Explain Current and Voltage to me

[erwin1978]

I'm having a little trouble grasping the voltage-current = mc² business. I hope someone can clarify it to me. Let's say I have a radio that runs on 1.5 volts & 100 mah. I have a transformer that outputs 1.5 volts & 300 mah. Could I use this transformer to power the radio or will it get destroyed due to the higher output current? Is the transformer pushing the current to the radio or is it the radio that's drawing the current? If it's the latter then the radio should be OK since it will only draw 100 mah from the transformer with 200 mah left in reserve? Did I get that right?

What if instead the radio draws 400 mah will that destroy the transformer because it's being maxed out or the radio?

 

[Rastus]

Could I use this transformer to power the radio or will it get destroyed due to the higher output current? You can use that transformer without problems.

Is the transformer pushing the current to the radio or is it the radio that's drawing the current? The radio is drawing the current.

If it's the latter then the radio should be OK since it will only draw 100 mah from the transformer with 200 mah left in reserve? Yes the radio will be OK.

Did I get that right? Yes.

What if instead the radio draws 400 mah will that destroy the transformer because it's being maxed out or the radio? The transformer rating of 300 mah is the limit that it will pass, not the limit that it can take, therefore if there is a short in the radio, the transformer will protect the input side. It's one of the nice things about transformers. They are used as current limiters all the time, even if they don't have to step voltage up or down.

 

[Rastus]

voltage-current = mc²

Power = voltage X current

E= mc² Einstein's theory of relativity.

 

[DominionSeraph]

1.5VAC doesn't make much sense. Are you sure you're not talking about a DC power brick?

 

[Cogman]

1.5VAC doesn't make much sense. Are you sure you're not talking about a DC power brick?

Its called a rectifier. Most actually have transformers in them .

 

[DominionSeraph]

Its called a rectifier. Most actually have transformers in them .

Yes, but then the transformer likely isn't pertinent.
If it's a regulated PS, above 300mah it won't be able to maintain rated voltage.
Unregulated, you're probably looking to smoke some diodes.
Bare transformer, now we're talking windings.

 

[bobsmith1492]

voltage-current = mc²

Power = voltage X current

E= mc² Einstein's theory of relativity.

Yeah, voltage-current = mc² does not mean anything. It's not a valid electronics equation.

Its called a rectifier. Most actually have transformers in them .

This doesn't make sense either. A rectifier is simply a diode. If you mean a bridge rectifier, than you would say that a wall wart typically contains a transformer AND a bridge rectifier plus regulation circuitry or, alternatively, a switching power supply. Wall warts are not called transformers but an AC adapter or wall-wart: http://en.wikipedia.org/wiki/AC_adapter


Erwin1978, can you describe the supply you are using? If you are powering a radio, you probably want a DC output. If it says AC out like this one: http://www.horrorseek.com/home/halloween/wolfstone/Power/powint_WallWart.jpg - "12.5VAC" then you do NOT want to connect it or things will fry.

You want your wall wart to show a DC output like this one: http://www.wonderhowto.com/images/gfx/gallery/633892460023124457.jpg Notice the "12V -- " The straight lines are the symbol for a DC output.

 

[Cogman]

Yeah, voltage-current = mc² does not mean anything. It's not a valid electronics equation.



This doesn't make sense either. A rectifier is simply a diode. If you mean a bridge rectifier, than you would say that a wall wart typically contains a transformer AND a bridge rectifier plus regulation circuitry or, alternatively, a switching power supply. Wall warts are not called transformers but an AC adapter or wall-wart: http://en.wikipedia.org/wiki/AC_adapter

Meh, we mean the same thing. I was not trying to call an adapter a transformer, only saying that it contains one paired with a rectifier.

 

[bobsmith1492]

Meh, we mean the same thing. I was not trying to call an adapter a transformer, only saying that it contains one paired with a rectifier.

And that typically being a bridge rectifier for full-wave rectification as opposed to just a rectifier, which would be half-wave and much less efficient and more noisy.

ED.: Read up!
http://en.wikipedia.org/wiki/Rectifier

 

[Modelworks]

A device will never use more than it needs , there are exceptions to this like LED, the reason goes back to ohms law and Kirchhoff. Basically a circuit has to equal the power it consumes and does not change when you increase supply.

Think of it like like a truck that can only go 50mph, even if you fill the tank up it can't go any faster than what it was designed for. truck = circuit, and gas =power

Exceeding a transformers rating usually results in a winding in the transformer burning out. The wire was selected to carry a specific current, using it for more than that increases the heat of the wire burning it out.

Transformer and wall adapter are not interchangeable. Wall adapters that are AC only are usually just a transformer in a plastic case, but not always, there are many other ways to convert wall AC to a lower voltage AC. Wall adapters that are DC sometimes have transformers, but again not always. Usually weight makes it easy to tell, transformers are much heavier and larger . The biggest benefit to a transformer is isolation from the home AC. I really prefer transformers in things like audio equipment over switching designs because the risk of ground loops is low due to the isolation.

 

[bobsmith1492]

A device will never use more than it needs , there are exceptions to this like LED, the reason goes back to ohms law and Kirchhoff. Basically a circuit has to equal the power it consumes and does not change when you increase supply.

