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Accuracy of Over-Current and Over-Voltage Protection of U8031A and U8032A

Question asked by glowitz on Feb 28, 2014
I just purchased two new Agilent PSUs -- one U8031A and on U8032A. When setting the over current mode, I find that the unit trips early -- about 30 mA or so, depending on voltage setting. This same phenomenon exists on both units, which are different models. I don't know how the trip point is determined internally -- however if it's based on the display reading, I would think it wouldn't trip until the display current reached the threshold. However, this is not the behavior. It trips early. Maybe this is a built-in safety mechanism, not sure -- but it's kind of odd since the purpose of the OVP and OCP modes is to set the voltage or current trip point. As a result, you are never quite sure at what point it will trip.

There is nothing I saw in the data sheet that states the accuracy of the trip point. There is information about the accuracy of the displayed voltage or current -- but I would think the trip point would be "digitally" tied to the reading on the display, even if the display is off by a few fractions of a percent from the "true" voltage or current. 

At first I thought maybe something was wrong with the unit -- but when the same issue appears in both models, it appears this is either a firmware issue or a design feature, not sure which.

I'd be interested in any one else who has these units to run similar tests. In my case, I just took a 100W power resistor (say 8 ohms) and set the over current to 1.00A exactly. Then I gradually increase the voltage from 0V to 8V. By the time it approaches 8V, the current will approach 1A (in this case with an 8 ohm load). As I dial it up carefully -- such as .95A, .96A, then it trips the output, and shuts off -- well before hitting 1.00A. 

Any suggestions? Thanks.