Science Buddies: "Ask an Expert"

Hello, I have been working on the "color mixing with LEDs" experiment from this website, (https://www.sciencebuddies.org/science- ... M&from=TSW), and my school has DC regulated power supplies (Extech Instruments 382210) with adjustable voltage and current, and a secondary output of +/- 5V, which seemed to fit the specifications. However, the experiment refers to a ground and the pictures are unclear on how the components are connected to the power supply. None of the DC regulated supplies we have here include a ground, does that refer to the negative 5 volts? Any additional info would be much appreciated.

I'm not familiar with that specific model of Extech power supply. It's a bit of an older model. It's replacement, the Extech 382213, has multiple sets of output terminals.

I recommend you find the user manual for that model and see just where the variable voltage comes out. Typically, a power supply like this will have sets of terminals for fixed voltage. In your case, it seems the model 382210 has fixed 5 volts - with a positive and negative terminal.

I suspect it also has an additional set of terminals for the variable voltage that you want. See if you can find that in the user manual.

I hope this helps.

Well, the problem is not with the voltage, this model has variable and fixed voltages, I have the variable voltage set at 15 as indicated in the lab, and the positive 15 goes to pin 7 of the ICs, the negative 15 goes to pin 4 of the ICs, and the fixed positive 5V goes to the potentiometers. Do I not need the -5V to complete a circuit? It is never mentioned in the experiment, and the pictures and description don't really help in telling where things should actually be hooked up. The other big question is the ground. It comes from the potentiometers and is in the same power bus as the resistors, and says to go to the ground, is that something regulated power supplies have? Is there a way to get in touch with the author of this experiment?

The experimental writeup and the schematics utilize a "ground" symbol instead of a "common" symbol or an "earth" connection symbol. These symbols and notation are simply a short-hand way of specifying that everything shown connecting to like symbols are all connected together. Connections marked +5v or +5 on the schematic are "understood" to connect to the positive terminal of the power supply. There is also a battery symbol on some of the schematics, The "long and thin" cross mark indicates the + terminal and the "short and thicker" cross mark indicates the - terminal of the battery.

Take a look at the markings on your power supply or utilize a DC volt meter (DVM on appropriate voltage scale and probe locations) to determine if there is a "common" connection between the various outputs. Typically the +5v/-5v supplies will share a "common" or "ground" connection. That connection may also be common with a metal case and connected to the "grounding" or "earthing" power plug connection (the longest one, in the US and Canada, typically a "U" ground prong).
The other output voltage may or may not have a common connnection to the common of the +5/-5v supply. If it does not, then if you want +15v with respect to the common of the +5v supply, you will have to connect the negative (usually black 5way binding post) of the variable power supply to the "common" of the +5v/-5v output.

Hope this helps.