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Advice page for lab 1
Here are some handy tips for lab 1. We might update this page with new tips, so
check back again just before your lab starts.
General
Using the multimeter
Multimeter probes have to make firm contact with the circuit to work reliably,
otherwise you introduce an extra resistance at the point of contact, which
creates an error in your measurement. Most of you only have two or fewer
hands, which isn't enough to hold everything down. Use your alligator clips
instead: it takes a few seconds of set-up but will save you time overall.
Remember: voltage meters go in parallel, current meters go in series!
The VΩmA port only goes up to 200mA. If you try to measure a higher current it
will just signal a “1” on the left, which means it’s above the range of the
display. (It might also blow the fuse in the multimeter, so it's best not to
even try.) To measure a higher current, you need to move the red lead to the
“10A” port, and turn the dial to 10A.
Remember that the current meter has a smaller voltage drop across it. In some
situations this is negligible, but ours isn’t one of them. So avoid the
temptation to put a current meter in series with the load resistor: this can
cause an underestimate as the current meter resistance adds to the load
resistance, particularly with the smaller load resistances. If you know the
voltage across the load resistor (which you should measure, because the
converter isn’t perfect), and you know the load resistance, then you can
calculate the current that the converter is supplying.
There’s a multimeter on the lab bench, which you can use if you like. The
multimeter on the lab bench is much, much more expensive, and much more
precise.
“Voltage meter” and “voltmeter” are synonyms. “Current meter” and “ammeter”
are synonyms.
Putting things together
It’s a good idea to plan how things will fit together before you start
sticking or soldering things together. Little things, like where your wires
will route or whether your battery aligns left, right or center, can make a
surprisingly big difference to the end result.
Stick to conventions with wire colors! If you get these the wrong way round,
you’ll confuse everyone, including (most importantly) your TA. Use black for
everything directly connected to the negative terminal of the battery. (You’ll
sometimes hear us call this “ground” or “GND”.) Use red for one of the
positive terminals (probably the battery one).
You have a bit more freedom with other colors, but be consistent with your
color scheme. Resist the temptation to make your wire arrangement colorful
for the fun of it: there should be some sort of logic to it. In particular,
wires that are directly connected should be the same color.
Cut wires to length. Wires that are unnecessarily long are surprisingly
cumbersome and make circuits harder to debug.
You’re allowed to clip the leads of the diode to make it shorter, if it helps
with your layout.
If you will cover your solder joints up, remember to show them to your TA
first!
Soldering
We’ll have some examples of good and not-so-good solder joints for you to
inspect.
Make sure the iron tip is tinned (has a bit of shiny new solder covering it)
before you solder a joint: this helps the solder melt faster, which will make
your joint neater.
Resist the temptation to use the soldering iron to melt the solder while the
components you're trying to solder are still cold. Soldering works much better
when the parts of the components that will be joined are hot as well, because
the solder will melt more evenly around the joint.
There should be enough solder to comfortably cover the joint, but not so much
that there’s a bulb on it.
It’s quite hard to hold things in place at the same time as you’re soldering
them, even if you appropriate your partner’s hands to do so. Another strategy
is to use tape to hold down wires or components (in some place away from the
joint), and remove the tape when you’re done.
If you have solder two wires together, this can be difficult. One strategy is
to arrange your wires so you don’t need to do this. But if you do, be
particularly careful not to put too much solder on the joint (that it bulges),
or too little (that it won’t hold mechanically).
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