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

[Ian Stirling]

In sci.electronics.misc Watson A.Name - 'Watt Sun' <alondra101@hotmail.com> wrote:

In article <bf4ah6$a2p$3$8300dec7@news.demon.co.uk>,
root@mauve.demon.co.uk mentioned...
In sci.electronics.misc Watson A.Name - 'Watt Sun' <alondra101@hotmail.com> wrote:
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.

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
none had approached 3V.

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

--
http://inquisitor.i.am/ | mailto:inquisitor@i.am | 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

 

[Cher]

the bottom line is that you are recreating an ohm meter except that it
has two meters.

a standard ohm meter does exactly the same thing using an internal
battery

 

[Tim Williams]

"Ratch" <Watchit@Comcast.net> wrote in message
news:BZmRa.80118$Ph3.9181@sccrnsc04...

While correct and true in all
cases, those formulas are NOT Ohm's law, and it is wrong to call them
that.

Dude, go back to school, Algebra 1. V=IR > I=V/R > R=V/I etc.
The equation can be rearranged to any of the others by simple
multiplication and division by whichever variable.

Tim

--
In the immortal words of Ned Flanders: "No foot longs!"
Website @ http://webpages.charter.net/dawill/tmoranwms

 

[Tim Williams]

"Watson A.Name - 'Watt Sun'" <alondra101@hotmail.com> wrote in message
news:MPG.1980214a6fc902f1989aa7@news.inreach.net...

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.

And besides that, it measures actual in-circuit conditions, because a
resistor is *supposed* to get hot, at least if it's doing its job...

Tim (can't believe the number of negative replies on this thread)

--
In the immortal words of Ned Flanders: "No foot longs!"
Website @ http://webpages.charter.net/dawill/tmoranwms

 

[Sofie]

How hot?
--------------

And besides that, it measures actual in-circuit conditions, because a
resistor is *supposed* to get hot, at least if it's doing its job...

 

[Sir Charles W. Shults III]

How hot? One solution is P=EI, where power in watts is equal to the product
of voltage and current. So the voltage drop across the resistor multiplied by
the current in amps through the resistor will yield watts of heat created by the
resistor.
Now, if you know the composition of the resistor, you can look up the
specific heat of the material and calculate how much energy it takes to change
its temperature. You will need to know how many joules of energy have been
applied. Calculate the joules by multiplying the time that the power was
applied in seconds by the power in watts. Now look at the specific heat table
and it will tell you how many joules per Kelvin it takes to raise the
temperature.
If you have any physics experience, it will be very simple.

Cheers!

Chip Shults
My robotics, space and CGI web page - http://home.cfl.rr.com/aichip

 

[Sofie]

Sir Charles W. Shults III:
That is a good in depth, overly complicated, answer but not the one I was
looking for..... obviously since the Tim Williams posting said ".And
besides that, it measures actual in-circuit conditions, because a resistor
is *supposed* to get hot, at least if it's doing its job..." ..... and
since the overly discussed "2 meters and power supply" ohm meter method can
overheat low-ohm resistors..... and the resistance of any resistor will vary
with temperature..... I was wondering at what exact "hot" temperature the
resistor should be at to make an accurate measurement..... LOL
Obviously I am not looking for any kind of answer here....... I just think
that this thread went bonkers when all the original poster wanted to do is,
as Chuck Harris indicated in his reply post, show off his new found
factoid.... however it seems clear that it may not be a very practical and
convenient method as evidenced by the length and tone of the majority of the
reply posts.
--
Best Regards,
Daniel Sofie
Electronics Supply & Repair
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -



"Sir Charles W. Shults III" <aichipREM@OVEcfl.THISrr.com> wrote in message
news:OFFRa.122068$ic1.2487130@twister.tampabay.rr.com...

