Measuring Current
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Measuring Current
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Jim Douglas
Guest
Posted: Sun Jan 30, 2005 3:00 am    Post subject: Measuring Current Reply with quote
I currently have one nice digital VOM. So I measure voltage, then current,
back and forth having to move the cable each time, which sucks. I want a
stand-alone ammeter that I can quickly place within a circuit, I think. I
would rather not purchase another meter just to measure current and am
thinking that this might be something easy to build but didn't find alot
searching so far.

Do you guy's use a separate amp meter for this stuff? I am just a hobbyist
that likes playing around with things. One goal is to eventually build a SW
transmitter. Should I try to purchase, build? I would prefer building. I am
thinking I would like to be able to measure down to the microampere range.

Thanks for any input!

--



Jim Douglas
www.genesis-software.com
Carrollton, TX USA 75006
Latitude 32.9616
Longitude 96.8916

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Art
Guest
Posted: Sun Jan 30, 2005 3:11 am    Post subject: Re: Measuring Current Reply with quote
Suggest checking into the Amateur News Groups regarding additional
information regarding home brew radio transmitting and receiving equipment.
S.A. rec.radio.amateur.homebrew, etc. Most of the equipment I've worked,
repaired, etc have an actual separate ammeter (milliamp meter).
"Jim Douglas" <james.douglas@genesis-software.com> wrote in message
news:9_idnYKM8aTDZGbcRVn-sg@comcast.com...
Quote:
I currently have one nice digital VOM. So I measure voltage, then current,
back and forth having to move the cable each time, which sucks. I want a
stand-alone ammeter that I can quickly place within a circuit, I think.
I
would rather not purchase another meter just to measure current and am
thinking that this might be something easy to build but didn't find alot
searching so far.

Do you guy's use a separate amp meter for this stuff? I am just a hobbyist
that likes playing around with things. One goal is to eventually build a
SW
transmitter. Should I try to purchase, build? I would prefer building. I
am
thinking I would like to be able to measure down to the microampere range.

Thanks for any input!

--



Jim Douglas
www.genesis-software.com
Carrollton, TX USA 75006
Latitude 32.9616
Longitude 96.8916

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John Woodgate
Guest
Posted: Sun Jan 30, 2005 3:30 am    Post subject: Re: Measuring Current Reply with quote
I read in sci.electronics.design that Jim Douglas
<james.douglas@genesis-software.com> wrote (in <9_idnYKM8aTDZGbcRVn-
sg@comcast.com>) about 'Measuring Current', on Sat, 29 Jan 2005:

Quote:
I want a stand-alone ammeter that I can quickly place within a circuit,
I think. I would rather not purchase another meter just to measure
current and am thinking that this might be something easy to build but
didn't find alot searching so far.

An analogue ammeter with a decent milliammeter movement is costly, and
you have to hand-craft the shunts. You could buy a bare digital panel
meter module, which will make an ammeter with 200 mV drop at full scale
on all ranges. This could cost less and be easier to make.

If it were me, I'd buy another inexpensive digital multimeter.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

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JeffM
Guest
Posted: Sun Jan 30, 2005 4:09 am    Post subject: Re: Measuring Current Reply with quote
Quote:
I would rather not purchase another meter just to measure current
Jim Douglas

Woodgate is right. Buy a cheapie.
Got one of these in Dallas? http://www.harborfreight.com/
$2.99 until 2/14/05.
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CJT
Guest
Posted: Sun Jan 30, 2005 4:35 am    Post subject: Re: Measuring Current Reply with quote
Jim Douglas wrote:
Quote:
I currently have one nice digital VOM. So I measure voltage, then current,
back and forth having to move the cable each time, which sucks. I want a
stand-alone ammeter that I can quickly place within a circuit, I think. I
would rather not purchase another meter just to measure current and am
thinking that this might be something easy to build but didn't find alot
searching so far.

Do you guy's use a separate amp meter for this stuff? I am just a hobbyist
that likes playing around with things. One goal is to eventually build a SW
transmitter. Should I try to purchase, build? I would prefer building. I am
thinking I would like to be able to measure down to the microampere range.

