- Category: Ham Spot Presents
- Published on Thursday, 09 February 2012 04:56
- Written by David L Norris, KG9AE
In this video I present a variety of common types of resistors used in the last 50 years. I do not cover current technologies such as surface-mount components as these will be the most familiar to new builders.
Most circuits only need simple low wattage fixed carbon resistors. However there are plenty of cases where you might need something more substantial. For example, a conventional DC power supply will need very low resistance high wattage ballast resistors, probably wire wound, to balance the current flowing through each pass transistor. For testing a transmitter you will need very high power carbon resistors; more common high power wire wound resistors are inductive at radio frequencies.
You may want a variable resistance in many circuits. This could be a simple carbon pot to adjust a signal level or high power wire-wound for adjusting brightness on a incandescent bulb. This is where potentiometers rule the world. But it would be a mistake to think that potentiometers are the only option for varying resistance. Many devices vary resistance based on environmental factors such as ambient light, temperature or pressure.
Carbon film resistors come in standard values. These values are based on a geometric progression of the tolerance. All resistors with a 5% tolerance have values spaced 5% apart. 10% and 20% resistors are fairly unheard of these days but were very common in the first half of the 20th century. So you are likely to find them in salvaged items or older gear. Here is a table of standard values that I suggest you keep on hand. ¼W carbon resistors are sufficient for most circuits. Sometimes you'll need a value not listed in my table and for that you can combine several smaller values at least until you can buy the exact value you need. For example, if you need a 150kΩ then a 100kΩ + 47kΩ is only 2% off.