Hello all:

I want to pass a current through a copper disc using a variable dc power supply. The disc measures 52.5 millimeters broadside and ~0.75 millimeters thick. It's like a big, flattened penny.

My first question is whether or not the placement of the alligator clips / electrodes changes the resistance.

My second question is if there is any way I can use the known mass of the conductor (the copper disc) to guesstimate my initial voltage and amperage settings. I just bought the thing (the power supply) and I don't want to wreck it, at least not on the first day I have it.

Any and all insight greatly highly appreciated.

Signed,

'lectric newb

 

My first question is whether or not the placement of the alligator clips / electrodes changes the resistance.

Sure it does... if you drive the current front to back you have a much shorter path, and hence a lower resistance, then if you run the current across the face side to side.

My second question is if there is any way I can use the known mass of the conductor (the copper disc) to guesstimate my initial voltage and amperage settings.

What settings could these be?

Anyway, it may not matter. If this is an adjustable power supply with current limiting just turn it down to zero, connect it, then turn it up slowly and observe what happens. The current limit keeps the supply safe from most things you can connect it to.

Exceptions to being safe are connecting to things like other power supplies.

 

Unless the copper is a special kind of copper, that we don't know of, the scenario your talking about is called a short circuit. And will effect your power supply. You need a series between on alligator clip and the power supply.

 

Sure it does... if you drive the current front to back you have a much shorter path, and hence a lower resistance, then if you run the current across the face side to side.

Thank you.

What settings could these be?

0-30 volts, 0-5 amps.

Anyway, it may not matter. If this is an adjustable power supply with current limiting just turn it down to zero, connect it, then turn it up slowly and observe what happens. The current limit keeps the supply safe from most things you can connect it to.

My power source is a Precision Variable DC Power Supply. The manufacturers blurb says it has "Short Circuit & Over Load Protection" but I would rather not rely on that.

Is it correct to say that there is an energetic threshold above which a circuit becomes a short-circuit?

Unless the copper is a special kind of copper, that we don't know of, the scenario your talking about is called a short circuit. And will effect your power supply. You need a series between on alligator clip and the power supply.

It's just copper. Nothing exotic. Because I can vary the power output, I don't think I'll need to put anything else in the circuit.

Rather, I can just add less energy to the circuit?

 

Even a flashlight battery will short out if connected with a copper wire. But, hey, it's your power supply so go for it.

 

Precision Power Supplies are not designed to heat flattened pennys. You will only be at a few millivolts when the current limit is reached. Thus you WILL be depending on the settings to defend the power supply from melting.

 

My power source is a Precision Variable DC Power Supply. The manufacturers blurb says it has "Short Circuit & Over Load Protection" but I would rather not rely on that.

"Short Circuit & Over Load Protection" means someone spent a lot of time designing a circuit that is nearly impossible to kill. It's designed to drive a short circuit like a big copper disk.

Now I have some personal mental reservations about slamming a short circuit on to these myself, so when driving a short I'll turn both voltage and current to zero, connect the short, then turn V and I up to what I want.

However, there is no reason for that. You can safely bang a short on them when set to the max... I'm just stressing them a but less.

Is it correct to say that there is an energetic threshold above which a circuit becomes a short-circuit?

Ummm... I'm not sure what that means.

 

Is it correct to say that there is an energetic threshold above which a circuit becomes a short-circuit?

Yes. The threshold would be the point when resistance becomes 0 across the terminals of the circuit's power supply; but, colloquially, it's also the term used to describe the phenomenon of replacing a normal circuit load with a large metal disc.

 

Even a flashlight battery will short out if connected with a copper wire. But, hey, it's your power supply so go for it.

Yes. I've never seen a 9v battery casing bend open and heat up to over 40deg C. Not once. Not me. Never.

Precision Power Supplies are not designed to heat flattened pennys. You will only be at a few millivolts when the current limit is reached. Thus you WILL be depending on the settings to defend the power supply from melting.

Okay, got it.

"Short Circuit & Over Load Protection" means someone spent a lot of time designing a circuit that is nearly impossible to kill. It's designed to drive a short circuit like a big copper disk.

Okay.

The power supply in question is a bottom-end, no-name brand. Its owner is a tinkering newbie hack. Hmmm... I forsee absolutely no potential problems here. Especially if the box the power supply came in says "Short Circuit & Over Load Protection" on it!

I'm being a smart-*ss; my point is, I don't want to find out if this power supply unit can protect itself or not.

Yes. The threshold would be the point when resistance becomes 0 across the terminals of the circuit's power supply; but, colloquially, it's also the term used to describe the phenomenon of replacing a normal circuit load with a large metal disc.

Ah, I get it. For some reason I was thinking the large metal disc made for a large resistance, but my big penny is offering a *small* resistance.

Let's say I employ a voltimeter at the alligator clips. I postulate that as long as there is a voltage drop, my power supply is safe. True or false?

 

So are you just connecting the item directly across your power supply? Perhaps you can add one resistor in series like this:
(+) terminal ----> 100 Ω resistor ----> magic orb ----> (-) terminal

If the resistance level between the terminals is within normal range, there should be a full voltage drop across the load; the supply voltage shouldn't change when you attach your load.

 

The only question remaining is, why would you want to pass some current through a large copper disk? The only thing you will achieve will be generating some heat.

 

So are you just connecting the item directly across your power supply? Perhaps you can add one resistor in series like this:
(+) terminal ----> 100 Ω resistor ----> magic orb ----> (-) terminal

Okay, I will do that. I think this is what shortbus was getting at as well.

Does the above series indicate a resistor between the magic orb and the alligator clip with the little plus sign stamped on it?

If the resistance level between the terminals is within normal range, there should be a full voltage drop across the load; the supply voltage shouldn't change when you attach your load.

So a voltimeter should read zero across the load?

 

Say, my power supply, cranked, goes up to 30V and 5 amps. That equates to 6 Ohms? Wouldn't a 100 Ohm resistor be out of the ballpark here?

 

Does the above series indicate a resistor between the magic orb and the alligator clip with the little plus sign stamped on it?

So a voltimeter should read zero across the load?

1st question: yes

2nd question: no. Let me restate it. If you put a voltmeter across the power supply and there's nothing attached at all, it will read a certain voltage, right? Say 12V. If you attach anything, whether resistors, orbs, or any other circuitry, it should still read 12V; if the voltage drops that means the supply cannot sustain the current it's putting out.

Sounds like 6Ω is the minimum resistance your power supply can handle. Any resistance above that is fine. If the orb is less than 6Ω and then you attach it alone then, zap, current overload is happening; if you add another resistor the supply will see XΩ + OrbΩ in series.

 

Say, my power supply, cranked, goes up to 30V and 5 amps. That equates to 6 Ohms? Wouldn't a 100 Ohm resistor be out of the ballpark here?

Not if you turn the voltage down.

Use 5 volts and 100 ohms limits the current to .05 amps even into a short circuit.

 

Okay.

I think I'm good to go.

Thank you all very much for your all your help!!!