is no R4 in your diagram, but if R4 = V3/I then R4 = R3.
#12 21671089 24 Feb 2014 01:26

Dir Olan Dir Olan

Anonymous

Post #12
21671089 24 Feb 2014 01:26

Thanks for your reply, I want us to forget about R4. I think what I need is like a METER to give me ratio of V3/I.
#13 21671090 24 Feb 2014 01:58

Steve Lawson Steve Lawson

Anonymous

Post #13
21671090 24 Feb 2014 01:58

I took a closer look at your diagram and it makes no sense if V1, V2 & V3 are voltmeters and A is an ammeter -- unless V1 = V2 = V3 = I = 0. Because there is no power source.

But, if you are looking for a meter to give you V3/I, then it sounds like you need an Ohmmeter, like some of the others have said.

In other words, V3/I = the resistance of whatever V3 is across. Now, if the component that V3 is across is not a linear positive resistance, or if it is, and at V3 it's being stressed in a way that would change it's resistance, then you will need two meters. One a voltmeter and the other an ammeter, and then some sort of dividing function -- since an ohmmeter will apply it's own voltage which will likely not be whatever V3 is.

So, maybe what you're looking for is the dividing function. There are several ways to do that, so let us know if that is what you want, and I/we will elaborate.
#14 21671091 16 Mar 2014 05:19

Aye Aye Soe Aye Aye Soe

Anonymous

Post #14
21671091 16 Mar 2014 05:19

Dear Olan,
the ampere meter should be connected in series, maybe something hidden in your circuit and where is the source? If V1, V2 and V3 are volt meters you will only get the PD and not supply voltage.
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Topic summary

✨ The discussion centers on calculating an unknown resistance labeled R4 in a circuit diagram. The original poster seeks a method or component to determine R4, proposing the formula R4 = V3/I, where V3 is a voltage measurement and I is current. Responses highlight issues with the provided circuit diagram, including missing power sources and incorrect voltmeter symbols, which affect measurement validity. It is clarified that an ohmmeter or a combination of a voltmeter and ammeter with a dividing function can be used to measure resistance by calculating the ratio of voltage to current. The importance of proper meter placement is emphasized, such as connecting the ammeter in series and voltmeters in parallel. The discussion also notes that the circuit as drawn lacks a voltage source, making current flow and resistance measurement impossible without modification. Suggestions include using a multimeter capable of measuring voltage, current, and resistance, and ensuring the circuit diagram is correctly formatted and complete for accurate analysis.

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]