Using only resistors in the E12,
E6,
E3 and
E1 ranges of
resistors, the calculator will find a pair of resistors which can be used to
create a resistance which is the closest match to that required, and will also
report any better match which can be obtained using three resistors ( or using
four resistors for the E1 and E3 series ).
Preference is generally given to the closest solution which uses resistors in
series, rather than in parallel, matched pairs of resistors, and resistors of
1K, 10K and 100K value.
The possible combinations of resistors used during calculation is not
exhaustive ( as the processing time would become excessively long ), but is
comprehensive enough to find good matches for almost any resistance required.
You can enter the resistance required in almost any format; 1234, 1234R, 1.234K,
1K234, M001234 and 0M001234 all represent the same resistance.
You can also enter resistance equations using '+' for resistors in series and
'|' for resistors in parallel. Resistance can be multiplied and divided by
using '*' and '/' respectively. Parenthesis, '(' and ')', can be used to group
resistor sets together.
For example; a required 24K5 resistance could be entered as '24K5', '24.5K',
'24.5E3', '24K+500R', '(48K|48K)+(1K|1K)', '12K25*2', '98K/4' and in many other
ways.
The calculator will not create a solution which uses a single, standard E series
resistor specified in the Resistance Required field, as it is assumed that this
value is unavailable; otherwise why would you be trying to find an alternative
solution to using a standard value ?
Actual resistor
tolerances are not taken into account when performing
the calculations.
The results of the calculations are given in one of the following formats ...
| R1 | | A single resistor |
| R1 + R2 | Two resistors in series |
| R1 + R2 + R3 | Three resistors in series |
| R1 + R2 + R3 + R4 | Four resistors in series |
| R1 | R2 | Two resistors in parallel |
| R1 | R2 | R3 | Three resistors in parallel |
| R1 | R2 | R3 | R4 | Four resistors in parallel |
| R1 + ( R2 | R3 ) | One resistor in series with a parallel pair |
| R1 | ( R2 + R3 ) | One resistor in parallel with a series pair |
| R1 + R2 + ( R3 | R4 ) | Two resistors in series with a parallel pair |
| R1 | R2 | ( R3 + R4 ) | Two resistors in parallel with a series pair |
| ( R1 | R2 ) + ( R3 | R4 ) | Two parallel resistor pairs in series |
| ( R1 + R2 ) | ( R3 + R4 ) | Two series resistor pairs in parallel |
For example, a 24K5 resistance can be formed from resistors in the E6 or E3
range as follows ...
| 2K2 + 22K | | with -1.22% error, or |
| 1K + ( 47K | 47K ) | an exact match |
A three or four resistor solution is only given if a better match than a two
resistor solution can be found, and is worthwhile checking for.
Resistor Tolerances
Although it is possible to find a combination of resistors which will create
any arbitrary resistance value, it is rarely necessary to do so.
All resistances have a manufacturing tolerance which is commonly
+/- 1%, but may be as much as +/- 10%
depending upon the type of resistor.
The original E series values were chosen to take advantage of this manufacturing
inaccuracy,
as any specifically required value will normally fit within the tolerance range
of the resistor values available. It is usually quite acceptable to choose the
closest resistor value to that required, without creating problems with the
operation of the circuit being designed; this is why having just the E3 or E1
range of values is acceptable in many cases.
It is usually pointless to create a resistor combination which matches
a required resistance exactly when the resistors being used have wide
tolerances themselves, although a perfect match is found, the tolerances of
the actual resistors used in the solution will often make the result less than
perfect.
Despite the issue of tolerance, the Resistor Calculator is still extremely
useful for those who keep an inventory of only E1,
E3 or E6 resistor values and need
to cater for a resistance value which is not readily available.
