General knowledge / electricity questions

Dr.EM · 23 Jul 2009 at 19:35

div0 said:
I disagree with Jez here. If your ipod is running from battery then changing the volume on the ipod changes the rate at which the battery is discharged. The higher the volume, the more power is drawn from the battery and the faster the discharge rate. Assuming your speakers stay at constant volume then the current drawn by the speakers from the mains will stay roughly the same. There may be a small change, but it will be negligible imo.

Jez is correct. When the iPod is connected to an amplified speaker system, changing the volume through whichever means will effect the power drawn from the speaker amplifier supply (the mains). Think of it like the signal from the iPod opening an elaborate tap in the speaker system, allowing more power to flow from the supply to the speaker

To be more technical, it depends on the amplifier topology. A class-A amp uses a great deal even doing nothing and is more efficient at higher volumes. A class-AB, typically employed by such a system (or class-D perhaps) uses more as the volume is advanced, as less power is dissipated as heat.

div0 · 23 Jul 2009 at 19:38

Nitefly said:
How does this different from a laptop charger? A laptop charger is always on full pelt, as far as I know, regardless of whether it's plugged into it.

I don't believe this to be true. Some current will probably be drawn due to the internal circuitry of the charger, but it will not be as high as when it is charging the device (phone/laptop).

Vonhelmet · 23 Jul 2009 at 19:40

The ipod thing will depend on the electronics involved, I think. Here's a slightly more elaborate guess...

The ipod has an amp in it that produces the sound that goes on the line out. Upping the volume on that will up the power consumption from the ipod.

The speaker dock also has an amp in it that amplifies the signal on the line in and sends it to the speakers. If the amp is set to run at a constant level then upping the volume on the ipod will not increase the amount of electricity consumed by the speakers, as the amp is still amplifying by a factor of 2 or whatever. It would only be when you increased the volume on the speaker dock that the mains consumption increased.

This is all conjecture really... We'd need someone with an in-line power monitor to check.

Vonhelmet · 23 Jul 2009 at 19:42

div0 said:
I don't believe this to be true. Some current will probably be drawn due to the internal circuitry of the charger, but it will not be as high as when it is charging the device (phone/laptop).

A charger has a transformer in it, that converts the mains voltage (240v) to the battery charging circuit voltage (6v or whatever). When the charger is plugged in and switched on but not charging, some power will still be lost in the transformer as heat in the coils.

Burnsy2023 · 23 Jul 2009 at 19:47

div0 said:
I don't believe this to be true. Some current will probably be drawn due to the internal circuitry of the charger, but it will not be as high as when it is charging the device (phone/laptop).

This is correct. A power supply will alwayd draw some current to keep the coils in the transformer charged so that there is enough power for the device to be turned on when the button is pressed. However, when the laptop is on there will be much more current draw.

Edit: should refresh the page before posting

Nitefly · 23 Jul 2009 at 19:56

Some really interesting answers, thanks guys

MikeHunt79 · 23 Jul 2009 at 19:57

Steve2000 said:
If I have a 600W PSU in my computer, but the components only amount to 400w total, does it draw 400 or 600 watts?

~ 450W

MikeHunt79 · 23 Jul 2009 at 19:58

Nitefly said:
How does this different from a laptop charger? A laptop charger is always on full pelt, as far as I know, regardless of whether it's plugged into it.

This is what is so great about having a multimeter, you can measure stuff like this.

My laptop charger is not always at full pelt, FWIW... It still uses a little power with no laptop plugged in however...

Nitefly · 23 Jul 2009 at 19:59

MikeHunt79 said:
This is what is so great about having a multimeter, you can measure stuff like this.

My laptop charger is not always at full pelt, FWIW... It still uses a little power with no laptop plugged in however...

Haha, shows what I know

MikeHunt79 · 23 Jul 2009 at 20:02

Nitefly said:
Haha, shows what I know

A laptop charger is pretty similar to a PC PSU is many respects, they are both a small SMPS, the PC PSU has many rails while the laptop PSU only has a single rail.

Laptop PSU's tend to be a little less efficient tho, but this isn't a huge problem as they rarely output more than 100W.

MikeHunt79 · 23 Jul 2009 at 20:06

vonhelmet said:
This is all conjecture really... We'd need someone with an in-line power monitor to check.

I've found those in-line power monitors are poor with when used with low power loads, anything less than 10W gives inaccurate readings. You're much better off with a good multimeter as they are much more accurate but the downside is that they are more hassle to connect up...

Jez · 23 Jul 2009 at 20:59

div0 said:
I disagree with Jez here. If your ipod is running from battery then changing the volume on the ipod changes the rate at which the battery is discharged. The higher the volume, the more power is drawn from the battery and the faster the discharge rate. Assuming your speakers stay at constant volume then the current drawn by the speakers from the mains will stay roughly the same. There may be a small change, but it will be negligible imo.

Think of the amp is a multiplier if it makes it easier. This is a simplistic view but its correct in theory.

