Hi there,

the tolerance band on a resistor is there to allow for inconsistencies in manufacturing, so a 100k resistor with 5% tolerance means that the actual value of the resistor could be between 95% and 105% of the 100k value (so the resistor is between 95K and 105K).

resistors with 5% tolerance are most often the carbon type, and are okay for most uses. if the resistor is required to be precise, metal film resistors usually have 1% tolerance (a 100k resistor would vary between 99k and 101k), but they are more expensive. (for cheap metal film resistors, check ebay for packs of resistors - i got 1080 metal film resistors for £10, ($20 US) in common values).

as far as checking the wattage of a resistor, your 'standard' size of resistor is usually 1/4 Watt for most low voltage applications. if the resistor needs to be 1/4 Watts, then this is a minimum, and any value above this can be used, but you have to take space into consideration, as they get very large.

you can tell the power rating of a resistor by measuring the length of the 'bead' or 'body' on the wire of the resistor:

1/8 (0.125) Watt resistors usually measure between 3mm and 3.4mm (these are the really tiny ones you get in walkmans etc)

1/4 (0.25) Watt resistors usually measure between 5.5mm and 6.5mm (these are 'standard' size resistors you see everywhere)

1/2 (0.5) Watt resistors usually measure between 8mm and 9mm

anything larger than 0.5 Watts is usually huge. to get the power rating go to an electronics website (i used

www.rapidonline.com) and view the datasheet for the resistor of that wattage - you get full measurements and dimensions.

to work out the minimum wattage required, you need to know the equation: P=VxI ,

i.e. power in watts = voltage x current

so if you know the voltage and current then you can work out power. if you do not know the voltage, the equation can be rearranged (using ohms law) to get P=I

^{2}R

if you dont know the current, you can use P=V

^{2}/R

I hope this is useful...

Dan