Circuit Debugging Hints
Troubleshooting a circuit is
not that different in approach from troubleshooting or debugging a program.
Some key points are:
1. Almost no one's circuit works the first time, so you should not be
surprised or discouraged when this happens to you. Related to debugging
is wiring your circuits neatly! This is more important than it first appears.
It will often help you avoid errors in the first place, and it will make
debugging the circuit much easier.
2. Generally you will want to take a divide-and-conquer approach.
This means that you will want to isolate the part of the circuit where
the problem is located. It is hard to write down general rules for doing
this. Three general strategies are to
- start from the input side and
work step by step towards the output, testing at each point in the circuit
to see if you have the expected signals,
- start from the output side
and work step by step towards the input, testing at each point in the
circuit to see if you have the expected signals, or
- divide the circuit into two
subcircuits somewhere in the middle, identify which, if either, is working
properly, then proceed by methods a, b, or c to further isolate the
problem.
One idea that should be clear from this description
is that to find the problem you will need to have a clear idea of how
your circuit should work.
3. Generally you will test your circuit by checking
voltages. This is because you can measure voltages without interrupting
the circuit, while to measure currents you need to break some connection
to insert the meter into the current path. If this difference is not clear
to you be sure to ask during the first lab. But measuring voltages can
lead to non-intuitive results until you gain some experience. For example,
if you measure the voltage across one element or subcircuit in a larger
series circuit, where that element is, for some reason, not conducting
properly, you will often measure a voltage across the non-functioning
element while you will measure zero voltage across the other elements.
Again, if this is not clear to you be sure to ask.
4. The two most common mistakes that students
make are to mis-wire a circuit and to use the wrong resistance values.
Before you get too wrapped up in detailed troubleshooting, be sure to
check and double-check your wiring and to very carefully check your resistance
values. Generally if one person in a team has done the wiring or selected
the resistances, it is a good idea for a different team member to check
things.
5. It is generally a good idea to turn off the
power before changing anything in a circuit.
6. When two parts of a circuit are connected
together, either part may behave differently than if they are operated
separately. Thus when troubleshooting you may want to disconnect the part
you are testing, test it, then reconnect it and test again, to see if
the problem is caused by the effect of the one part on the other.
7. When you measure resistance it is very
important to make sure the element you are testing is disconnected
from the rest of the circuit. Also, never
measure resistance in a circuit when the power is on. If the element is
in the circuit, you will usually get the wrong answer. When measuring
resistance the meter supplies its own power; if your circuit also supplies
power, it can damage the meter.
In summary: have fun; take some risks; take
notes; be prepared; review; be careful in the lab.