Say the amp switched to full pelt multiplies the input by 10*

Ipod at low volume has a value of 1, ipod at full has a value of 10.

At the same amp volume setting (10* multiplier), at 1 it will be outputting at 10. At full ipod volume it will be outputting at 100, a clear 90 more than before, requiring a lot more power regardless of the ipods volume level.

The ipod uses more internal battery because it is itself outputting at a higher volume, that has no impact on the external output of the speakers though, it is just the feed into the external amp which the amp then takes and multiplies.

As i say, simplistic with full of holes and discrepancies, but basically explains why an amp will use more power at various input levels.

MikeHunt79 · 23 Jul 2009 at 21:35

Yep, agreed with Jez, unless you're running a class A amp.

Jonnycoupe · 23 Jul 2009 at 21:41

Nitefly said:
-If a plug switch is turned on, but there is nothing plugged into it, does it consume electricity?

Possibly if you have a transformer doing a voltage conversion. You have galvanic isolation as you have basically two large coils of wire with a ratio of coil windings to transform the voltage, so the circuit could draw current even though the appliance isnt. This is why a typical power adaptor, or phone charger will be warm even if its plugged into nothing.

Jonnycoupe · 23 Jul 2009 at 21:44

vonhelmet said:
The speaker dock also has an amp in it that amplifies the signal on the line in and sends it to the speakers. If the amp is set to run at a constant level then upping the volume on the ipod will not increase the amount of electricity consumed by the speakers, as the amp is still amplifying by a factor of 2 or whatever. quote]

Line out on a ipod is for unamplified speakers, ie headphones.

The actual dock interface will just be data including a volume request to the amplifier on the dock.

So it depends how its connected to the station. headphone jack or ipod interface on the base.

MikeHunt79 · 23 Jul 2009 at 22:33

Jonnycoupe said:
Possibly if you have a transformer doing a voltage conversion. You have galvanic isolation as you have basically two large coils of wire with a ratio of coil windings to transform the voltage, so the circuit could draw current even though the appliance isnt. This is why a typical power adaptor, or phone charger will be warm even if its plugged into nothing.

Quick question for you about transformers, as I think I've killed one yesterday!

I have a car radio, and I plugged it up with the wrong polarity, smoke came out, etc... Now the backlight does not work get the radio still makes sound.

Long story short, the backlight uses a step up transformer to make 200Vac, and it seems the resistance of the primary winding on the transformer is a few ohms, while the resistance of the secondary winding is around 3.3Kohms... Is the transformer dead or could it be the thing feeding the transformer (switcher)...

Jonnycoupe · 23 Jul 2009 at 23:24

Reversing AC current shouldnt really cause a problem, if its 200Vac?

Not really sure on that one tbh, what type of backlight is it to need 200V?

MikeHunt79 · 23 Jul 2009 at 23:36

Sorry for the thread hijack!

Jonnycoupe said:
Reversing AC current shouldnt really cause a problem, if its 200Vac?

Not really sure on that one tbh, what type of backlight is it to need 200V?

It's an electroluminescent (EL) backlight, and when the transformer was working it used to put out 200Vac at 1khz and the display lit up.

After connecting up the radio with the wrong polarity from the car, the transformer outputs nothing, and there is now no display and I have a feeling the secondary winding is fried because of the high resistance (3.3KOhm)... I think maybe the output from transformer got shorted to ground when I plugged the radio up to the car with the wrong polarity...

The car was providing 12V dc, which was then getting converted by the radio to 200Vac for the EL display.

I'm going to borrow an oscilloscope tomorrow, to see if the transformer is getting an input on the primary winding, and hopefully I'll know for sure.

Amp34 · 24 Jul 2009 at 02:19

vincent1989 said:
400, it's rated to supply up to 600 (at peak load)

Also most PSU's are designed to be most efficient at around 75-90% usage, so in fact it's better to buy a PSU that suits your components and is utilised at around 75+%. For example it is better/more economical to buy a 600W PSU to power a 500W (max) system than a 1000W PSU to power it.

Kaladan · 24 Jul 2009 at 02:35

Amp34 said:
Also most PSU's are designed to be most efficient at around 75-90% usage, so in fact it's better to buy a PSU that suits your components and is utilised at around 75+%. For example it is better/more economical to buy a 600W PSU to power a 500W (max) system than a 1000W PSU to power it.

A slight deviation off of this topic, but a point I've always wondered, but is there any reliable way to estimate the power draw of a system before you have all the parts to hand, like an online calculator? I know I've seen these kinds of things before, but they were never really up-to-date with components and/or used far too generic components/terminology.

How do people get a feel for what level of PSU they need, or is it just one of these things you make an educated stab at? Do people get an idea for power draw through looking at benchmarks of similar systems when comparing product reviews and so on? It's for this very reason I've always overestimated the power draws of my system, but after reading this it seems kind of counter productive if you aren't going to utilise that extra power with a future upgrade.