Thanks for any input!

I agree with the other posters that it would probably be cheaper to buy

another multimeter. However, I would add that your desire to measure in
the microampere range is probably wishful thinking. The equipment and
techniques involved in _accurately_ dealing in those ranges are, IMHO,
beyond what those of what you're likely to encounter in a typical
hobbyist setup.

--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form che...@prodigy.net.
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Roger Johansson
Guest
Posted: Sun Jan 30, 2005 4:55 am    Post subject: Re: Measuring Current Reply with quote
CJT <abujlehc@prodigy.net> wrote:

Quote:
I agree with the other posters that it would probably be cheaper to buy
another multimeter. However, I would add that your desire to measure
in the microampere range is probably wishful thinking. The equipment
and techniques involved in _accurately_ dealing in those ranges are,
IMHO, beyond what those of what you're likely to encounter in a typical
hobbyist setup.


Or learn to avoid current measurements as much as possible.
It takes time to open a connection and insert the amperemeter, and then
you have to restore the connection after measuring.

In most cases you can measure voltages and calculate the currents.

Make a few homemade shunts, like 1 Ohm and 0.1 Ohm, so you can insert the
shunt and measure the voltage over it, when there is no other way to find
out the current. The shunt can be a resistor or a suitable piece of wire.


--
Roger J.
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BFoelsch
Guest
Posted: Sun Jan 30, 2005 5:07 am    Post subject: Re: Measuring Current Reply with quote
"Jim Douglas" <james.douglas@genesis-software.com> wrote in message
news:9_idnYKM8aTDZGbcRVn-sg@comcast.com...
Quote:
I currently have one nice digital VOM. So I measure voltage, then current,
back and forth having to move the cable each time, which sucks. I want a
stand-alone ammeter that I can quickly place within a circuit, I think.
I
would rather not purchase another meter just to measure current and am
thinking that this might be something easy to build but didn't find alot
searching so far.

Do you guy's use a separate amp meter for this stuff? I am just a hobbyist
that likes playing around with things. One goal is to eventually build a
SW
transmitter. Should I try to purchase, build? I would prefer building. I
am
thinking I would like to be able to measure down to the microampere range.

Thanks for any input!

Well, I do approximately what you want. I have around the shop 3-4
miscellaneous meters in plain old sloping front panels. The meters are
almost free at the swap meet, the cabinets are the expense, but you could
fashion something yourself. Let's see, I have a 50 microamp, 1 milliamp, 500
milliamp and 5 amp units.

Leads are brought out through the top through a dual banana jack. For some
frequently used current ranges, I put some shunt resistors across a
stackable dual banana plug. By plugging the right shunt plug into the meter,
I get some ranges I commonly use; 10 mA, 100 mA, 1A, 15A.

You can laugh, but it sure beats wiring up range switches. Too, using the
dual banana jack forces you to unhook the meter from the source of power
before changing the shunt, which protects the meter from an open shunt
(burn-out) situation.
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Jim Douglas
Guest
Posted: Sun Jan 30, 2005 5:40 am    Post subject: Re: Measuring Current Reply with quote
Thanks for all the good input. I will be picking up another VOM for my
current measurements.

"Jim Douglas" <james.douglas@genesis-software.com> wrote in message
news:9_idnYKM8aTDZGbcRVn-sg@comcast.com...
Quote:
I currently have one nice digital VOM. So I measure voltage, then current,
back and forth having to move the cable each time, which sucks. I want a
stand-alone ammeter that I can quickly place within a circuit, I think.
I
would rather not purchase another meter just to measure current and am
thinking that this might be something easy to build but didn't find alot
searching so far.

Do you guy's use a separate amp meter for this stuff? I am just a hobbyist
that likes playing around with things. One goal is to eventually build a
SW
transmitter. Should I try to purchase, build? I would prefer building. I
am
thinking I would like to be able to measure down to the microampere range.

Thanks for any input!

