OT: 7a vs 10a universal power cord?

[k2500x]

Hope this is a very easy question for someone that knows about electricity.

 

I've been rebuilding my studio from the ground up recently and always put all those 'universal' 3-pronged power cords in a pile and don't pay much attention. This time I took a closer look and saw that some of them say 7amp 125volt, others 10amp 125volt, while some others are not even marked at all. I've blown some PSUs in the past just trying to look for plugs that 'fit' so this made me a little nervous to just pick random cords and plug everything in again. Any tips? Or no need to worry?

[mate_stubb]

Put the 10A cords on the power amps. Don't worry about the rest.

[kazper]

 

Put the 10A cords on the power amps. Don't worry about the rest.

 

 

Add mixer's and some large older keyboards and organs.

 

Look at the plates or the manual of your heavy pieces of gear.. if it lists more than 7.5 Amp, and or fuze is larger than that then you will probably need the larger cable.

 

My mixer needs a 10A cable, as well as a couple of my PSU's

 

Power supplies do go out in gear.

[k2500x]

So if I understand correctly, said piece of equipment is only going to draw the amount of power it needs to run, up to the amps limit listed on the cable?

Would an underpowered cable kill the PSU?

Where to watts come into play here?

Is there an idiots guide to understanding electricity on the web?

 

I have one of the older Roland JD990s with universal 2-prong input. So I lost the Roland cable one day and decided to try one that fit perfect from an old DVD player. Well it worked, except every few minutes the speakers let out a horrid electrical zapping sound. Needless to say I won't try that again. Got the correct cable and now all is well, but I wonder how much damage might I have done to my beloved JD990?

[kazper]

http://en.wikipedia.org/wiki/Watt%27s_Law

 

Watt's is a measurement of power as seen as VA or KVA on equipment.

1.8KW=1.8KVA=1800W=1800VA

 

http://en.wikipedia.org/wiki/Volt-ampere

 

It's like this, It all relative and equals out or has a cause and or effect on another variable.

 

1+1=2...

 

If there is variances there may be changes like this..

1.5 + 0.5 =2

 

AC is tricky because it's not simple math like DC. There is phasing, lead, lag etc.. But it works the same in basic terms.

 

Say your fuse is a 0.4A (just for kicks) and your equipment has a nameplate rating of 50W.

 

So if your Keyboard or module normally runs at 12W usage at idle (just plugged in and on) and you have 120V wall power... you have 0.1A of current flow.

 

If your equipment at full normal draw is 0.3A at 120V it's using 36 Watts of power. Thats using the Watt's law formula of Power = Current (Amps) x Potential (volts), P=IE.

 

Now you have a transistor/Diode that is on it's way out towards causing a short. The machine draws more current than it should when pushed hard.

Say it's now pulling 0.5A, this would make a 0.4A fuze blow (not going into slow/fast and other fuse types)

 

 

Where the cord issue lies in this here... You have a load (semi-fixed resistance/reactance). If your amp draw is higher than the cable permits. the voltage or resistance will need to adjust to keep up for the current demand and to make the math equal. Ohms law now..

 

Potentially but highly unlikely you could end up with one or two failures... Device burnout because it was specified for a minimum voltage and like the cable adjust till it fails. Cable opens up under power dissipation in the cable at a weak spot, which is less likely. You could also end up with both of those situations...

 

Don't worry, this is all unlikely unless it's a big ass amp or mixer. Most the gear in my place uses less than a 1/2 or 500 or 0.5A fuse I can say I have a couple items that must use a larger gauge cable than normal for it's power.

 

http://en.wikipedia.org/wiki/Ohms_law

 

Hopefully that makes sense...

[k2500x]

My understanding is a whole lot better now.

 

It seems that if none of my gear uses over 875 watts (7x125) of power I'm safe.

Well I'm sure that is the case since I'm only running MIDI tone modules and my computer PSU is 350 or 400w as I remember.

 

Thanks!

[mate_stubb]

Not to mention that if you are running all this on typical 15A circuit breakers, you could only have a couple of items pulling 7A on one circuit.

[TheB3Nut]

And I guarantee your PC power supply isn't drawing anywhere near its max load, espeically if it's a newer 80%-efficiency or higher unit. I've built systems with Core 2 Quad CPU, 4GB RAM, 600W Seasonic modular PSU, and multiple hard drives and struggled to draw more than 200W on the Kill-A-Watt meter I use on my bench running burn-in software (I'm a PC/network tech by day.)

 

IOW, for your applications, don't sweat it.

[k2500x]

I think where I got into trouble in the past was when I was messing with OEM wall wart power supplies and looking for "anything that fit", and I must have put a 12v power supply on something that only needed 9v. Then it fried. So in the end it looks like I've just got to make sure that the volts match up, and that the amps is at least as high as the gear requires. Then I'm safe?

 

Wait that still sounds like I might be confused because I didn't distinguish internal from external PSU.