From the Lab March 1997 [Archive]

skranish

Mar 01, 1997, 01:00 AM

Beginner's Corner: Measuring Current 

Do you know how much current your motor draws at full throttle? You should! There has
been a lot of discussion on the eflight listserv lately about current measurement,
and a number of people have responded with something to the effect of 'use a shunt', often
with little or no explanation. 

Why not 'use a meter'? Well, as we discussed in previous columns, the currents in our
electric flight system are considered to be very high - much higher than any household
appliance - so typical current measuring equipment - especially consumer grade equipment -
is just not suitable. A standard multimeter may have a 10 Ampere current scale - with
a fuse - and this is almost high enough to measure the current on a tiny Speed 400 motor.
To measure the current on something larger, like the mighty Speed 500 or Speed 600 motors,
we need to use something else. 

This month we will discuss the hows and whys of current measurement. First you will
need to understand voltage, current, and resistance, which were covered in detail in the <a href="/articles/ezonemag/fromlab/kran1296.htm">December 96 'From the Lab'</a>. If you have not read that column, or
are not well versed in these concepts, please review it before continuing. 

Are you SURE you understand voltage, current, and resistance? If not, PLEASE go back
and review the <a href="/articles/ezonemag/fromlab/kran1296.htm">December 96 'From the Lab'. </a>

Ok, now that you understand voltage, current, and resistance, we will introduce the
concept of schematics, those strange looking drawings used by electrical engineers to
describe and define a circuit. A circuit is a collection of components connected
in a way to do something (hopefully) useful. What we call a schematic is a schematic
representation of a circuit. The drawing is a way of showing the circuit: what
the components are, and how they are interconnected. The schematic may look very different
from the physical implementation, but the components and connections should be the same. 

An Introduction to Schematics 

<img src="http://static.rcgroups.com/articles/ezonemag/fromlab/../../images/krschem1.gif" border="1" width="313" height="99">

The drawing above shows typical schematic representations, or symbols, for some circuit
elements - devices - that are relevant to our discussion. There are no real hard and fast
'rules' about schematics, and some people use different symbols for devices - note
the US and UK representations of a resistor. These basic devices may be connected together
in a variety of ways to make different circuits. 

A Little Bit of Circuit Theory: Ohms's Law 

The most basic law that governs electrical circuits is Ohm's Law, which describes the
relationship between voltage (V), resistance (R) , and current (I). The common form is: 

V = I * R         volts = current * resistance 

but it is equally valid to rearrange the equation (remember your basic algebra?) 

I = V / R           current = volts / resistance 

This form of the equation is the one we are interested in here. Think about what this
equation says: if we measure the voltage across a known resistor, we can determine the
current through the resistor. This is in fact how most current measurements are done - a
resistor is added to the circuit to cause a voltage drop, and the voltage across the
resistor is measured. (For you know-it-alls, we will discuss measuring current from
induced magnetic fields in a later column. There are definitely some advantages, but also
a major disadvantage - cost!) 

<img src="http://static.rcgroups.com/articles/ezonemag/fromlab/../../images/krschem2.gif" width="144" height="107">

In the above circuit, the battery on the left is a voltage source, and the same
voltage appears across the resistor on the left. The current goes round and round and
round. The components are connected to make a complete current loop, and we have a simple
circuit. 

What is a 'Shunt', anyway? 

A lot of the eflight traffic about current measurement mentions the use of 'shunts'.
Let's take a look at what a shunt is (and is NOT!) and how it works. 

To understand what a shunt is, you first must understand why they exist at all.
According to the American Heritage Dictionary, a shunt is 'a low-resistance connection
between two points in an electrical circuit that forms an alternative path for a portion
of the current', 

The traditional type of electrical measuring device is the 'D'Arsonval moving coil
meter'. This is the 'old style' analog type of meter, and you may have seen multimeters
that are based on one. It is also common to find ones that are not multimeters, but rather
single purpose meters. D'Arsonval meters are electrically a resistance, so they draw
a current proportional to the voltage across them. This also means that as a current
passes through the meter, a voltage will appear across it. The more current , the
more voltage, so you don't want much current flowing through the meter - and they are
designed to use very little current, generally in the range of hundreds of microAmperes to
milliAmperes. 