--



Jim Douglas
www.genesis-software.com
Carrollton, TX USA 75006
Latitude 32.9616
Longitude 96.8916

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John Smith
Guest
Posted: Sun Jan 30, 2005 6:17 am    Post subject: Re: Measuring Current Reply with quote
"Jim Douglas" <james.douglas@genesis-software.com> wrote in message
news:9_idnYKM8aTDZGbcRVn-sg@comcast.com...
Quote:
I currently have one nice digital VOM. So I measure voltage, then current,
back and forth having to move the cable each time, which sucks. I want a
stand-alone ammeter that I can quickly place within a circuit, I think.
I
would rather not purchase another meter just to measure current and am
thinking that this might be something easy to build but didn't find alot
searching so far.

Do you guy's use a separate amp meter for this stuff? I am just a
hobbyist
that likes playing around with things. One goal is to eventually build a
SW
transmitter. Should I try to purchase, build? I would prefer building. I
am
thinking I would like to be able to measure down to the microampere
range.

Thanks for any input!

--



Jim Douglas
www.genesis-software.com
Carrollton, TX USA 75006
Latitude 32.9616
Longitude 96.8916


Hi, Jim -

For my current measurements, I bought an HP 428B (toob-type) off Ebay and
repaired it. The reason I wanted it was so I would not have to break
connections to measure current. Of course, one must have enough slack in
the wire to allow inserting the clamp-on probe.

Just an idea. Good luck.

John
(kes is not required)
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DaveM
Guest
Posted: Sun Jan 30, 2005 9:05 am    Post subject: Re: Measuring Current Reply with quote
"John Smith" <kd5yikes@mindspring.com> wrote in message
news:cth937024ca@enews4.newsguy.com...
Quote:

"Jim Douglas" <james.douglas@genesis-software.com> wrote in message
news:9_idnYKM8aTDZGbcRVn-sg@comcast.com...
I currently have one nice digital VOM. So I measure voltage, then current,
back and forth having to move the cable each time, which sucks. I want a
stand-alone ammeter that I can quickly place within a circuit, I think.
I
would rather not purchase another meter just to measure current and am
thinking that this might be something easy to build but didn't find alot
searching so far.

Do you guy's use a separate amp meter for this stuff? I am just a
hobbyist
that likes playing around with things. One goal is to eventually build a
SW
transmitter. Should I try to purchase, build? I would prefer building. I
am
thinking I would like to be able to measure down to the microampere
range.

Thanks for any input!

--



Jim Douglas
www.genesis-software.com
Carrollton, TX USA 75006
Latitude 32.9616
Longitude 96.8916


Hi, Jim -

For my current measurements, I bought an HP 428B (toob-type) off Ebay and
repaired it. The reason I wanted it was so I would not have to break
connections to measure current. Of course, one must have enough slack in
the wire to allow inserting the clamp-on probe.

Just an idea. Good luck.

John
(kes is not required)


Yup!!! I bought three of those HP 428B meters on ebay in one auction. One
of them had a problem, but was easily repaired. Old technology, but they
sure save the day when you need to jump around in a circuit and measure
currents quickly. They are capable of current measurement from 1ma full
scale to 10A full scale. Great equipment survives the ages.
--
Dave M
MasonDG44 at comcast dot net (Just subsitute the appropriate characters in
the address)

Never take a laxative and a sleeping pill at the same time!!
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Ken Smith
Guest
Posted: Sun Jan 30, 2005 5:03 pm    Post subject: Re: Measuring Current Reply with quote
In article <9_idnYKM8aTDZGbcRVn-sg@comcast.com>,
Jim Douglas <james.douglas@genesis-software.com> wrote:
[...]
Quote:
Do you guy's use a separate amp meter for this stuff? I am just a hobbyist
that likes playing around with things. One goal is to eventually build a SW
transmitter. Should I try to purchase, build? I would prefer building. I am
thinking I would like to be able to measure down to the microampere range.

Harbor Freight has DMMs on for $3.99. Buy one for each thing you want to
measure.