How hot? One solution is P=EI, where power in watts is equal to the
product
of voltage and current. So the voltage drop across the resistor
multiplied by
the current in amps through the resistor will yield watts of heat created
by the
resistor.
Now, if you know the composition of the resistor, you can look up the
specific heat of the material and calculate how much energy it takes to
change
its temperature. You will need to know how many joules of energy have
been
applied. Calculate the joules by multiplying the time that the power was
applied in seconds by the power in watts. Now look at the specific heat
table
and it will tell you how many joules per Kelvin it takes to raise the
temperature.
If you have any physics experience, it will be very simple.

Cheers!

Chip Shults
My robotics, space and CGI web page - http://home.cfl.rr.com/aichip

 

[Sir Charles W. Shults III]

"Sofie" <sofie@olypen.com> wrote in message
news:vheca77sefdtd5@corp.supernews.com...

Sir Charles W. Shults III:
That is a good in depth, overly complicated, answer but not the one I was
looking for..... obviously since the Tim Williams posting said ".And
besides that, it measures actual in-circuit conditions, because a resistor
is *supposed* to get hot, at least if it's doing its job..."

Thanks, I think.
Well, for most applications, the variation does not really matter. But
"overly complicated" is a relative thing. Without all those steps and facts,
you can't really derive an answer at all. That is, unless somebody has made a
chart like a nomograph where you pick out the resistor type and go to a table,
start at the room temperature, drop a ruler across to a power value, and derive
the reading from a scale.

Cheers!

Chip Shults
My robotics, space and CGI web page - http://home.cfl.rr.com/aichip

 

[Lizard Blizzard]

Ian Stirling wrote:

In sci.electronics.misc Watson A.Name - 'Watt Sun' <alondra101@hotmail.com> wrote:

In article <bf4ah6$a2p$3$8300dec7@news.demon.co.uk>,
root@mauve.demon.co.uk mentioned...

In sci.electronics.misc Watson A.Name - 'Watt Sun' <alondra101@hotmail.com> wrote:

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.

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.

But, But.. Doesn't the FR in FR4 mean flame resistant? If so, how
could it burn? Char?

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.,

--
----------------(from OED Mini-Dictionary)-----------------
PUNCTUATION - Apostrophe
Incorrect uses: (i) the apostrophe must not be used with a plural
where there is no possessive sense, as in ~tea's are served here~;
(ii) there is no such word as ~her's, our's, their's, your's~.

Confusions: it's = it is or it has (not 'belonging to it'); correct
uses are ~it's here~ (= it is here); ~it's gone~ (= it has gone);
but ~the dog wagged its tail~ (no apostrophe).
----------------(For the Apostrophe challenged)----------------
From a fully deputized officer of the Apostrophe Police!

<<Spammers use Weapons of Mass Distraction!>>

I bought some batteries, but they weren't included,
so I had to buy them again.
-- Steven Wright

FOR SALE: Nice parachute: never opened - used once.

F
o
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f
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..

 

[Ratch]

"Tim Williams" <tmoranwms@charter.net> wrote in message
news:vhe6u95nd8l13d@corp.supernews.com...

"Ratch" <Watchit@Comcast.net> wrote in message
news:BZmRa.80118$Ph3.9181@sccrnsc04...
While correct and true in all
cases, those formulas are NOT Ohm's law, and it is wrong to call them
that.

Dude, go back to school, Algebra 1. V=IR > I=V/R > R=V/I etc.
The equation can be rearranged to any of the others by simple
multiplication and division by whichever variable.

Tim

I believe everyone on this newsgroup can isolate each of the terms
algebraically. What is your point? What don't you agree with? What is
incorrect? Be specific. Ratch

 

[Lizard Blizzard]

Tim Williams wrote:

"Watson A.Name - 'Watt Sun'" <alondra101@hotmail.com> wrote in message
news:MPG.1980214a6fc902f1989aa7@news.inreach.net...

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.


And besides that, it measures actual in-circuit conditions, because a
resistor is *supposed* to get hot, at least if it's doing its job...

