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

W

#### Watson A.Name - 'Watt Sun'

Jan 1, 1970
0
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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###Got a Question about ELECTRONICS? Check HERE First:###
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My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
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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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B

#### buck rojerz

Jan 1, 1970
0
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.

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.

M

#### Mark D. Zacharias

Jan 1, 1970
0
Congratulations. You proved Ohm's Law works. Of course it's only as accurate

Sorry, but give me a decent digital multimeter.

Your method is certainly worth remembering in a pinch.

Mark Z.

C

#### Costas Vlachos

Jan 1, 1970
0
Watson A.Name - 'Watt Sun' said:
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.

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

A

#### Al

Jan 1, 1970
0
Costas Vlachos said:
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.

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 S #### Sofie Jan 1, 1970 0 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. I #### Ian Stirling Jan 1, 1970 0 In sci.electronics.misc Watson A.Name - 'Watt Sun' said: 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. 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.

M

#### Mark D. Zacharias

Jan 1, 1970
0
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.

R

#### Ratch

Jan 1, 1970
0
Mark D. Zacharias said:
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

A

#### AC/DCdude17

Jan 1, 1970
0
X-No-Archive: Yes

Watson A.Name - 'Watt Sun' said:
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.

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

R

#### Ratch

Jan 1, 1970
0
Mark D. Zacharias said:
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

W

#### Watson A.Name - 'Watt Sun'

Jan 1, 1970
0
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

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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###Got a Question about ELECTRONICS? Check HERE First:###
http://users.pandora.be/educypedia/electronics/databank.htm
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 <at> hotmail.com
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
@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@

W

#### Watson A.Name - 'Watt Sun'

Jan 1, 1970
0
Watt Sun:
Just a little cumbersome.... isn't it??

Less cumbersome than removing the DMM leads and then reconnecting
them.
As you indicated, there is a risk of smoking low ohm, low wattage resistors

Yes, as I indicated.
unless you already know the value... if that is the case then why are you
measuring it??......

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.
....and the accuracy is compromised because you are measuring the voltage
and then measuring the current.....

It still finds the value with a reasonable accuracy.
and you are at the mercy of the regulation of your power supply.

The regulation of the power supply makes no difference.
2 meter operations instead of one... the inherent innaccuracy of one of the

You're repeating yourself. As I said above, it still finds the value
with reasonable accuracy.
give me a DMM or VOM with a dedicated OHMS function any time..

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.
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.

Right. Now you've stated something useful.

--
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###Got a Question about ELECTRONICS? Check HERE First:###
http://users.pandora.be/educypedia/electronics/databank.htm
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 <at> hotmail.com
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
@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@

W

#### Watson A.Name - 'Watt Sun'

Jan 1, 1970
0
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. I bought a few of those$5 DMMs from Futurlec a few months ago,
actually I think they were about $6. 9V vattery included(!) -- @@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@,@@[email protected]@[email protected],@@[email protected]@[email protected]@[email protected]@ ###Got a Question about ELECTRONICS? Check HERE First:### http://users.pandora.be/educypedia/electronics/databank.htm My email address is whitelisted. *All* email sent to it goes directly to the trash unless you add NOSPAM in the Subject: line with other stuff. alondra101 <at> hotmail.com 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 @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@ R #### Ratch Jan 1, 1970 0 Watson A.Name - 'Watt Sun' said: http://www.launc.tased.edu.au/online/sciences/PhysSci/done/electric/resistnc/Resistance.htm 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. 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 W #### Watson A.Name - 'Watt Sun' Jan 1, 1970 0 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. Or use a constant current PS and short the far end of the cord, and measure across both conductors on the near end. 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 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. -- @@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@,@@[email protected]@[email protected],@@[email protected]@[email protected]@[email protected]@ ###Got a Question about ELECTRONICS? Check HERE First:### http://users.pandora.be/educypedia/electronics/databank.htm My email address is whitelisted. *All* email sent to it goes directly to the trash unless you add NOSPAM in the Subject: line with other stuff. alondra101 <at> hotmail.com 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 @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@ W #### Watson A.Name - 'Watt Sun' Jan 1, 1970 0 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. 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] -- @@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@,@@[email protected]@[email protected],@@[email protected]@[email protected]@[email protected]@ ###Got a Question about ELECTRONICS? Check HERE First:### http://users.pandora.be/educypedia/electronics/databank.htm My email address is whitelisted. *All* email sent to it goes directly to the trash unless you add NOSPAM in the Subject: line with other stuff. alondra101 <at> hotmail.com 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 @@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@[email protected]@[email protected]@@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@[email protected]@@ C #### Chuck Harris Jan 1, 1970 0 Watson said: 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] It doesn't matter what your original post said, the OM found a factoid, and wants to show it off! -Chuck, WA3UQV I #### Ian Stirling Jan 1, 1970 0 In sci.electronics.misc Watson A.Name - 'Watt Sun' said: I bought a few of those$5 DMMs from Futurlec a few months ago,
actually I think they were about $6. 9V vattery included(!) 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

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

--
http://inquisitor.i.am/ | mailto:[email protected] | Ian Stirling.
---------------------------+-------------------------+--------------------------
Lord, grant me the serenity to accept that I cannot change, the
courage to change what I can, and the wisdom to hide the bodies
of those I had to kill because they pissed me off. - Random

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