# Meter To Measure Current / Calculator Suggestions

Discussion in 'General Electronics' started by Jim Douglas, Feb 7, 2005.

1. ### Jim DouglasGuest

I have been looking at a new meter to measure current. I wanted another
method to measure instead of moving cables back and forth between
voltage/amps.

I was thinking that as a hobby builder I would want to measure between
1uA-5A? I am not having alot of luck with finding a meter that will go that
low. The ones that do go that low don't go anywhere near what I would want
on the high side. What range do others use, I am just playing around now
learning transistors, filters, active antenna's and such with the goal of
building a shortwave transmitter in the future.

Also I want to get another calculator that I could program to perform
functions. I have been checking out quite a few on Ebay and don't want to
spend the HPXX-C model prices, any recommendations there?

Thanks for taking the time reading this!

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

2. ### Peter BennettGuest

I find that I very rarely need to measure current (and I've been
working in electronics for over 40 years).

However, most DVMs these days will measure currents from 200 uA to 10A
full scale, which should cover almost any requirement.

--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter

3. ### Joe McElvenneyGuest

Hi,

So what you do is to use more than one meter to cover the range
you need. 50uA (or lower) analogue meters are easily found in
surplus and at reasonable prices. Alternatively make one using a
fixed resistor, an op-amp (or FET) voltage amplifier and an el-cheapo
DVM. Generally, low current circuits are high resistance, so the
extra series resistance won't matter.

Cheers - Joe

4. ### BanGuest

Here is a circuit that looks a bit like an Instrumentation Amplifier with
the property of no voltage drop. It is of course good only for small
currents <10mA. With a FET-input opamp really small currents can be measured
with minimum impact on the DUT.
___
+-|___|-+----+
| R |
I | |\ |
o----->----+-|+\ | ___ ___
| >- +----)-+-|___|-+-|___|-+
| +-|-/ | | | R1 | R1 |
| | |/ .-. | | | ===
| | | | | | | GND
| | | |R | | |
| | '-' | | | |\
Ue=0 | | | | +-|+\ Uout
| +-------+ | | | >--+----o
| | | | | +-|-/ |
| | .-. | | | |/ | Uout=2*R*I
| | | | | | | |
| | | |R | | | |
| | |\ '-' | | | |
| +-|-\ | | | ___ | ___ |
V | >- +----+-)-|___|-+-|___|-+
o-----<----+-|+/ | R1 R1
I | |/ |
| ___ |
+-|___|-+------+
R
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
view\fix font

5. ### beastGuest

I don't know about measuring low amps, but I though of a way to
measure high AC voltages: By having a rated induction coil pushed
against the measured voltage, the induced voltage would be
proportional to the voltage present...

Isn't that so?

6. ### mikeGuest

HP3478A will do 3 amps. Has 1uA resolution on 300mA scale.
My experience has been that whenever I really, really needed ACCURATE
low current
measurements, there were a bunch of other issues. The meter resolution
was the least of my worries.

HP calculators are nice if you have dedicated calculations and
absolutely need portability. But a laptop is cheaper. Desktop cheaper
still. There are some mighty nice math packages available.
mike

--
Wanted, PCMCIA SCSI Card for HP m820 CDRW.
FS 500MHz Tek DSOscilloscope TDS540 Make Offer
http://nm7u.tripod.com/homepage/te.html
Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
Bunch of stuff For Sale and Wanted at the link below.
http://www.geocities.com/SiliconValley/Monitor/4710/

7. ### DaveMGuest

How accurate is ACCURATE? What kind of resolution is acceptable? High
accuracy usually means high resoultion also.
The HP 428B DC Current Meter is an older vacuum tube technology instrument,
but it has a clamp-on probe that will let you measure current without having
to open the current path to measure. Full-scale ranges from 1ma to 10A.
+/- 3% full scale on all ranges. Find them on Ebay from \$25 - \$150.
--
Dave M
MasonDG44 at comcast dot net (Just subsitute the appropriate characters in

Never take a laxative and a sleeping pill at the same time!!

8. ### Dimitrios TzortzakakisGuest

I have a hung chang analogue meter that measures (dc) current from 5 mA to
10 A, which I think is sufficient.In all my life I needed to measure current
only once, when I needed to find out if a battery charger provided the right
charging current.I trashed my digital multimeter, after finding out it was
incapable of measuring leaking resistance (hundreds of kohm) correctly.5 mA
is quite a small current, and 10 quite a large.There are meters with a clamp
(a small current transformer) that you put around the live conductor in AC
and measures current indirectly.There are few meters that can measure ac
current directly, thus it passes through them.

9. ### SteveBGuest

Just get a cheap meter that measures the low end. All you then need to
expand the top range is a shunt resistor, you should be able to wind your
own as the value will be very low for fsd readings of 5A or more.

10. ### ctyguyGuest

E÷R has always done it for me, but I did use a current range checking for
car battery leakage with ignition off once.

FWIW forget using the current ranges, you'll only end up wrecking the DAMN
DMM

11. ### SteveBGuest

A reasonable DMM will have an internal replaceable fuse for the current
range to protect the meter.

12. ### Guest

Get a DMM with current measuring capability up
to 10 or 20 amps. (It seems that those are the
two most common sizes for the high side of the
current measuring capability.) On the low side
you don't need to measure down to 1 uA. Standard
for these meters seems to be 200 uA. Almost any
decent DMM you buy has current measuring capability.
Even the \$2.99 (on sale) Harbor Freight meter
that Roy mentioned measures 200 uA to 10 Amps. I
think it may be on sale now. By the way, that is
a great buy - it even comes with a 9V battery
included. It's great for non-critical measurements,
and for "kick-around" use where if it gets damaged
or lost it doen't matter. And the thing is
surprisingly accurate.

If you do enough messing around as a hobbyist, you
will certainly run into occasions where you want to
measure the current.

Ed

13. ### Jim DouglasGuest

OK, you guy's with the Harbor Freight got to me and I just completed the
order for (5) of the \$2.99 meters. I was going to order more and put em on
Ebay. I will have some to give to other's when they ask some questions about
electronics. Thanks!