# Homework Help: Why does a CD player operate at 24V not 240V

1. Sep 28, 2011

### AHUGEMUSHROOM

1. The problem statement, all variables and given/known data
A portable CD player could be designed to operate at 240V. Explain why a much lower voltage is normally used. Provide a number of reasons.

2. Relevant equations
V=IR
P=VI

3. The attempt at a solution
The reason a CD player does not operate at 240V is because at 240V there would a lot of power being wasted.

Guys, I don't really know how to answer this one. My teacher was telling me today that if theres a high voltage; theres a low amount of current. And if theres a low voltage; theres a high amount of current. This got me all confused, why would a CD player need a lot of current, surely because its a CD player it would only require a small amount of current and therefore would operate 240V.

2. Sep 28, 2011

### NewtonianAlch

Well most appliances don't run direct off 240V, they have some kind of transformer to step down the voltage, because of the high current associated with larger voltages. So what your teacher said can't be right. V = IR says it all.

3. Sep 28, 2011

### Staff: Mentor

What considerations other than power might be involved? For example, would you feel comfortable with possibly exposed 240V terminals in your pocket? What would supply the voltage? Look at some typical electronic parts (transistors, op-amps, digital IC's, etc.). What are their typical operating limits for voltage (do you know why)?

4. Sep 28, 2011

### NewtonianAlch

I believe it's for such reasons that the USA uses 110V, so there's a better chance of survival from getting electrocuted.

5. Sep 28, 2011

### phinds

I think the first part of that is nonsense. There is no high current automatically associated with larger voltages. You just have to use a smaller resistance so that the amount of POWER is the same whether you use high voltage or low voltage. So, yes, V=IR DOES say it all, but not the way you interpreted it.

I agree w/ gneill's post. High voltage is dangerous and harder to supply and THAT's the reason we don't use it for small appliances.

High voltage isn't needed to get high power, but the problem is that if you want high power with low voltage, you have to run so much current that the size of the wires just gets to be ridiculous to avoid too much voltage drop in the lines. That's why power is DISTRIBUTED over power lines as very high voltage and then stepped down to provide house power.

220V is used for some large machinery for eactly that reason --- to supply the needed power with lower voltage requires wires that are too big and/or overheat too much. Three-phase is used for the same reason (and others) --- higher power without higher current because you're using 3 wires instead of one [well, really 4 instead of 3 when you count earth ground].

6. Oct 14, 2011

### NewtonianAlch

It wasn't a hard and fast rule, it was more of a generalization. High voltage is dangerous, why? It's not the voltage that kills people, it's the current. So yes, high voltages pose a threat to people, but it's not the voltage in itself that is dangerous.

Yes, V = IR does say it all. If you have a high voltage, generally there is a high current associated with it, unless it just happens to have a ridiculously high resistance and a low current. I don't see what's wrong with what I said as a generalization. Maybe I should have pointed that out, so I didn't have to cop a large rant; most of what you were babbling on about is nothing I disagree with, but it's nothing that I contradicted either, you're just going off on a tangent by misinterpreting what I said.

7. Oct 14, 2011

### Staff: Mentor

Minaturising is difficult at 240v. Thickness of insulation on wires is a factor, and I believe you'd need extra copper strands simply to give the heavier wire the strength to withstand heavy bumps. Bulky insulation makes bending and routing difficult, and needs stronger pads on PC boards. Tracks on PC boards have to be wider spaced, and safety margins must be more conservative. High voltage components are invariably many times more expensive than the low voltage equivalent. High voltage components are usually larger than the low-voltage versions (connectors, switches, capacitors, diodes, semiconductors). A lot more exposed voltages would need to be covered out of safety considerations for repairmen, too, even though most of us regard consumer electronics as disposable and non-repairable. Wasn't the first minature mp3 player designed to be discarded when its battery went flat? The battery was sealed in and not even accessible by owners.

Last edited: Oct 14, 2011
8. Oct 14, 2011

### phinds

Yeah, but that would have deprived me of a perfectly good rant.