PHY 2049L
Lab 0

Electrical Measurements

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Updated: 12/27/2009

We will work our way through several exercises to gain some experience using the DC power supply and the Mulitmeter that will be used for all of our DC and AC measurements this semester. A separate page (originally set up as background information for the measurements in the Equipotential lab) contains additional details about the use of this equipment.

Lab Zero setup
This shows you the way it should look when you arrive. We will use an almost new BK Precision Multimeter, the Kenwood DC Power Supply, and an old (as old as the lab building) Keithley Multimeter along with a resistor that you see in the foreground. The BK Precicision Multimeter should have the cables connected for use as a voltmeter, as you see it here.

Kenwood Power Supply

The photo below shows the power supply in operation. You can see many of the controls and other details, but one needs to be pointed out first: the red "output" button at upper right. This power supply does not put out power when it is turned on. Instead, it goes into a standby mode where it produces a regulated voltage (or current); you have to press the red output button to send the voltage (and current) out to the connectors at the bottom. This ensures that the supply is warm and stable at all times, even when you want it "off".

Kenwood DC power supply

The leads are connected at the bottom. Notice that the GND and COM are connected by a metal plate. This is the way we will use it for every lab that needs a regulated DC source. The large knob at bottom right is what we use to set the voltage (or current) when the supply is in "variable" mode. The green light above the button labeled "variable" (just above the knob) shows that the supply is operating in this mode.

The buttons labeled "V" and "A" determine whether the large knob controls the voltage or the current being produced. The green light above the "V" button in the picture means that the this power supply is set to vary the voltage (presently 5.21 V) while putting out as much current as necessary up to a preset limit (see below). Pushing the "A" button will allow you to vary the current being produced, in which case the voltage will be whatever it needes to be (again, up to a preset limit).

Load limiting feature:
An important feature of this power supply is that it will automatically limit the current (or voltage) to less than what has been set by the user. For some labs, this can lead to an extremely annoying situtation where the power supply stubbornly refuses to do what you want it to do! It works as follows:

The actual settings of the "V" and "A" levels are displayed when the power supply is on but the "output" button has not been pushed. (When the output is enabled, the displays show the supply's internal measurement of the actual voltage and current output, what we usually read with our multimeter.) If you are operating in the voltage mode but need to change the current limit (say you find that you need 2.00 A rather than the 1.04 A displayed in the photo above), you first disable the output, then push the "A" button, adjust the current to the limit you want to use, then push the "V" button to go back to voltage mode and enable the output. (And vice versa for changing the voltage limit when in current mode.)


BK Precision Multimeter

We will use this multimeter for every electrical lab, so you must become familiar with its features.

The photo below shows the BK Precision Multimeter we will use for every electrical experiment this semester.

BK Precision Multimeter

In addition to changing the push button settings when you change it from operating as a voltmeter (voltage measurements) to an ammeter (current measurements), you also have to connect the cables differently. Those details are identified on a separate page.

For reasons we will go into later in the course when we look at how meters work, it is best to keep the range the same for all measurements in one part of an experiment unless it is impossible to do so and get good data.

Important note: You always want to use the range setting that gives you the most sensitivity. You lose significant figures if you use a range that is greater than you need. We will experiment with this during Lab Zero. We will set the power supply to a specific voltage, then see what is measured when different ranges selected. You will see three things: (1) that you lose significant figures if you use a range that is higher than you need; (2) that the last digit is not rounded, so the less significant value is also wrong; and (3) what gets displayed when the voltage or current is higher than the top end of the range.

Keithley Multimeter

We will only use this older meter if we want an independent measurement of voltage or current while using the BK Precision meter for some other measurement. At present, that will be the case only during our Lab Zero experiments, for practice and a comparison of these two types of meters.

Most of the Keithley (or Micronta) meters we have are battery powered. Be sure to turn it off when it is not in use. The red button is pushed in to turn it on, and pushed and released to turn it off.

This photo shows the Keithley meter set to measure a DC voltage greater than 2 V and less than 20 V. We will also use it to measure DC current as well as AC voltage or current depending on the requirements of the lab.

Keithley multimeter

The blue arrows show the buttons used to select this particular DC voltage range. The leftmost arrow points to a gold-colored button that chooses between DC and AC mode. It is out to select DC. (Notice the white "out" and "in" icons at far left that show which is which.) The middle arrow points to the button used to select Voltage mode. Pushing it in makes this into a voltmeter and causes the amp and ohm buttons to pop out. The right arrow points to the selector for the range of the meter. The range is shown in the light tan band that is connected to where the V button is.

By the way, this particular meter is reading -0.05 V when it should have been reading 0.00 V. (The probes were not connected to anything.) This meter has been thrown away and replaced by the Micronta meter in our Lab Zero experiments.


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