The D'Arsonval meter is actually current responding, that is a small current through
the meter causes the meter needle to move, and the deflection is proportional to the
current. If such a meter is to be used to measure a larger current, most of the current
must be sent somewhere else, rather than through the meter - and that is what a shunt is
for.  A shunt is a resistor in parallel with the meter. Two resistors in
parallel (remember, the meter appears as a resistance) will share the current that
passes through them: 

<img src="http://static.rcgroups.com/articles/ezonemag/fromlab/../../images/krschem3.gif" width="159" height="152">

If a shunt resistor is placed in parallel with the meter, the shunt and
the meter will share the total current. The current sharing is actually in proportion
to the ratio of the meter and shunt resistances, but we do not need to go into the
math here. Below is a schematic showing how a moving coil meter with a shunt would be used
to measure the current through a motor: 

<img src="http://static.rcgroups.com/articles/ezonemag/fromlab/../../images/krschem4.gif" width="293" height="199">

Now that you know what a shunt is, you can forget about it. Despite all of the email on
the listserv from folks who should know better telling everyone to use a shunt to measure
current, what they are describing is not a shunt at all. Unless you are using
a (current responding) moving coil meter, you do not need a shunt. What you need is a
current sensing resistor, also called a voltage drop resistor. They look
about the same, but they work differently. 

Current Sensing Resistors or Voltage Drop Resistors 

You should remember our rearranged form of Ohm's Law: 

I = V / R           current = volts / resistance 

If we measure the voltage across a known resistance, we can calculate the current
flowing through the resistance. This is the technique used by digital multimeters (DMMs),
the Astro Flight Model 100, and a lot of other equipment. The difference between this
method and the above described shunt is that all of the current being measured flows
through the current sensing resistor, whose sole purpose is to provide a voltage to
measure. 

<img src="http://static.rcgroups.com/articles/ezonemag/fromlab/../../images/krschem5.gif" width="293" height="198">

The above schematic shows the current sensing resistor on the positive side of the
motor. There is really no reason it cannot be attached at the other (negative) side of the
 motor: 

<img src="http://static.rcgroups.com/articles/ezonemag/fromlab/../../images/krschem7.gif" width="292" height="197">

The value of the current sensing resistor must be very small. The misnamed 'shunts'
sold by Aveox are 0.001 Ohms (1 milliOhm), so a voltage of 1mV will appear across it
for every Ampere that flows through it. The current sensing resistor actually has an
effect on the circuit, because it adds resistance to the circuit. Astro Flight likes
to give a strange description of their Model 100 taking 300 RPM off at 50 Amps or
something like that, but there is a simpler way to look at it: the current sensing
resistor dissipates power, rather than letting it go to the motor, which is where we
really want it. In the <a href="/articles/ezonemag/fromlab/kran0197.htm">January 97 'From The Lab'</a> we discussed
power - P = I * I * R. For this example, consider the case of a 25 Amp system - the
power dissipated in the current sensing resistor will be P = 25 * 25 * 0.001 = 0.63 Watts,
or about 0.4% of the total power in a 170 Watt system. This may or may not be significant,
depending on where else you are wasting power. 

Instrumentation: What you REALLY need 

<table border="0" cellpadding="5" width="100%">
<tr>
<td><img src="http://static.rcgroups.com/articles/ezonemag/fromlab/../../images/multim.jpg" width="128" height="186"></td>
<td>Face it: you need a multimeter. If you are operating electric airplanes without one,
you are flying blind, plane and simple. If you don't have one yet, consider investing in a
good one. Most folks want digital multimeters (DMMs), and there are very good reasons for
buying digital instead of an analog meter. In general, analog meters are still better for adjusting
things, but digital meters are generally superior for measuring things. </td>
</tr>
</table>

A multimeter is one that can make several types of measurements, usually AC and DC
voltage, AC and DC current, resistance, and continuity. Some meters add capacitance,
frequency, and component testing. The current ranges on a typical multimeter will
typically be less than 10 Amps, so it will only be useful for measuring charging currents
- not discharge currents. This is why we are having all this discussion about current
sensing resistors, anyway - because most moderate price to cheap meters will not measure
beyond 10 Amps. 