--
--
kensmith@rahul.net forging knowledge
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Ian Stirling
Guest
Posted: Mon Jan 31, 2005 2:53 am    Post subject: Re: Measuring Current Reply with quote
In sci.electronics.design CJT <abujlehc@prodigy.net> wrote:
<snip>
Quote:
I agree with the other posters that it would probably be cheaper to buy
another multimeter. However, I would add that your desire to measure in
the microampere range is probably wishful thinking. The equipment and
techniques involved in _accurately_ dealing in those ranges are, IMHO,
beyond what those of what you're likely to encounter in a typical
hobbyist setup.

I've found common multimeters fairly accurate for measuring very small
currents.
The typical 1Mohm input resistance at 200mV full-scale drops 200nA.
(when I checked it, it seemed to be accurate).
Even my cheap $4(us) meter has a 200uA range.
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Spehro Pefhany
Guest
Posted: Mon Jan 31, 2005 7:26 am    Post subject: Re: Measuring Current Reply with quote
On Tue, 1 Feb 2005 00:36:54 -0000, the renowned "Jim Douglas"
<james.douglas@genesis-software.com> wrote:

Quote:
I saw some of those on Ebay today, It seems to have a scope type thing that
simply wraps around a wire??

Like these ones I put up (these particular ones are sold)?

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=3870013201&ssPageName=STRK:MESO:IT
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=3867723467&ssPageName=STRK:MESO:IT

The clamp opens up and you can put it round the wire. They are *not*
for small currents-- scores of mA up to hundreds of A, depending.

If you put multiple wires through the core (like an AC line cord) they
will read about zero, unless there is a fault to ground, as they read
the difference between the two currents.

They are great for working on power stuff- you can poke around in a
panel and check motor currents etc., but for electronics you really
want some DVMs.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
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Rich Grise
Guest
Posted: Mon Jan 31, 2005 9:09 am    Post subject: Re: Measuring Current Reply with quote
On Sun, 30 Jan 2005 20:53:16 +0000, Ian Stirling wrote:

Quote:
In sci.electronics.design CJT <abujlehc@prodigy.net> wrote:
snip
I agree with the other posters that it would probably be cheaper to buy
another multimeter. However, I would add that your desire to measure in
the microampere range is probably wishful thinking. The equipment and
techniques involved in _accurately_ dealing in those ranges are, IMHO,
beyond what those of what you're likely to encounter in a typical
hobbyist setup.

I've found common multimeters fairly accurate for measuring very small
currents.
The typical 1Mohm input resistance at 200mV full-scale drops 200nA.
(when I checked it, it seemed to be accurate).
Even my cheap $4(us) meter has a 200uA range.

It's still a PITA to break the circuit to interpose the meter. If it's
something that I'll want to read the current for calibration or something,
I just put a 0R1 or 0R01 resistor in the circuit, and measure its voltage
drop.

I've also seen that technique used to increase the range of 0-1 or 1-0-1
milliammeters - a 1K resistor turns it into a 1K ohm/volt voltmeter - and
you measure the drop across the shunt. Much easier than trying to match a
shunt to a given meter's internal resistance.

Cheers!
Rich
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Robert Baer
Guest
Posted: Mon Jan 31, 2005 1:10 pm    Post subject: Re: Measuring Current Reply with quote
Ian Stirling wrote:
Quote:

In sci.electronics.design CJT <abujlehc@prodigy.net> wrote:
snip
I agree with the other posters that it would probably be cheaper to buy
another multimeter. However, I would add that your desire to measure in
the microampere range is probably wishful thinking. The equipment and
techniques involved in _accurately_ dealing in those ranges are, IMHO,
beyond what those of what you're likely to encounter in a typical
hobbyist setup.

I've found common multimeters fairly accurate for measuring very small
currents.
The typical 1Mohm input resistance at 200mV full-scale drops 200nA.
(when I checked it, it seemed to be accurate).
Even my cheap $4(us) meter has a 200uA range.

Err...most DVMs have a 10Meg input resistance, not 1Meg.
However, one can use a resistive shunt (previously suggested) and
measure the voltage across it (to 200mV full scale).
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