yanpsq (yet another newbie power supply question)

No - you'll get the higher voltage across the higher value resistor - remember Ohm's Law, and the fact that the current is the same in both resistors.

Yes, it isn't :-)

It would be best to use a wall wart and a voltage regulator - otherwise, you'll go through a lot of batteries (and with batteries, you'll still need a voltage regulator.)

You should look for a wall wart that will produce 3 - 4 volts above the regulated voltage you want - otherwise, the regulator will get hot dissipating all the extra voltage.

Even better that a random wall-wart and regulator would be a regulated

5 volt power supply

--
Peter Bennett, VE7CEI  
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Yes, you should invest in a voltage regulator. But an adjustable one will be more versatile. Look for an LM317. It can be programmed with a pair of resistors for any voltage between 1.2 volts and 2 volts less than the input.

Check the data sheet for the capacitors required to guarantee stability and for the formulas for the resistors.

--
John Popelish

======================== Everybody builds a couple of power supplies at some time. The 20V wart would run a 5v regulator no prob. BUT... just go buy a regulated 5v supply... use this to run your projects. You test equipment needs to be working so you dont need to fiddle with that to get the other stuff working.

Thanks for all the help. We're moving our offices to another building and people are throwing lots of stuff out; I now have several wall warts and some are even 5v. I got two that were for external scsi drives and use that round Mac ADB connector (which I'll just cut off) and they put out both 5v and 12v. I'm ready to start cooking. Now if only my breadboard and stuff would arrive...

Here's another question; for these dual output wall warts, what's the best way to figure out which wires are which? I.e., is it safe to just put the leads of my meter on 2 wires and see what it says? Or should I put something like a 1k a resistor in series or parallel? Should I expect it to have separate ground wires for each voltage or one that's common?

Good thinking, the 1k resistor. But even better is knowledge about how a multimeter works and how to use it.

A voltmeter needs to have a very high inner resistance, so you can measure voltages without influencing the circuit you are masuring.

An amperemeter needs to have a very low inner resistance, because you use it by inserting it into a circuit and masure the current throughthat part of the circuit.

A resistance-meter works in a third way, it puts a voltage, usually

200mV, over a component, and measures how much current it can push through it. That is how it finds out how much resistance the component does against the current.

For voltmeters and ampmeters there is also a choice of DC and AC measurement.

A multimeter combine all these instruments into one unit.

If you happen to set the MM to current and connect it to the mains it will be destroyed, or at least you will have to replace the fuse inside the MM. Work only with battery voltages until you know well how the MM works. Read a manual on the usage of a multimeter carefully.

You can find out what function the output connections of a wallwart have by plugging it in an carefully explore what voltages, both AC and DC, there are on every connection.

With the wallwart disconnected from the mains you can test what resistance there is between each possible pair of outputs. This will tell you which wires are in contact with each other.

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Roger J.