Brads Electronic Projects Forum

Welcome to the first installment to the brads projects university!

This lesson is a small introduction and one use (out of the many) for resistors.

Resistors are very handy little devices that come in a variety of packages such as:

Through hole:


Surface mount:


Dual inline package (DIP):


The most handy for prototyping is the through hole and dip type.


Resistors basically restrict current flow in a circuit. For example if we wanted to make an LED shine (without blowing it up) we would grab ourselves a battery or power supply, (lets say 6v) and then we would get an LED and a resistor.

You would connect the LED anode to the +ve end of the power supply, the cathode would goto one end of the resistor and the other end of the resistor would goto the -ve end of the battery.

A typical voltage drop of a red LED would be 2volts. So if we have 6volts total, and the LED has grabbed 2 of that, then we have 4volts left across the resistor. If the resistor that we have chosen is 1kohm (one thousand ohms) then we can work out how much current is flowing in the circuit:

4volts / 1kohms = 4mA

So in this case the LED would be quite dim.

How could we make it brighter? That's right - we would REDUCE the value of the resistor!

so lets get rid of the 1Kohm resistor and throw in a 100 ohm instead.

how much current is flowing in the circuit now?

4volts / 100ohms = 40mA

now that will be a nice and bright LED

what about if we had a blue LED with a voltage drop of 2.5volts and we wanted 25mA to flow through this LED with a power supply of 6volts.

What value resistor would we need?

Hi, as we are on the subject of resistors I just thought I would share a little more info
on resistors and will try not too get to complicated hehe!

Resistors are used to restrict current flow in a circuit and to a component.
as Brad pointed out above, Resistors used in electronics can have resistances as
low as 0.1 Ohm or as high as 10 M.

Resistors can be connected in Series -

When resistors are connected in series their combined resistance
is equal to the individual resistances added together.
For example if resistors R1 and R2 are connected in series their
combined resistance, R, is given by:


Combined resistance in series: R = R1 + R2

This can be extended for more resistors: R = R1 + R2 + R3 + R4 + ...



Note - that the combined resistance in series will always be greater than any of the individual resistances.

Resistors can also be connected in Parallel -

When resistors are connected in parallel their combined resistance
is less than any of the individual resistances.
There is a special equation for the combined resistance of two resistors R1 and R2:

Combined resistance of
two resistors in parallel: R = R1 × R2
***********************R1 + R2
(ignore the * symbol as this was used to get the correct spacing)
(for brad website)



For more than two resistors connected in parallel a more difficult
equation must be used. This adds up the reciprocal ("one over") of
each resistance to give the reciprocal of the combined resistance, R:

1 = 1 + 1 + 1 + ...
R**R1*R2*R3
(ignore the * symbol as this was used to get the correct spacing)
(for brad website)

The simpler equation for two resistors in parallel is much easier to use!

Note - that the combined resistance in parallel will always be less than
any of the individual resistances.

Excellent!

now we're getting somewhere

I came across this and thought it would be good to put here

its for a 4 and 5 band resistor colour chart

A very welcome addition!