This is the "Basics" group, and yeah, I have a question that's most definitely basic...

I want to purchase a small widget for a 12V DC setup to keep the voltage constant but control the *amperage* from 0 to a maximum of 3. I'm having a difficult time pinning-down what it's called - I *think* what I need is a "varistor" or a "potentiometer", but the info I've turned up in my searches sems contradictory.

Many thanks to any kind soul who will let me tknow the correct name so I can order the right thing =;)

To answer your question, we really need more information. To us, your question is somewhat confusing. Let me explain why.

A basic equation of electricity is: V = I * R where V = voltage (measured in volts) I = current (measured in amps) R = resistance (measured in ohms)

This equation can also be rearranged as: or I = V / R or R = V / I

If I have a resistor that is 12 ohms and I run a 1 amp current then the voltage across it will be 12 ohms x 1 amp = 12 volts.

If I have 4 ohm resistor and I put 12 volts across it, the the current will be 12 volts divided by 4 ohms = 3 amps.

Now if I have a 3 ohm resistor and I put 12 volts across it, then the current will be 12 volts divided by 3 ohms = 4 amps.

If I have a 3 ohm resistor and I run a 3 amp current through it, then the voltage across the resistor will be 9 volts.

Now, what do you want to happen when we connect a 3 ohm resistor to your

12 volt source that you also want to limit to 3 amps? There are two possible answers:

1) The current will be limited to 3 amps. This means that the voltage will not be held at 12 volts, instead, it will have to drop to 9 volts. This contradicts your statement that the voltage remains at 12 volts.

2) The voltage will be held at 12 volts. This means that the current will increase to 4 amps which contradicts your desire that the current is limited to 3 amps.

We can give a better answer if you really describe what the application is that you have in mind.

Dan Coby wrote in news:55mdnSToxfV snipped-for-privacy@earthlink.com:

Thank you, Dan,

I understand the V to I to R relationship in thory. THe practical application I'm looking at is doing some small-scale metal etching using a

12VDC source using the emthod described here:

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although I'll be ethching much smaller items than kettles.

It's my understanding (which is likely to be entirely wrong...) that I need to maintain the Voltage steady at 12VDC, while being able to dial-down the Amperage.

If that's the case, I can't order the Amp-control-widget-thing because I don't know what it is called - and unfortunatley, there is no listing at Digikey for "Amp-control-widget-thing", Heh! ;)

Hopefully, that clarifes why I'm asking about what I need to vary Amperage :)

However what you said here will not be true, " I want to purchase a small widget for a 12V DC setup to keep the voltage constant but control the *amperage* from 0 to a maximum of 3."

The output voltage will vary to keep the current constant. But that's OK, that's the way it works.

It's called a "magic box", because the load itself will determine how much current will flow for a given voltage.

Think about a resistor: E = IR means that the voltage is _always_ the resistance times the current. Conversely, the current is _always_ the voltage divided by the resistance. So a 10 ohm resistor on a 12 volt supply will flow 1.2 amps -- the only way to change the current is to change the voltage.

You want a constant-current power supply; the voltage will vary (up to some limit) depending upon the resistance.

These are often used for driving LEDs, but the current tends to be fixed (and they're likely to be designed for a stable load).

Bench power supplies often have a constant-current mode, and the current and (maximum) voltage can be varied; but they're somewhat more expensive than a wall wart or battery pack.

amdx wrote in news:njif1e$ffl$ snipped-for-privacy@dont-email.me:

OH! THanks, Mike,

So amps cannot be controlled in and of themselves. Hmm. I think I need to print out these comments (and the suggested links!) and give a good close read in the morning when my mind is fresh :)

Nobody wrote in news: snipped-for-privacy@nowhere.invalid:

Thanks! Yeah, I really need to study all the info. I thought I could be varied while keeping V constant OR V veried with I constant, but it doesn't seem to work that way at all.

I think I need to go to the page about steel etching and just *exactly* follow the directions for using the 9V battery, and take a lot more time to think about adapting the wall wart.

Tim Wescott wrote in news: snipped-for-privacy@giganews.com:

Oh....! OK, Thank you, Tim!, I had thought one could be varied while keeping the other constant....

I'll be collecting all the info that the group members have generously given, and print it out for study in the morning - and meanwhile, I think I'd better just *exactly* follow the instructions at the website for the salt+vinegar steel etching using a 9V battery, and make sure I'm *much* more clear on all of this before I even think about fooling around with a wall-wart =:o

That link does not talk about current, so I suppose that your 0 - 3 amps was found somewhere else?

Anyhow, in that application, the load resistance is going to vary a lot, depending upon the electrolyte and your technique. So, I would say that your need is to regulate the current. You can use your 12v source, but the voltage doing the etching will vary as the load resistance varies. And you don't care if it does! Your current will be constant at whatever value you set. In the end, it is the current that is doing the etching.

As everyone knows atleast two of the 3, volts, current, resistance, has to change. Assuming you want to keep a constant current when the resistance is changing, you set the voltage to the maximum you want without a load connectd. YOu then adjust the current for the maximum value with the otuput shorted or into a very low resistance. The current will stay constant as long as the voltage is set high enough. The voltage will automatically be reduced to whatever it takes to provide the constant current.

You can also set the current to a high value and the voltage will remain constnt as long as the current is under the set value when the load changes. When the load gets to the current limit , the voltag will automatically drop.

I see what you're looking for and using solid state devices means you could be burning things out, unless you have proper arc protection.

For etching and current limiting, the easier thing you can do is use a series of incandescent lamps in parallel, each in a socket to pop them out of circuit to adjust current. Or use a switch on each one.

A 25Watt 12 volt lamp will give you 2 amps, a smaller one less and if you combine them in parallel you can beef it up or have a smaller one only for low current.

Lamps change their resistance when loade.

That is about as simple as I can come up with and you can get the parts at an auto parts store or Walmarts.

Of course you could go the route of using a Rheostat in series, plan to pay money for a large one..

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