The hand-held multimeter market is basically broken into two segments: Fluke and
everyone else. I have several Flukes, and would be hard pressed to consider anything else.
(the above picture is of Jim Bourke's meter, because he has a scanner!) You basically get
what you pay for - Fluke meters probably have a useful life of 15 to 20 years, and a cheap
meter will have a service life of one accidental trip to the floor. 

Make sure that you get one with a DC voltage scale that can resolve milliVolts, because
the current sensing resistors will provide a voltage of 1mV per Amp of current
flowing. Cheap meters may not go this low. 

<img src="http://static.rcgroups.com/articles/ezonemag/fromlab/../../images/astro100.jpg" width="220" height="114">

You may also want to consider the Astro Flight <a href="http://www.astroflight.com/">http://www.astroflight.com/</a>
Model 100, except that they are no longer being made. This unit is simply a digital panel
meter (with a voltage doubler, because they typically run on 9V) and a current sensing
resistor, in a nice little box, with a pair of Astro Flight connectors attached. There
is a switch to allow selecting between measuring voltage and current.
Astro Flight supposedly has a new version coming out that will also measure power, but if
you are interested in building the equivalent of the Model 100, email me at at <a href="http://rcgroups.com/shared/nospam.php?u=skranish&d=ezonemag.com">skranish@ezonemag. com </a>and I will consider
presenting the plans in a future column, and making a kit of parts available from New
Creations RC. 

If you are using a multimeter, you need a current sensing resistor from one of the
sources listed below. YOU WILL NOT FIND ANYTHING LIKE A CURRENT SENSING RESISTOR AT RADIO
SHACK, SO PLEASE DO NOT ASK. You will not find them at Digi-Key or Mouser or pretty much
anywhere else, either. One milliOhm is an unusual value. There are some vendors who make
them, but they are not going to sell them to hobbyists, so if possible I will arrange for
New Creations to carry one. The schematic below shows how to use a current sensing
resistor with a multimeter. 

<img src="http://static.rcgroups.com/articles/ezonemag/fromlab/../../images/krschem6.gif" width="293" height="199">

In the above circuit, the DMM reads 1mV for each Ampere of current flowing through the
sense resistor. 25mV => 25 Amps, and so on. 

Sources 

Tom Hunt at ModelAir-Tech sells the 'Kowalski Shunt' which probably is not a shunt, and
I am told is made from nothing more than a piece of 14 gauge house wiring. I have not
seen this item. Contact him at <a href="http://rcgroups.com/shared/nospam.php?u=thunt95147&d=aol.com">THunt95147(at)aol.com</a>.


Aveox at <a href="http://www.aveox.com/">www.aveox.com</a> sells what they call a
'shunt', but is actually a current sensing resistor, and the supplied instructions show it
being used as one. The current sense resistor kit comes with Anderson/Sermos connectors,
which I really do not like. 

<img src="http://static.rcgroups.com/articles/ezonemag/fromlab/../../images/kranbox.jpg" align="left" hspace="0" width="315" height="166">I
packaged the  Aveox current sense resistor in a small box with banana jack
connections for measuring voltage and current, with a set of Astro Flight connectors so
that it can easily be attached into motor systems as needed. 

This photo shows the box during the process of attaching the Astro Flight connectors -
note that everything is held in place with pins for soldering. If you would like more
details on how to construct one of these boxes, please email me, and I will put a
description in a future column. 

The January 97 Ampeer, available from Ken Myers web site at <a href="http://members.aol.com/kmyersefo/">http://members.aol.com/kmyersefo/</a> contains a
discussion of current measurement, and a description of how to make a shunt (really) out
of zip cord. 

Digital Multimeters are available from a variety of sources. I really recommend Fluke
meters, but they are available from a variety of manufacturers, including (yuck!) Radio
Shack. Digi-Key at <a href="http://www.digikey.com/">http://www.digikey.com/</a> sells
some inexpensive meters. Contact East, Future/Active, and many other electronic suppliers
also sell a variety of meters. If you pick up a copy of Electronics Now magazine,
you will find ads for many of them. 

