Check my math?

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J

Jalex420

Member
Joined 2006
I've been reading up on Ohm's Law, and i made up some equasions here for an amp / psu i'd like to make, im not sure how exact the math has to be, so i figured i'd run it past you folks, maybe you can give me a hand :)

========================
= From Wall Outlet =
========================
Input:
Voltage: 120 Volts
Power: 2400 Watts
Current: 20 Amps

========================
= PSU Output =
========================
R = Resistance (Ohms)
P = Power (Watts)
C = Current (Amps)
V = Voltage (Volts)

R = 12
V = 54 Volts
C = 4.5
P = 243 Watts

Output:
Power: 243 Watts
Current: 4.5
Volts: 54
========================
= Out From Amp =
========================
I = Current (Amps)
R = Resistance (Ohms)
V = Voltage (Volts)
P = Power(Watts)

P = 100 Watts

V = 50 Volts

R = ((V * V) / P)
R = ((50 * 50) / 100)
R = (2500 / 100)
R = 25 Ohms

I = V / R
I = 50 / 25
I = 2 Amps

Output:
Channels: 2
Power: 100 watts / Channel
Current: 2 Amps / Channel
Click to expand...
 
poobah

poobah

Member
Joined 2005
Looks good... I don't understand why you have R listed in your power supply calc's.

Use I for current in the future... not C. C is used for capacitance.

And don't expect to pull 20 Amps from a wall outlet. If you need that much power... change your equipment to 240 V... like a dryer or stove.


;)
 
poobah

poobah

Member
Joined 2005
It might help to know what you are doing?

If your power supply is putting out 243 Watts, it is probably requiring 243 / .95 = 256 Watts at its input. This assumes 95% efficiency... could be lower like 85 - 90%

If 256 Watts were going into your supply; what would the current be?

:cool:
 
poobah

poobah

Member
Joined 2005
Yes!

It is important to understand that the wall socket doesn't just cram 20 Amps into anything you plug in.

The input to the power supply has a resistance (not really a resistor) that determines what anount of current flows. With what you know... what is the "effective input resistance" of your power supply?

:)
 
preiter

preiter

Member
Joined 2006
Home electrical circuit usually put out either 15 or 20 amps (if we are talking US here) . Unless you are building a 1500+ watt amplifier, however that's not what you should be worried about.

Try going backwards from your amp wattage to determine what size transformer you need.
 
J

Jalex420

Member
Joined 2006
Ok, using the following variables
P = 256 Watts (What im drawing from my outlet)
E = 120 Volts (What im drawing from my outlet)

Now that i have 2 Set variables, i can now find the other 2
I (Current)
R (Resistance)

R = (E * E) / P
R = 260.86 Ohms

I = V / R
I = 0.46 Amps

So my effective input resistance is 206.86 Ohms?
 
J

Jalex420

Member
Joined 2006
preiter said:
Home electrical circuit usually put out either 15 or 20 amps (if we are talking US here) . Unless you are building a 1500+ watt amplifier, however that's not what you should be worried about.

Try going backwards from your amp wattage to determine what size transformer you need.
Click to expand...

Great idea. Thanks!


--------------------
edit
--------------------
Correct me if im wrong, but i should have the output of the powersupply give more power (Watts) than what my Amp calls for right?

I know for computers, you should always have more wattage than your computer draws.

Thanks!
 
J

Jalex420

Member
Joined 2006
Ok, i've created yet another one. Here it is.

Channels: 2
Output / Channel: 100 Watts
==================================
= PSU -> Amp =
==================================
P = 250 Watts
V = 120 Volts
I = P / V = 2.08 Amps
R = V / I = 57.69 Ohms
=================================
= Outlet -> PSU =
=================================
P = 250 / .8 = 312.5 Watts
V = 120 Volts
I = P / V = 2.6 Amps
R = 46.15 Ohms
 
poobah

poobah

Member
Joined 2005
Looking good!

Now... don't equate power out of your amp directly to the power coming from your supply.

Your power supply has an efficiency. But separate from that, your amplifier has an efficiency too... amplifiers make heat, and that energy has to come from somewhere. An audio amp could be anywhere from 10 - 95%... all depends on the type (don't quote me on the 10)

Now... if we knew more about your amp...
 
J

Jalex420

Member
Joined 2006
poobah said:
Looking good!

Now... don't equate power out of your amp directly to the power coming from your supply.

Your power supply has an efficiency. But separate from that, your amplifier has an efficiency too... amps make heat, and energy has to come from somewhere. An audio amp could be anywhere from 10 - 95%... all depends on the type (don't quote me on the 10)

Now... if we knew more about your amp...
Click to expand...

I havent started making an amp yet, i've been tring to learn some of the basics, so i would know what i was doing, and won't brun my house down.

Here is what i've read about so far
Ohm's Law
How Semi-Conductors work
How Amplifers Work (Very basic)

Would you happen to know where a good amp to start from would be?
 
poobah

poobah

Member
Joined 2005
Jalex,

Sounds like your off to good start.

I think a good place for you to start would be a chip-amp. This will teach alot of basics... and get your iron wet.

Peter Daniels, and others, makes some nice reasonably price kits that have a good reputation. Building a kit won't give you alot of design experience... but you can analize the hell out of it to build your design skills.

There is a wealth of info here... search "newbie wants a chip amp" or something like that.

As you start out, don't get caught up in silver speaker wires, $40 capacitors and that ****. Stick to the basics, you learn a ton, make some nice sound, and keep your money for Saturday night!



:D
 
A

AndrewT

R.I.P.
Joined 2004
Hi,
and try to analyse the answers.

Sometimes you need to separate the wheat from the chaff.

For an amplifier the output power P=Vrms^2/R = Vpk^2/2/R
from this you can find that Vpk=root(2) times Vrms ~=1.414*Vrms.
(^ means raised to the power of, so ^2 = squared, ^3 = cubed).

If you know your load impedance and your output power you can calculate the Irms and Ipk and Vrms and Vpk.

I and a few others refer to Vrms as Vac, for a sinusoidal waveform they are identical. For our PSU purposes they can be considered interchangeable.
 
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