Turn Your Power Supply into an Ohmmeter - It's Free!

Watson A.Name - 'Watt Sun' wrote in news: snipped-for-privacy@news.inreach.net:

Also, you must now consider the combined accuracy, of two meters, instead of just one.

Yes it works, but, so do pliers on a hex-nut. Something about the right tool for the job.

Congratulations. You proved Ohm's Law works. Of course it's only as accurate as your meters, then you have to use your calculator.

Sorry, but give me a decent digital multimeter.

Your method is certainly worth remembering in a pinch.

Mark Z.

In article , snipped-for-privacy@outerspace.org mentioned...

Actually it works better than a DMM on low resistances below an ohm. But hey, it's still more accurate than a 5% resistor.

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Yep, that's how ohmmeters work actually. I'm currently designing a digitally controlled PSU which includes a dot-matrix LCD that shows a lot of info about the state of the PSU. For e.g., it multiplies V * I so the user can see the load power in real-time. Simple, but very handy. I suppose I could also have it display V / I to show the load resistance in real-time. I think the PSU approach is good for measuring very small resistances (when you need to generate a lot of current to have a voltage drop large enough to measure accurately.

cheers, Costas

At a local electronics store, there was a sale of multimeters for $5. I bought a bunch and have velcroed some to my workbench. I set them for voltage, current or resistance and leave them there. Good cheap way of doing some quick and dirty measurements. Surprisingly accurate too. If I smoke one, I just toss it. After all, it is a toss away world nowadays.

Al

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There's never enough time to do it right the first time.......

Actually it is the resistance formula that works. What you think is Ohm's (V=IR) is not. See the links below and a good physics book. Ratch

Watt Sun: Just a little cumbersome.... isn't it?? As you indicated, there is a risk of smoking low ohm, low wattage resistors unless you already know the value... if that is the case then why are you measuring it??...... ....and the accuracy is compromised because you are measuring the voltage and then measuring the current..... and you are at the mercy of the regulation of your power supply.

2 meter operations instead of one... the inherent innaccuracy of one of the readings is further compromised by the inaccuracy of the 2nd reading..... give me a DMM or VOM with a dedicated OHMS function any time.. Actually, the much more used and handier version of this is the "flip-side" where you measure the voltage across a known-value resistor in the circuit to determine the approximate current.... most techs do this all the time while routinely troubleshooting.

-- Best Regards, Daniel Sofie Electronics Supply & Repair

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Amazing. The possibilities are endless. Using a thermister you could turn your power supply into a temperature meter.

How bout a diode checker. If the voltmeter reads 0.7 volts its good (watch that current limit).

I just tossed my Fluke in the garbage. Now lets see you turn your power supply into a frequency counter.

This isn't especially usefull usually. However, with low ohm resistors, it can be.

Given a constant current of an amp, the $5 meters mentioned elsewhere can now measure with a resolution of .1mohm.

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Yes, and Ohm's Law describes the interaction of resistance, voltage, and current. Not just resistance. I'm not an engineer, granted, but I don't require an education on Ohm's Law.

Mark Z.

/Resistance.htm

Law.

I belive that you are missing the point. The resistance (or impedance) formula V=IR (or V=IZ), describes the describes the interaction of resistance (impedance), voltage, and current. While correct and true in all cases, those formulas are NOT Ohm's law, and it is wrong to call them that. As shown in the second link I gave, Ohm's law is a property of resistive linearity in a material. Just as the specific gravity of a material is a property. If it conforms to Ohm's law, it is ohmic. Otherwise it is nonohmic. Ratch

is

X-No-Archive: Yes

"Wats> I got tired of switching the leads of my DMM. Suddenly if dawned on

Dangerous and inaccurate. Inaccurate, because the resistor will warm up and the resistance will increase.

Using a volt meter and ammeter is certainly cheap and practical for measuring a very low resistance that the typical DMM can not.

Say you have a 25ft 12AWG extension cord and you want to know the total resistance. The average DMM has a resolution down to 100mOhm. The resolution is nowhere near what you need.

Connect it to ~12V DC(you don't want to use 120V, because high floating voltage will throw off accuracy on the low voltage meter range) and connect an ammeter in series.

Connect a load on the other end that takes about a few amps.

Measure the voltage across both ends of one conductor in mV range.

If you read 54.6mV and the ammeter reads 2.12A, you can figure out the resistance by:

0.0546/2.12=0.02575

three sig dig=0.0258ohms=258mOhms x 2(to accomodate for return path)=506mOhms

Law.

I belive that you are missing the point. The resistance (or impedance) formula V=IR (or V=IZ), describes the describes the interaction of resistance (impedance), voltage, and current. While correct and true in all cases, those formulas are NOT Ohm's law, and it is wrong to call them that. As shown in the second link I gave, Ohm's law is a property of resistive linearity in a material. Just as the specific gravity of a material is a property. If it conforms to Ohm's law, it is ohmic. Otherwise it is nonohmic. Ratch

is

In article , snipped-for-privacy@Comcast.net mentioned...

And both Mark Z and Ratch are barking up the wrong tree. If they were to reread my post below, they would see that I used the formula, R=V/I, which in both URLs above was shown first and named Ohm's Law. Doh.

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In article , snipped-for-privacy@olypen.com mentioned...

Less cumbersome than removing the DMM leads and then reconnecting them.

Yes, as I indicated.

If you already know the value, then there'd be no point in measuring it. So you would be measuring the unknown value to determine it.

It still finds the value with a reasonable accuracy.

The regulation of the power supply makes no difference.

You're repeating yourself. As I said above, it still finds the value with reasonable accuracy.

If you reread my post, you would see that I already have the DMM. I was using an alternate method.

If the VOM you mention above is an analog wiggle stick meter, it may be less accurate - maybe only 3% - than using my PS method.

Right. Now you've stated something useful.

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In article , snipped-for-privacy@mauve.demon.co.uk mentioned...

I bought a few of those $5 DMMs from Futurlec a few months ago, actually I think they were about $6. 9V vattery included(!)

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is

No, both the above URLs make a point of saying that R=V/I is not Ohm's law, and the first refers to R=V/I as the resistance formula. In other words, Ohm's law referring to V=IR is a misnomer. The second URL points out that Ohm's law really and truly refers to the resistive linearity of a material. Dah.

By the way, you did not turn your electrical energy supply into a ohmmeter. You applied a method of using the energy supply to determine resistance. Ratch

and

In article , snipped-for-privacy@prontoREMOVETHISmail.com mentioned...

Or use a constant current PS and short the far end of the cord, and measure across both conductors on the near end.

Or you can just look up the resistance in a wire table and find that

12 AWG has 1.59 milliohms per foot. Then multiply by twice the cord length.

As for being "dangerous and inaccurate", electronics experimenting is frought with danger, one being 'letting the smoke out.'

As for "Inaccurate, because the resistor will warm up and the resistance will increase", you could have the same problem if you were measuring a resistor with a DMM, and the equipmewnt with the resistor had been powered on before you did the measurement. The resistor could already be hot.

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In article , snipped-for-privacy@Comcast.net mentioned...

This is *still* pointless. I *never* claimed that I was doing

*anything* with _Ohm's_Law_!! Quit trying to put words in my mouth! See my OP below. [snip]

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