question about volts..amps

[mhn3773]

i have a question about volts and amps and such i have recently been thinking of putting a massive stereo setup in my car with expo 5.0 =) but when i read things like for amps they say rated at 12.2volts and 14.5volts

now if it car is a 12v system wouldnt it only be able to pump out 12v constant power to the car? so like the amps would only get like 6v wouldnt they? also when u buy an alternator u can get like a 200amp or a 220amp how does amps figure into volts? i know these are kinda dumb questions but it would help if i understood these things

o...also how can i measure the rms of my amp? like a volt meter or something??

 

[Alec]

Most auto 12V systems put out about 11.5 with the motor off (depending on battery) and about 14-14.5 with the car on (if the alt is working normally).

The amp rating of the alternator is the max it can put out at a certain RPM. This number should be higher than the sum of all the amp draws of everything electrical in the vehicle, and remember that you should always go overkill on the alt so it will put out enough at idle.

As for the RMS of the audio amp, it's easiest to look on the manufacturer's website.

 

[expo5.0]

not many explorers actually have more than 14.0 volts at idle.

also, in a "12" volt system (whether its running at 12 with the car off or 14 with it on) provides 12 volts to everything connected to it in parellel. you'd never wire your amps in series! so everything will be getting the full voltage- the issue is getting enough amps from the battery/alt/or capacitors and little enough resistance.

 

[expo5.0]

one more thing- an easy way to guess the rms of your amp is to take the recommended fuse rating and multiply by the voltage. for example 20 amp fuse times 14=280 watts max power or 140 watts rms- then you can just spread that out over the channels.

sometimes you'll see an amp that say 1800 watts but only has two 20 amp fuses... (cough sony)

 

[Chew_12]

Power = amps x volts

So u have 2 amps and 9 volts that is a total of 18 volts

 

[Robb]

Originally posted by Chew_12
Power = amps x volts

So u have 2 amps and 9 volts that is a total of 18 volts

HUH?

PIE: P=IE
P (power) = watts
I (current) = amps
E (voltage) = volts

Ford reccomends alt. output voltage at 14.1-14.7 at no load. Both my Xs are dead on 14.5v.

 

[expo5.0]

but power is measured in watts not in amps

so your example would be P(watts)=2amps times 9 volts

18 watts and if this were an amplifier that would be the max power rating.

 

[expo5.0]

is that 14.5 taken at the battery while its running? or where?

mine is taken at my amps and it ranges from 12.0 with it off to 14.6 at the very high end.

 

[mohog]

for people who don't know there is a difference in the types of "amps" that these guys are talking about. you have power amps and then you have fuse amps. in the equation PIE the I or current is fuse amps not amp amps. if that makes sense. it looked to me like you confused chew_12. but every amp has fuses and you take the amps allowed by the fuses (it will say right on the fuses how many amps of current it allows) multiplied by the volts coming from your battery/alt. = max watts. sony/lanzar are exceptions

 

[brendan]

Yeah, but there are more ways...

P=I^2R
P=V^2/R

If you have 3phase, Real power P=3*Vphasor*Iphasor*cos(theta)

There are many ways to find the power ;-)

 

[Robb]

expo...that is measuring at battery. At an amp (plifier), with only about 15-20' of properly sized wire, I wouldn't imagine the drop to be very much at all.

mohog.....yea, sorry about that. How about using amplifier from now on when refering to the car audio gizmos, and amp(s) when refering to current?

brendan....yes, power can be derived by various equations, we could go into HP equations if we wanted to dig. We hadn't brought resistance into the subject, so PIE should be fine when only discussing voltage and current draws and capabilities. 3-phase really has nothing to do with 12V DC.

 

[brendan]

Yeah, I know... I'm just so used to doing this stuff within my 17 credits of EE studying that it all started pouring out of my brain when I saw PIE!!!

Quiz tomorrow in Power class too! Just had an exam in Emag, we just find the average power in there though... Exam Thurs in Controls, we don't worry about power there... 3 interviews next week, hopefully the people with power will give me a job!!!

 

[KPT]

Originally posted by expo5.0
one more thing- an easy way to guess the rms of your amp is to take the recommended fuse rating and multiply by the voltage. for example 20 amp fuse times 14=280 watts max power or 140 watts rms- then you can just spread that out over the channels.

sometimes you'll see an amp that say 1800 watts but only has two 20 amp fuses... (cough sony)

Thats nowhere near accurate. I have a RF amp rated 20x2 RMS, its got a 15 amp fuse, using your calculation the RMS is 105x2.

Have another RF rated 40x2 RMS with a 20 amp fuse. 140x2 RMS.... I think not.

 

[expo5.0]

my ratings are the maximum possible that the fuse will allow. it doesn't apply to low output amps real well. what it does do is put up a red flag when some company claims 1,000 rms but only has two 20 amp fuses. there is no reason the output has to be as high as that method predicts- just proof that it isnt higher.

 

[expo5.0]

btw- are you interested in selling either of those amps? what models are they? i don't see em in your system and i could use one.

 

[debutis]

Not a very reliable method of determining max power output.. Some amps are fused internally or have some on the outside and some on the inside. When you deal with higher end amps, some of the companies put them on the inside because of the circuitry designs they use. Like JL for instance.. You see no fuses outside of the amp. You look inside and they're all over. One of my Zapco amps is the same. Higher end companies figure people know what they're doing when they have expensive components like that in their systems. RF and Sony and Hifonics etc. all know people will be more at risk to blow fuses and put them on the outside of their amps. Plus, when amps are modified, (the D2 or D3 for instance) fuses aren't what changes, it's internal circuitry and resistors. That equation will work with the cheaper, lower end amps, but not the expensive audiophile grade equipment.

 

[expo5.0]

i only was trying to suggest it as a very general rule
for some situations

none of the amps i use have external fuses either, i'm not dumb i was just trying to help the guy out with his question.

 

[debutis]

Sorry to get a little off topic, but, Expo 5.0 - what audio do you run? Just curious, I always see you reply to thread but I dont recall seeing what you have in your xploder. Just wanna see what fellow explorers have. I'm always changing my stuff so I want to know what others have tried.

 

[expo5.0]

right now i run

a strange combination of things for my hu-

the 11" lcd displays track # and time
my cd/dvd/cd-rw etc. reader is the clarion 715
and i use my eclipse 2100 both as an eq and as level control. next (on the rear channels only) is an epic150 bass eq with bass restoration circuitry and spl meter/volt meter the signal is split by an mtx electronic crossover and sent back to my amp rack.

currently the amp rack has three rf power amplifiers- 800a2 (for subs) 500a2 (3-way components) and 150a2 (for 8" sub and rear door speakers- these use 12 db crossovers i built and act as "components" together"

the front speakers are abc6003q audiobahn 3-ways in kickpanels and the doors.

rear doors are 5 by 7 cerwin vegas right now (these change all the time- will soon be audiobahn 3-ways like up front- maybe?)

the 8" is a mtx 6000

and the current subs are he2 15s


soon the subs and sub amp can be forgotten though. i sold my stuff last night and i'm going to be running an mtx 1501 amp to two 9500 mtx 15s.

 

[debutis]

You must really like MTX. Never been a big fan of them, but then again never heard any of their higher end stuff. Sounds like a solid system you got goin tho.