Hello,
I am taking a class on DC Circuits, and I would like to be able to re-create a lab we did at school, at home. I have a 24 V power supply (AC Adaptor), but it's output is more like 30 volts....

How can I wire in an old volume control type switch to turn down the voltage?

The outside of the AC Adaptor plug is +, the inside is -.
My old volume switch has three wires coming out of it, and it's only ID marks are M 5KD.

See bitmap attached.

Thanks,
Gary

 

Oh yes, and how can I protect my power supply from an accidental short?

Where would I put a fuse, and how large a fuse do I need?

 

You would need to use a device like an LM317 to do the actual control, but your 5K pot is potentially (little joke there) useful to change the output level of the LM317.

Here is a link to a data sheet that tells you how to use it - www.national.com/ds/LM/LM117.pdf.

 

I skipped trying to control the voltage, and moved on to re-do the lab, substituting my voltage for theirs.

When I treid to measure current, it gave me a brief reading that quickly began to drop. The AC adaptor is rated at 24V 500mA.

Do such adaptors have a safety mechanism built it to cause them to drop like that?

I tried for several hours, using different meters, all with the same result.

I was unable to measure current for each resistor in the parallel circuit.

Any clue as to what went wrong?

Thanks,
Gary

 

http://www.fairchildsemi.com/pf/LM/LM7824.html