Amps and Power Supplies

Thread Starter


Joined Dec 9, 2004
Hi to the forum. I am new and have just found the online book and this forum. Looks very good. My interests are hobby work, and since I don't have an electronics background, I want to try and get some basic information as I move into CNC machining and robotic controlled telescopes. I am tired of putting resistor A into slot A without knowing the why of a circuit.

All the published and online information has been very good. I am wading through boolean math right now (headache city) in preparation for building a PIC programmer, but overall it is going well.

However I find myself stumbling over really basic concepts sometimes, a newbie situation I guess.

For instance, I would like to build a foam cutter for my RC model hobby. This is just a length of stainless wire stretched tight between two supports and connected to a variable power supply. I have built a few little 5 volt 50 ma power supplies but this time we are talking more power, about 12 volts and around 5 to 10 amps depending on the length of the wire. The simplest design is a doorbell transformer and dimmer switch. Better is to add a full wave rectifier.

However I find myself stumbling over Ohms Law in the design of this supply. One of my texts warns about the inverse nature of amperage through a transformer. A 2 amp 110v input results in an 8 volt 28 amp output (225 watts!). According to Dr. Ohm, I=V/R. In this case, as R drops to 0, voltage equals amperage. So if I drop a nail across the outputs of my little transformer, I would expect an 8 amp shower of sparks. Apparantly I would get a 28 amp shower, much more spectacular. So if I connect my transformer output to a full wave rectifier and run this to my 18" stainless wire, I can calculate the resistance of the wire and the voltage that should be required for my target amperage to cut the foam for a model wing. But what if I want to cut a 25' glider wing and increase my wire length (and resistance) dramatically? Dr. Ohm says the voltage must go up. Or does it, with 28 amps on tap?

If I have a 30 amp alternator in my car and I start turning on accessories, the resistance in the system goes down because all the electrical accessories are wired in parallel in the vehicle ( I noticed that this conundrum of decreasing resistance in parallel resistances is discussed in another forum topic ). As I flip switches the amperage draw on the alternator increases so the voltage regulator ups the voltage output of the alternator until it gets to a maximum ( too high and all the 12 volt accessories get very unhappy) at which point I run out of steam. I could drop down to Ford and buy a 50 amp, 80 amp, or 100 amp alternator, all producing the same voltages. Now I have more juice for that 500 watt stereo, but how does this fit with the relationship between voltage and amperage in Ohm's law? The voltage stays the same in all these alternators, but the amperage goes up dramatically.

I can choose 50 amp, 100 amp, and 200 amp service for my house in our area, all at 110 volts. I can also wire together 70 AA batteries in series and get the same voltage, but I am not going to get the same current flow for my house.

This blindness to the nature of the power supply confuses me in Ohm's law. It seems that the supply voltage should be matched to the supply amperage somehow. Amperage can change dramatically independent of voltage.

Sorry for the ramble. Typical newbie, stumbling over the basics. I enjoy the web site and my readings in the forum. Hope to participate in future.


Joined Apr 20, 2004

A resistance of 0 implies an infinite current. But, every voltage source has a practical limit to the current it can supply, so that is never a real-world case. A battery can transport only so many ions, a generator can only produce a finite number of lines of force to swing through the coils, and so on. If you try to pull 10 amps out of a power supply rated at 5, the source voltage will drop and limit the current produced. Then either a fuse will blow or some component will get hot and fail.

As far as your foam cutter goes, you will need more voltage to get the same current as the wire length increases. I can't imagine even stainless wire will have a great inclrease in resistance even as the length doubles, though. I don't have any experience with foam cutters, but there have been many articles in the modlers magazines. There is very likely to be an on line archive you can get plans from.