Re: DC Power Supply Message #5 Posted by Eric Smith on 27 Apr 2004, 6:49 p.m., in response to message #1 by Wayne Stephens
I don't know anything about that power supply specifically,
but if the current limit is adjustable, it should be OK.
The current limit serves to prevent damage to the DUT (device under test) if there are fault conditions. For instance, if you know that the DUT should draw no more than 200 mA at 2.5V and set the power supply to those limits, but there is a bad component causing it to draw more, the power supply will go into current limiting. This will prevent other components of the DUT from being stressed beyond their ratings.
A current-limited DC power supply has two independent limits, voltage and current, and two mode. Normally the power supply operates in constant voltage mode, where it provides the set voltage at a very low impedance, so it maintains voltage regulation with a variable load. But if the load exceeds the current limit, the power supply switches to current limit mode, in which it has a higher output impedance, and regulates the current. This results in the voltage being lower than the voltage limit. Many bench power supplies have "CC" and "CV" indicators, though the same information can be inferred from the meters.
If you were to draw a voltage-vs-current graph, it would have line segments at V=Vlimit for C=0 to Climit (CV mode), and at C=Climit for V=0 to Vlimit (CC mode).
Of course, this description is for an ideal power supply. A real power supply will deviate slightly due to imperfect line and load regulation and finite transient frequency response. In particular, highly inductive or highly capacitive loads may significantly reduce the regulation performance of the power supply. (For testing calculators this should not be an issue.)
For non-printing LED models, set the current limit to
about 200 mA. Printing models need much more current while they are printing (>500 mA), but they should initially be tested without printing at the 200 mA limit. Once you've determined that the non-printing functionality works correctly, increase the current limit to 600 mA and test printing. Models equipped with a card reader will also draw more than 200 mA when the card reader is in operation; I haven't yet done any characterization of the current requirements.
The later all-CMOS calculators (41C family, Voyager family) should be tested at a much lower current limit, except when using the 82104A card reader. However, I normally just test those with disposable alkaline batteries.
If the power supply doesn't have a direct method of setting the current limit, you can do it by shorting the output (which puts the supply into constant current mode), then adjusting the current limit knob until the measured current reads the desired limit.
I use an HP E3631A programmable triple power supply, but that is much fancier (and more expensive) than what you need.
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