Excellent point. If you adjust the voltage (and current) to the same as
the V (and current) in circuit, then your measurement reflects a more
realistic value of resistance.

And another thought. If you are trying to ferret out an intermittent
problem, then subjecting the resistor to similar V and I conditions
found in the circuit can help make the problem appear, so it an be
isolated to a certain component.

Tim (can't believe the number of negative replies on this thread)

--

--
----------------(from OED Mini-Dictionary)-----------------
PUNCTUATION - Apostrophe
Incorrect uses: (i) the apostrophe must not be used with a plural
where there is no possessive sense, as in ~tea's are served here~;
(ii) there is no such word as ~her's, our's, their's, your's~.

Confusions: it's = it is or it has (not 'belonging to it'); correct
uses are ~it's here~ (= it is here); ~it's gone~ (= it has gone);
but ~the dog wagged its tail~ (no apostrophe).
----------------(For the Apostrophe challenged)----------------
From a fully deputized officer of the Apostrophe Police!

<<Spammers use Weapons of Mass Distraction!>>

I bought some batteries, but they weren't included,
so I had to buy them again.
-- Steven Wright

FOR SALE: Nice parachute: never opened - used once.

F
o
d
d
e
r

f
o
r

s
t
u
p
i
d

n
o
t

e
n
o
u
g
h

i
n
c
l
d
u
d
e
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t
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x
t

e
r
r
o
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m
s
g
..

 

[Lizard Blizzard]

Sofie wrote:

How hot?
--------------

I squared R hot, in watts or fraction thereof. And that value should be
(somewhat) less than the maximum rating for that component.

You could be running just a few volts across a 1/8W resistor, and it may
not feel so hot to the touch, but it may be running at or above its
rated dissipation. And that's not cool, no pun intended.

And besides that, it measures actual in-circuit conditions, because a
resistor is *supposed* to get hot, at least if it's doing its job...

--
----------------(from OED Mini-Dictionary)-----------------
PUNCTUATION - Apostrophe
Incorrect uses: (i) the apostrophe must not be used with a plural
where there is no possessive sense, as in ~tea's are served here~;
(ii) there is no such word as ~her's, our's, their's, your's~.

Confusions: it's = it is or it has (not 'belonging to it'); correct
uses are ~it's here~ (= it is here); ~it's gone~ (= it has gone);
but ~the dog wagged its tail~ (no apostrophe).
----------------(For the Apostrophe challenged)----------------
From a fully deputized officer of the Apostrophe Police!

<<Spammers use Weapons of Mass Distraction!>>

I bought some batteries, but they weren't included,
so I had to buy them again.
-- Steven Wright

FOR SALE: Nice parachute: never opened - used once.

F
o
d
d
e
r

f
o
r

s
t
u
p
i
d

n
o
t

e
n
o
u
g
h

i
n
c
l
d
u
d
e
d

t
e
x
t

e
r
r
o
r

m
s
g
..

 

[Lizard Blizzard]

Sir Charles W. Shults III wrote:

How hot? One solution is P=EI, where power in watts is equal to the product
of voltage and current. So the voltage drop across the resistor multiplied by
the current in amps through the resistor will yield watts of heat created by the
resistor.
Now, if you know the composition of the resistor, you can look up the
specific heat of the material and calculate how much energy it takes to change
its temperature. You will need to know how many joules of energy have been
applied. Calculate the joules by multiplying the time that the power was
applied in seconds by the power in watts. Now look at the specific heat table
and it will tell you how many joules per Kelvin it takes to raise the
temperature.
If you have any physics experience, it will be very simple.

Whew! Sounds like an experiment in calorie whatever, with all the
thermometer measurements, etc. Yeah, now I remember. It's called Cold
Fusion!!

Cheers!

Chip Shults

--
----------------(from OED Mini-Dictionary)-----------------
PUNCTUATION - Apostrophe
Incorrect uses: (i) the apostrophe must not be used with a plural
where there is no possessive sense, as in ~tea's are served here~;
(ii) there is no such word as ~her's, our's, their's, your's~.