Think of it like like a truck that can only go 50mph, even if you fill the tank up it can't go any faster than what it was designed for. truck = circuit, and gas =power

Exceeding a transformers rating usually results in a winding in the transformer burning out. The wire was selected to carry a specific current, using it for more than that increases the heat of the wire burning it out.

Transformer and wall adapter are not interchangeable. Wall adapters that are AC only are usually just a transformer in a plastic case, but not always, there are many other ways to convert wall AC to a lower voltage AC. Wall adapters that are DC sometimes have transformers, but again not always. Usually weight makes it easy to tell, transformers are much heavier and larger . The biggest benefit to a transformer is isolation from the home AC. I really prefer transformers in things like audio equipment over switching designs because the risk of ground loops is low due to the isolation.

Any Class 2 power supply, switchers included, are always isolated from AC hot and neutral. However sometimes the output negative will be connected to earth ground (easily fixable via cheater plug!)

 

[TuxDave]

Awww, I got to this thread too late. Just when I was about to dish out the water pipe analogies.

 

[johnnydrinkawater]

Voltage won't kill you, I will kill you.

(I is the variable for current)

 

[DrPizza]

Voltage won't kill you, I will kill you.

(I is the variable for current)

I is dependent on voltage and resistance though. Without a sufficient voltage, you're not going to get a current. That's why I'm not afraid to touch one terminal on my car's 12V battery with one hand & touch the other terminal with the other hand. Yet that battery is capable of providing more current than you are limited to in most household circuits.

 

[herm0016]

Awww, I got to this thread too late. Just when I was about to dish out the water pipe analogies.

me too!

voltage is like pressure, you can only push what the pipe is rated for until it blows, current is like flow rate, the size of the pipe at the prescribed pressure determines the flow rate.

 

[erwin1978]

So if I short my DC wall adapter example, it will quickly overheat and die? How much current is going through my shorting wire?

 

[PM650]

So if I short my DC wall adapter example, it will quickly overheat and die? How much current is going through my shorting wire?

Largely depends on the resistance it's flowing through, and to some degree the smoothing cap's ESR & capacitance (at the instant of shorting); suffice it to say, the current will be large enough to blow a transformer winding if nothing else fails first. The energy stored in the smoothing cap could provide the bulk of the energy to cause the shorting wire to fuse - if it doesn't, the rectifier will be next to go (or a pcb trace, or another wire, who knows). Overheating only occurs over a longer period of sustainable overload, it's not at all what you need to worry about in the event of a short.

 

[DominionSeraph]

So if I short my DC wall adapter example, it will quickly overheat and die?

Consumer devices are usually designed to avoid arcin' and sparkin' and smokin'. I'd bet that, at the very least, it has a current limiting resistor.

 

[BrownTown]

Consumer devices are usually designed to avoid arcin' and sparkin' and smokin'. I'd bet that, at the very least, it has a current limiting resistor.

not to mention the wall socket has a circuit breaker.

 

[DominionSeraph]

You can smoke a lot of dope on a 20A circuit.

 

[erwin1978]

So this statement is wrong? The wall adapter doesn't limit the current as you can short it and the current will surpass 300 mA and blow it to smithereens? The 300 mA rating is simply the allowable load to be connected to it without failure?

 

[DominionSeraph]

Why are you so keen on shorting a wall wart?

 

[bobsmith1492]

So this statement is wrong? The wall adapter doesn't limit the current as you can short it and the current will surpass 300 mA and blow it to smithereens? The 300 mA rating is simply the allowable load to be connected to it without failure?

First it's not a hard limit; it could probably deliver 400 or 500mA for a short time, though the voltage may sag and the ripple would get out of spec. If you kept drawing that much for a longer amount of time, components would start to fail.

It would not blow to smithereens though; probably a fuse or thermal fuse would open first, if you drew enough current (350mA might not trigger a fuse). A regulator would probably fail open next. The transformer and its winding would probably be the last to go unless it is a simple unregulated device.

Engineering of this kind is not very exact, it's more statistical. Statistically speaking the device was designed so that 99% of them would last for at least 5 years while on 10 hours/day with a 300mA load. Of course I made up the numbers and most designers can't give you these numbers with much confidence anyway. Also, depending on the quality of the device, it could be more like 50% of them will last 1 year with a 300mA load for 2 hours a day. It's really hard to say.

 

[johnnydrinkawater]

I is dependent on voltage and resistance though. Without a sufficient voltage, you're not going to get a current. That's why I'm not afraid to touch one terminal on my car's 12V battery with one hand & touch the other terminal with the other hand. Yet that battery is capable of providing more current than you are limited to in most household circuits.

Just out of curoisity - do you know what resistance a typical human body is? I thought it was around 10K or so...

I think that number goes way down for AC because the skin becomes less resistive, so at 12VDC, it's even less of a concern.

 

[erwin1978]

Why are you so keen on shorting a wall wart?

I don't really like shorting wall warts. I have destroyed a couple doing just that. My main motive is that I have an LED flashlight I wanted to upgrade to a better LED and I just wanted to learn a bit more on how the driver is providing the power to it and whether or not the new LED or driver will be safe after the upgrade.