Additional Reading 

Dover Publications, Inc of New York publishes Basic Electronics, Prepared by the
Bureau of Naval Personnel. This is a basic textbook on electronic circuits, rather
than the electrical devices covered in the companion book Basic Electricity, and
one of the early chapters covers meters and measurements. This book dates from the 1950s,
so it covers only moving coil type meters - no digital stuff in sight. It is
available at Barnes & Noble for about $11, which is pretty cheap for an engineering
text. 

Next Time 

Is there something that you don't understand that you would like to see explained?
Please email me at <a href="#skranish(at)ezonemag.com">skranish@ezonemag.
com</a> 

<hr>

Workshop Storage 

If you are a serious modeler like me, you probably have a lot of 'stuff' in your
workshop. Actually, if you are like me, you have an absolutely ridiculous amount of
stuff in your workshop. And unless you are blessed with an immense workshop, you probably
have problems storing and organizing all that stuff. I know I do. 

If you have tried to do something about this clutter, you have probably looked into
workshop storage systems, such as the workbenches with drawers sold by Sears (that's
Roebuck's to our friends in the UK). These workbench bases are great - mine has 10 drawers
of varying sizes, and my building board is on top. Sears also sells metal storage shelving
in a variety of sizes. Your local hardware store may have 12" x 36" x 60"
steel shelving units for cheap,. but these are really little more than an ugly bookcase.
I have several shelving units with 24" x 36" shelves, which will
probably have to be special ordered. They were worth the trouble - one is full of
carpentry tools and the other is full (I am not making this up) of model kits, which will
take me about the next 30 years to finish. But it is not enough storage - as if anything
ever is. So I am always on the lookout for other storage systems. 

Rubbermaid is a sort of remarkable company that makes everything from children's toys
to yard storage sheds to tool boxes to industrial storage systems, all of plastic and by a
variety of manufacturing methods. A few years ago, they introduced a set of storage
cabinets - sort of  plastic kitchen cabinets - in a very limited set of sizes.
Two are floor standing and one is wall mounted. The wall mounted cabinets are 24" x
24" x 12" and can be had for about US$30 when on sale. The floor standing
cabinets are about 24" x 18"; one is 36" high (about US$50) and the other
is 72" (about US$90).  Each of the cabinets has one or more shelves in it.
My workshop has two of the wall mounted cabinets above the workbench, a tall floor
standing unit full of transmitters, chargers, and batteries, and a short floor standing
cabinet full of field boxes. This keeps the balsa dust off of everything, and the exterior
is very easy to vacuum when it gets dusty. The cabinets take perhaps 10 minutes to
assemble and are quite sturdy. 

The next problem is how to organize all of this storage space. Workshop 'organizers'
are usually meant for storing nuts and bolts, so I looked to the kitchen, where we use a
variety of plastic containers to organize almost everything. Rubbermaid, Tucker, and a
number of other manufacturers make a huge selection of storage containers in a wide
array of sizes. The big discount chain stores regularly put these on sale at as much as
50% off retail prices, so if you watch for the sales, they can be very economical. I have
a vertical container for epoxies, another for CAs, and still another for various other
glues. I use flat containers for clamps, small paint jars, and glue in tubes. It is easy
to fill up the storage cabinets and workbench drawers with these containers, and it is
much easier to get at things. Instead of hunting through all sorts of stuff for the
matching bottles of epoxy, I just pull down the box of epoxy, take out what I need,
and put it back when I am done. No muss, no fuss. 

I even keep a few containers around to store the radio gear an every thing else that I
remove from an airplane when it needs, well, modifications. This keeps everything
together, and I don't lose any of the small mounting screws. 

Zipper - closing  plastic bags are also very useful workshop storage, whether
storing covering scraps, those last few parts from a kit, or the few reusable parts from a
wreck. Stay organized! 

<hr>

The Reader's Survey 

Ok gang, did you actually read this far? If so, please email me at <a href="#skranish(at)ezonemag.com">skranish@ezonemag. com</a> and
tell me if any of this actually makes sense to you. 

<hr>

Reference Material 

See source of the Month below if you need something to read. 