Confusions: it's = it is or it has (not 'belonging to it'); correct
uses are ~it's here~ (= it is here); ~it's gone~ (= it has gone);
but ~the dog wagged its tail~ (no apostrophe).
----------------(For the Apostrophe challenged)----------------
From a fully deputized officer of the Apostrophe Police!

<<Spammers use Weapons of Mass Distraction!>>

I bought some batteries, but they weren't included,
so I had to buy them again.
-- Steven Wright

FOR SALE: Nice parachute: never opened - used once.

F
o
d
d
e
r

f
o
r

s
t
u
p
i
d

n
o
t

e
n
o
u
g
h

i
n
c
l
d
u
d
e
d

t
e
x
t

e
r
r
o
r

m
s
g
..

 

[Peter Gottlieb]

When I need to power some of my smaller creations I use an ohmmeter as a
power supply...

 

[Sir Charles W. Shults III]

Well, if you think about it, doing cold fusion experiments will teach you
everything you need to know about calorimetry and heat measurement! So even if
the effect is never shown to yield any anomalous heat, you still got an
education. Funny how pursuing even an odd question can teach you worlds.

Cheers!

Chip Shults
My robotics, space and CGI web page - http://home.cfl.rr.com/aichip

 

Tim Williams wrote:

Tim (can't believe the number of negative replies on this thread)

Exactly. It is un-eff-ing believable how some people just love
to snipe.

I wonder how many of the "snipers" ever had to brew a meter shunt?
You can use the "Watson ohmmeter" for that - and, as another
poster mentioned - toss your Fluke in the dustbin.

 

Ian Stirling wrote:

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.,

YES!!!! I had a situation where I needed to continually and
simultaneously monitor current and voltage into and out of a
dc-dc converter with two outputs, while varying the loads and
the input supply. Some asshole said he could do it with two
meters - another "sniper" who obviously did not understand
the word "simultaneously". Anyway, there's a catalog outfit
named Harbor Freight, and they had multimeters on sale for
$2.99 including the 9V battery. I bought 4 of them. As you
correctly pointed out, it's amazing how many you can use
at the same time when they are just sitting around waiting
for use!

--
http://inquisitor.i.am/ | mailto:inquisitor@i.am | 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

 

[Mark Mcmillan]

After all that I still can't get my ohmmeter to work as a power supply!!!

 

[Mark Jones]

Ratch wrote:

"Tim Williams" <tmoranwms@charter.net> wrote in message
news:vhe6u95nd8l13d@corp.supernews.com...
"Ratch" <Watchit@Comcast.net> wrote in message
news:BZmRa.80118$Ph3.9181@sccrnsc04...
While correct and true in all
cases, those formulas are NOT Ohm's law, and it is wrong to
call them that.

Dude, go back to school, Algebra 1. V=IR > I=V/R > R=V/I etc.
The equation can be rearranged to any of the others by simple
multiplication and division by whichever variable.

Tim

I believe everyone on this newsgroup can isolate each of the
terms algebraically. What is your point? What don't you
agree with? What is incorrect? Be specific. Ratch

I think, for "practical" electronics, the "ohmic" and "non-ohmic"
properties of resitance and its implication in the Ohm's Law equation is
largely depreciated. All resistors are likely assumed ohmic except for
specialty devices or applications, most of which deal with temperature and
not voltage. Personally, I find the voltage-dependant-negative-coefficient
resistances (such as carbon fiber) particularly interesting.

 

[Mark Jones]

Mark Mcmillan wrote:

After all that I still can't get my ohmmeter to work as a
power supply!!!

I was going to ask you to check out an image, but I can't find it...
anyways it was a simple circuit I drew and simulated in proteus. It was a
battery, a lightbulb, and an ohmmeter across the lamp. The lamp was lit and
the resistance was 120 ohms...

Oh well it might have been funnier with the actual drawing. Man I need to
get out more...