Hi I'm Bill Griffith, an application engineer with Agilent Technologies. Today we're going
to look at two different methods for protecting our DUT when using a power supply. We're going
to set a current limit. In the first method we're going to switch from having the power
supply be a constant voltage source to a constant current source. In the second method we'll
use the overcurrent protection method. So let's get started. We'll go ahead and set
up a current limit such that when the limit is exceeded the power supply will switch from
being a constant voltage source to a constant current source. So I set the limit at 20 milliamps.
So now we'll go ahead and we'll increase the voltage to our DUT and as we increase the
voltage it will start to pull some current. At this point you will see that the power
supply is a constant voltage source. So whatever voltage we program it for that's the voltage
that it's outputting. As we increase it, now you can see it's switched to being a constant
current source, so it can no longer increase or decrease the voltage. The power supply
just stays as a constant current source. If we do decrease the voltage enough that the
current drops below the limit, it will go back into being a constant voltage source.
In addition to that is if we remove our DUT and we program it for a higher voltage, say
3 volts, when we go ahead and connect our DUT the power supply instantly goes into the
constant current mode. So it will just toggle back-and-forth as the DUT is connected and
disconnected. Moving onto the second method, we're going to use the overcurrent protection.
So from the front panel I can only set a level of 100 milliamps or more. So I've gone ahead
and programmed the overcurrent limit from the PC and I can store it as a state and recall
it at any time. So I've gone ahead and programmed the overcurrent for 20 milliamps. The next
thing is is we need to turn it on. So now with the overcurrent turned on we'll go ahead
and increase the voltage of our DUT and see what happens. So as we increase it, it starts
to pull some current. You can see the power supply is operating as a constant voltage
source. Then once it exceeded the 20 milliamps the overcurrent protection gets tripped. At
this point in time the output has been set to zero and even if we remove the DUT so the
condition is no longer there, it still stays at zero. So in order to use the output again
what we have to do is we have to clear the condition. So I'm going to go ahead and recall
our storage state, which sets up the power supply for a lower voltage and then I'm going
to go ahead and clear this condition. So I go to "Overcurrent" and "Clear." Okay, overcurrent,
clear. Okay, now the output is enabled again and we can increase our voltage and as long
as our current doesn't exceed our overcurrent condition, it will continue to operate as
a constant voltage source. So at this point in time you can see some of the differences
between using the overcurrent condition and the setting a limit and switching between
constant voltage and constant current. So if you're programming we'll take a look at
what that looks like really quickly. The one thing you can do is in your program is you
can actually query the status registers to see what state the -- or if there is a change
of state in the power supply. So I wrote two programs, one uses the first method to switch
between constant voltage and constant current. So in the program it starts off in and all
the measurements are in the constant voltage until we exceed our 20 milliamp limit and
the power supply switches to the constant current mode. In our second program we're
going to use the overcurrent protection mechanism. As we run the program the power supply stays
in the constant voltage mode until we exceed the 20 milliamps and it switches to the overcurrent
protection. Once that happens the output drops to zero and will stay there until we clear
that condition. So we're able to tell what mode the power supply is in or when the mode
changed by looking at the status register and querying it after each time we set the
output of the power supply. So we looked at a couple of different methods for protecting
our DUT. We set a current limit and had the power supply switch from being a constant
voltage source to a constant current source. The second method we went ahead and set the
overcurrent protection so that when the power supply output more than 20 milliamps it just
shut down the output until we went ahead and cleared that condition and cleared the overcurrent
protection. So I hope you found that interesting. Thanks for watching.