NCP1117 Board Load Test (Part 2)

In my last post, I tested my NCP1117 5v voltage regulator board with a couple of power resistors. In those tests, I used a wall plug rated at 9v 1A. Since then, I received a new variable power supply which I used to see how different voltages and currents would affect the regulator board.
IMG_20150704_181821For these tests, I stuck with the 5-ohm resistor. I set up the power supply to 9v and “unlimited” current so that the board would draw whatever it wanted. It drew roughly 890mA, read at both the input and output of the board. (The multimeter is measuring the output current.) Unlike the previous experiments, the regulator was able to stay steady for the three or so minutes I left it going. Even though it was able to stay on, the board becomes too hot to handle with bare hands. The current seemed to hit a ceiling at 890mA. Going any higher than 9.5v would cause the regulator to quickly hit its thermal limit and the current would start dropping rapidly. I suspect the wall power supply is slightly higher than 9v printed on it which is why it did the same in my tests with that.



To calculate the power dissipation that the regulator is dealing with:

Power (P) = Voltage (V) * Current (I)

P = (9v – 5v) * 0.890A

P = 3.560W

From the datasheet, the thermal resistors junction-to-ambient, RθJA, and junction-to-case, RθJC, is 67°C/W and 6°C/W, respectively. Together, it’s 73°C/W, which can tell us how hot the regulator should get:

73°C/W * 3.560W = 259.88°C

Yeah… It needs a heatsink, though the current design doesn’t really allow for a proper one that screws into the circuit board.

I’ve used a similar board that uses the AMS1117 regulator on many projects that were running 24/7 for months. I noticed that the regulator did get very warm but I wouldn’t really call it alarmingly hot as this board was during these tests. I didn’t have the bench power supply by then and I didn’t do any current measurements (doh!), but I can estimate that none of those projects ever pulled more than 200mA from the regulator board. They’ll still be good for those types of projects where I can’t get an already regulated wall power supply. However, I was also using them for prototyping but, now that I have the variable bench power supply, I won’t be using them for that anymore.

I hope these experiments were interesting to you. Thanks for reading!


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