How to Buy Electronic Components

If you are replacing an electronic component in an existing circuit, the best bet is to purchase an exact replacement. The circuit was engineered as a whole and a different component may cause expected results.
When buying a component for a new circuit or to modify an existing circuit, there are a few key parameters to keep in mind. This article will focus on the hobbyist or technician building a new circuit from an untested schematic.

Difficulty: Moderately Easy

Instructions

Things You'll Need:

• Electronic circuit schematic with component values, voltage and current estimates.

How to Buy Electronic Components

1. Calculate the expected voltage across and current through each component in the schematic. Multiplying the voltage by the current will give the wattage the component may see as well.

2. Consider the most important buying parameters for resistors: resistance value (no cost impact); tolerance (how close to the resistance value it is--more precise is more expensive); and wattage rating (always get a rating twice or more expected wattage--more watts are more expensive).

3. Determine if the capacitor must be polarized. A polarized capacitor can only operate where one side is a higher voltage than the other side. For those situations (usually DC), a polarized capacitor is often the most cost-effective way to achieve high capacitance at reasonable cost.

4. Determine for each capacitor (polarized or non-polarized) the capacitance value (higher capacitance usually costs more) and voltage rating (a higher rating costs more and is larger).

5. Realize if the circuit uses inductors, the tolerance and wattage considerations are much the same as with resistors. The value (in Henrys) will have some impact (more Henrys are more expensive and larger, as a rule).

6. Know for diodes the voltage seen, current carried, voltage drop (almost always 0.7 VDC for silicon diodes) and Peak Inverse Voltage (PIV) rating. PIV is the limit of voltage the diode can be take when reverse biased without breaking down. Higher voltage, current and PIV generally cost more. Knowing the voltage, current and PIV the diode will see are needed when choosing your diode.

7. Choose any other active components, such as transistors, FETs and integrated circuits by the function needed. Considerations usually include switching speed (faster costs more, but uses less power); voltage and current rating (more usually cost more); and power rating.

Tips & Warnings

• For passive components (resistors, capacitors, inductors), there will often be quantity discounts from most wholesalers. For active components (transistors, ICs, etc) older designs are usually cheaper. Silicon components will be cheaper than active components made of other materials such as gallium arsenide (GaAs).

References

• About resistors
• About capacitors
• A component datasheet archive