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

I got tired of switching the leads of my DMM. Suddenly if dawned on
me that I can just set the power supply to 10.0V for exaample, and
read the current, and then divide the voltage by the current to find
the resistance. Like I put a resistance on the PS, it reads 10.0V and
the current is .018A, so 10 / .018 gives 555.6 ohms. Must be a 560
ohm resistor.

I turned my PS into an ohmmeter - FREE!

Hee-hee - Work smarter, not harder!

Of course, make sure the current stays low so the resistance doesn't
overheat. For low resistances use a volt or less.

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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.

 

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

 

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
http://maxwell.byu.edu/~spencerr/websumm122/node50.html
http://www.launc.tased.edu.au/online/sciences/PhysSci/done/electric/resistnc/Resistance.htm

 

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.

 

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.

 

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.

 

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

 

X-No-Archive: Yes

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

 

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

 

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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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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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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Just when you thought you had all this figured out, the gov't
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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

 

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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###Got a Question about ELECTRONICS? Check HERE First:###
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Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
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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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###Got a Question about ELECTRONICS? Check HERE First:###
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Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
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It doesn't matter what your original post said, the OM found a factoid,
and wants to show it off!

-Chuck, WA3UQV

 

Very handy indeed.
The ones I bought were 3 pounds 99p, ($6us?) I have around 8.
Soon after buying one, I thought I'd discovered that they have an
overvoltage LED.
However, the smell of burning FR4 soon made me realise otherwise.

It's amazing how many multimeters you can use when you have them free.
I was just discharging a series string of Li-Ion batteries, to measure
capacities.
In the past I would have taken measurements every 5 minutes to ensure
none had approached 3V.

Why bother, just hook up 5 of them, and glance over every once in a while.,

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