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dave_Edwards

Ohm's Law explained

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OHM'S law

There are 2 base formulae which will help you to understand the relationship between current , voltage , resistance and power . If you have any two of the parameters, you can calculate the other two parameters.

OHM'S LAW

BASE FORMULAS P=I*E E=I*R

TO FIND VOLTAGE E=P/I E=I*R E=SQR(P*R)

TO FIND CURRENT I=P/E I=E/R I=SQR(P/R)

TO FIND POWER P=I*E P=E2/R P=I2*R

TO FIND RESISTANCE R=E2/P R=E/I R=P/I2

P = Power in Watts

E = Electromotive Force in Volts

I = Electrical Current in Amps

R = Electrical Resistance in Ohms

SQR = Square Root

Note:

I use 'E' to represent voltage most of the time but sometimes you'll see 'V' used for voltage. Don't let it confuse you.

Remember

Fuses:

I've been in the electronics repair business since about 1996 and have come to believe that most people don't understand the function of a fuse, or they just like to let the smoke out of electronic devices (transistors, resistors...).

Note:

Letting the smoke out of an electronic device is a process which converts a useful piece of electronic equipment into a paper weight.

Function:

A fuse is generally inserted into an electrical circuit for 1 of 2 reasons, either to protect the power source which includes the wire that connects the power supply to the electrical device, or to protect the electronic equipment. The electronic equipment manufacturers specify a fuse rated to open the electrical circuit before damage can be done to the device or open the circuit if the electronic device fails in some way (electronic devices may pull excessive current when they fail). If a fuse larger than the specified fuse is used, a small mistake when installing the equipment may cause catastrophic failure of the equipment. WHEN, not if, WHEN you're thinking of replacing a blown fuse with a higher rated fuse ask yourself if you know more than the engineer who designed the equipment. Don't get in a hurry when installing electronic equipment. Take the time to go get the right fuse. 50 cents for a fuse is better than $50 labor plus the cost of the replacement parts for a repair job.

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In most cases, the wire size is reduced at the point of distribution. ANY time that the wire size is reduced, you must add a fuse in the line.

Using multiple small wires in place of a larger wire:

Some people may want to use a bunch of smaller, individually insulated, wires (like ten 14g wires) in place of one larger wire (like a 4g wire). This may be OK as far as current carrying capacity is concerned but the problem comes in when you have to fuse it. A 4g wire can handle about 125 amps. A 14g wire can handle about 15 amps. If one of the strands of the 14g wire is shorted to ground (like where it runs through the firewall), the main 125 amp fuse would not blow and the wire would burn. To properly protect the multiple strands of insulated wire, you'd have to use ten 15 amp fuses in individual holders (each wire would have its own fuse). I know that this may be an 'off the wall' situation but I've had several emails about this (generally concerning two or three 8g wires and a large wafer fuse) so there are, at least, a few people who don't fully understand this.

Suggested Fuse Sizes:

Wire Gauge Recommended

Maximum Fuse Size

00 awg --------400 amps

0 awg--------- 325 amps

1 awg--------- 250 amps

2 awg--------- 200 amps

4 awg--------- 125 amps

6 awg--------- 80 amps

8 awg--------- 50 amps

10 awg------- 30 amps

12 awg------- 20 amps

14 awg-------- 15 amps

16 awg-------- 7.5 amps

These are the recommended maximum fuse ratings for the corresponding wire size. Using a smaller fuse than what's recommended here will be perfectly safe.

Fuse Opening Time:

A fuse does not blow when the current reaches its rated current. It is designed to pass its rated current without opening. A fuse will take varying times to blow under different conditions. A fuse will pass significantly more than its rated current for a very short time. It may take 10 minutes or more to blow a fuse at 25% over its rated current. The table below is an example of the specifications for a slow blow fuse. You can see that a 20 amp fuse may pass 40 amps of current for as long as 5 minutes before blowing although it probably wouldn't take a full 5 minutes to blow. The times for other fuses will be slightly different.

%of amp rating Opening time

110% 4 hours minimum

135% 1 hour maximum

200% 5 minutes maximum

Circuit Breaker:

A circuit breaker's function is, like a fuse, to break a circuit path when a predetermined amount of current is passed. In my opinion, circuit breakers should never be used to protect electronic devices such as radios, amplifiers or crossovers. Most common circuit breakers (thermal snap action) take far too long to open the circuit path. This does not mean that they are not useful. When they are properly selected they do a good job of protecting wiring and devices such as electric motors. Some breakers are self resetting. Others require manual resetting. I strongly recommend using a manual reset type. This will allow you to watch for any problems when the circuit path is restored.

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is there an archive section on the site that we could list this and other 'tutorial' things in?

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