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I want to use a lab power supply to run simple ohm's law experiments instead of using a 9v alkaline battery. One of the experiments uses a resister in series with an LED to show the effects of how a resister limits current. I can set my lab power supply to 9V but I don't know what current to set it to in order to be the equivalent of the 9V battery. Can anybody enlighten me please? I'm just getting into learning electronics.

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Jack A Lopez (author)2017-02-24

A battery can be modeled as ideal voltage source in series with a resistor, R, and the current-voltage (I-V) curve for this looks like a downward sloping line. Also this resistance, imagined to be inside the battery, has a name. Students of electrical engineering call it, "internal resistance".

A lab power supply, the kind with independent limits for I and V, can give you this same curve, if you wire a resistor R in series with its output, and set the voltage limit equal to Voc (Vocstands foropen circuit voltage), and set the current limit to a value higher than Isc = Voc/R (Isc stands for short circuit current) When choosing this resistor, make sure to pick one that could comfortably dissipate Voc^2/R watts of power, because, you know, it might have to in the event the rest of the load is a short. Or maybe you just want to the current limit (Ilimit) kick in, in which case the most power R would have to dissipate would be Ilimit^2*R

By the way, the I-V curve of this kind of lab power supply, by itself, is NOT a downward sloping line. It is two straight lines, one vertical (at low currents where it is in constant voltage mode), and one horizontal( in constant current mode). Usually there are some little indicator lights to tell the user which mode the supply is trying to give, at that moment, sometimes labeled CV for "constant voltage" and CC for "constant current".

The only question that remains is, how big (or small) of a resistor is needed, and I am hoping this page,

will answer that question. It has a little table of different kinds of batteries, and a number for typical internal resistance for each of these. It says, "1 to 2 ohm" for a "9-V alkaline"

woodyd (author)2017-02-24

Thanks Jack. I appreciate your prompts reply. A lot for me to learn about. I'm just beginning.

Downunder35m (author)2017-02-24

If the supply has current limiting than do what I do:
Adjust the voltage to your needs and the amps so that the current limiting is just off.
If your project requires more amps you adjust them higher until you get the light out or realise smething must draw too much.
So you get some extra protection for your connected electronics ;)

woodyd (author)2017-02-24

thanks