If you would like to recommend something, please email me at <a href="#skranish(at)ezonemag.com">skranish@ezonemag. com</a> 

<hr>

Tool of the Month 

Most modelers use paint masking tape for a variety of purposes when building,
finishing, or (gasp!) repairing airplanes. I frequently use masking tape to protect part
of the structure when I am sanding, or to hold things together while glue is drying.
Although I have a large collection of clamps, there are some tasks that are easier to do
with tape, such as holding the leading edge to a foam wing while the glue dries. 

The biggest problem with using masking tape is that it often leaves a lot of crud
behind when you remove it. An the cheaper the tape, the more crud it leaves, and the
quicker it leaves it. 

3M has introduced a line of 'Long Mask' tapes that are based somewhat on the adhesive
technology used in the ubiquitous 'Post-It Note'. The tape is considered to be medium
stick, and 3M claims that it will not leave residue if left in place for up to a week,
even in bright sunlight. I have used it for household painting, and found that it is easy
to remove and leaves no residue after being in place for months. 

3M product # 2090, in 3/4" wide and 2" wide by 60 yard rolls should be
available at any hardware store or home center. I have bought it at Home Depot and the
local True Value Hardware. The tape is colored blue, so you can easily distinguish it from
the cheap stuff - because it is NOT cheap. Expect to pay about US$2.50 for the 3/4"
roll, and US$6.50 for the 2" wide roll. Nonetheless, the lack of residue or damage to
fragile structures and sheeting make this a very good tape to work with, and worth the
money. 

If you would like to recommend a 'Tool of the Month', please feel free to contact me at
<a href="http://rcgroups.com/shared/nospam.php?u=skranish&d=ezonemag.com">skranish(at)ezonemag.com</a> 

<hr>

<h2>Source of the Month </h2>

Wise Owl Worldwide Publications, 4314 West 238th Street, Torrance CA 90505-4509,
 310-375-6258  <a href="http://rcgroups.com/shared/nospam.php?u=wiseowl&d=sprintmail.com">wiseowl(at)sprintmail.com</a>
is the US subscription agent for a number of UK model magazines, including more airplane
magazines than you are likely to find the time to read:

<ul>
<li>Aeromodeller  - freeflight models of all varieties </li>
<li>Aviation Modeler International - previously mentioned here, this new magazine covers everything,
and does it in colour. Free plans every issue. </li>
<li>Flying Model Designer and Constructor - sort of a blast from the past, mostly
freeflight, with hand drawn plans. Very few ads. </li>
<li>Radio Modeller - seems to be less technical or detailed than RCM&E (below).
Some issues include free plans. </li>
<li>RC Models & Electronics - covers all things RC, and frequently includes electronics
projects. Some issues include free plans. </li>
<li>Scale Models International - I don't do scale, so I haven't looked </li>
<li>Silent Flight - Gliders and Electrics, and lots of them, in color. A direct competitor
to Electric Flight International. </li>
<li>UK Books from the publishers of the above magazines. </li>
</ul>

As if I don't get enough magazines already! Each of these holds my attention far longer
than the US magazines. There are nowhere near as many ads, and many more interesting
articles. There are also magazines on model boats, live steam railway locomotives, plastic
models, and military miniatures. 

The folks at Wise Owl respond promptly to email. Please tell them I sent you. 

The above list does not include all of the Traplet publications
(RCMW, EFI, QFI, etc.) that can be found at  <a href="http://www.traplet.co.uk/traplet/">http://www.traplet.co.uk/traplet/</a>. Regular
eflight contributor Dereck Woodward writes for Traplet. 

Our readers in the UK may laugh at all of this, because you can find these
magazines at a newsstand or model shop, but good newsstands are hard to find here in the
US, and very few carry European model magazines. 

If you would like to recommend a 'Source of the Month', please feel free to contact me
at <a href="http://rcgroups.com/shared/nospam.php?u=skranish&d=ezonemag.com">skranish(at)ezonemag.com</a> 

<hr>

<h2>COPYRIGHT </h2>

This document is copyrighted (c) 1997 by Steven Kranish, and may not be copied or used
in other forms of publication (electronic or paper) without written permission from the
author. I will probably grant permission, but I would like to know about it, so go ahead
and ask. 

<hr>

<h2>CONTACTS </h2>

If you have any questions, please feel free to contact me at <a href="#skranish(at)ezonemag.com">skranish@ezonemag. com</a>